Dynamics isocapnic
of respiratory VT response to pH, forcing in chemodenervated
H. L. BORISON, S. F. GONSALVES, S. P. MONTGOMERY, AND L. E. MCCARTHY Department uf Pharmacology and Toxicology, Dartmouth Medical Hanover, New Hampshire 03755
B~RISON, H. L., S. F, GONSALVES, S. P. MONTGOMERY, AND L. E. MCCARTHY. Dynamics of respiratory VT response to isocapnic pH,, forcing in chemodenervated cats. J. Appl. Physiol.: Respirat. Environ. Exercise Physiol, 45(4): 502-511, 1978. -Arterial blood pH (pH,) was continuously monitored in decerebrate or pentobarbital-anesthetized cats with a rapidly responding hydrogen ion sensor inserted into the aorta. Alveolar carbon dioxide partial pressure and pH, were controlled independently during infusions of 1 N NaHCO:, or 0.5 N HCl into the inferior vena cava. Shifts in pH, up to 0.3 unit were effected isocapnically within 2.5-20 s over a working pH, range of 6.9-7.7. Before carotid sinus neurotomy, average onset latency of the tidal volume (VT) response to acid and alkaline pH, shifts was less than 5 s and the average VT response half time was less than 8.5 s regardless of whether the vagus nerves had been interrupted. After carotid sinus neurotomy, the AVT onset latency was approximately doubled, whereas the response half time was prolonged about eightfold on the average. Subsequent vagotomy tended further to increase the responding time lag. Nevertheless, the minimum response latency after peripheral chemodenervation was less than the ApH, forcing rise time. It is concluded that the central chemoreceptors promptly sense pH change in the arterial blood and that neural processes adjust the time course of the respiratory response through the VT controller.
indwelling pH sensor; ramp-step pH, forcing; carotid neurotomy; vagotomy; central hydrogen ion receptors; volume controller
sinus tidal
ATTEMPTS IN THE PAST to study ventilation response dynamics to systemic forcing of arterial blood carbon dioxide partial pressure (Pa& and pH (pH,) have been hampered by inadequate means of monitoring pH, in situ. To circumvent this problem, Fitzgerald et al. (11) devised the technique of administering intra-arterially a transient infusion of tonometered blood with known gas and hydrogen ion composition on the assumption that the chemical stimulus was thereby transmitted abruptly to the intended target. Their first effort was concerned with intracarotid infusions of hypercapnic and hypocapnic blood uncorrected for associated change in pH. They found that the carotid body reacted to PacOz (and pH,) displacements as promptly as to cyanide. Employing the same technique, Domizi and Perkins (5) then showed that acidified normocapnic blood infused into the carotid arteries elicited respiratory stimulation as fast as did hypercapnic blood; moreover, 502
cats
School,
alkaline blood depressed the respiration. By contrast, when Fitzgerald et al. (9) made infusions into the vertebral arteries to bypass the peripheral chemoreceptars, they found that acidified normocapnic blood caused no significant change in ventilation during a 100-s delivery period, whereas infusions of hypercapnic blood produced an increase in ventilation but only after a delay of 20-30 s. They concluded that increased acidity of the arterial blood does not stimulate the central chemoreceptors in the absence of an increase in C02. In the present work, we have attempted to exercise more strict control over pH, forcing transients with the use of a recently developed fast responding pH electrode suitable for intravascular measurements (15). We found in apparent, conflict with the observations of Fitzgerald et al. (9) that, after peripheral chemodenervation, the respiratory controller responds with minimal delay in a direction-specific manner to isocapnic acid and alkaline shifts in the arterial blood pH. Nevertheless, the time course of the ventilatory response is markedly prolonged by inactivation of the peripheral chemoreceptors. It has been our practice to use tidal volume (VT) as the sufficient criterion of respiratory chemosensitivity to CO, (3). We have reported that pH,, like Pacq , has no direct influence on respiratory frequency, which is affected reflexly through the vagus nerves as a function of tidal volume (2). Accordingly, all chemogenic ventilatory responses presently described are expressed solely as changes in tidal volume. Furthermore, since this paper deals only with the dynamics of chemorespiratory behavior, no attention is given to actual respiratory amplitude. In this regard, the influence of arterial blood pH change on breathing amplitude under steady-state conditions remains largely unaffected after section of the vagus and carotid sinus nerves in cats (2, 14). Hence, the changes in response dynamics that resulted from the peripheral denervations, as reported herein, were not accompanied by any notable loss in response gain. METHODS
Experiments were performed on 17 well-nourished, fully grown cats unselected for sex. All of the cats were initially anesthetized with pentobarbital sodium, 40 mg/kg, given intraperitoneally. In six of the animals,
0021-8987/78/0000-0000$01.25
Copyright
0 1978
the American
Physiological
Society
Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (129.237.035.237) on January 1, 2019.
ARTERIAL
HYDROGEN
ION
AND
TIDAL
VOLUME
503
CONTROL
anesthesia was maintained to completion of the experiment in the same work day. The remaining animals were decerebrated supratentorially and were kept overnight at constant body temperature, regulated by rectal probe and heating lamp through an on-off controller. The decerebration procedure consisted of ligation of the external carotid arteries followed by suction removal of the forebrain rostra1 to a precollicular midbrain transection. The acute experiment was performed on the following day when for all practical purposes the cat was unanesthetized. The monitored experimental variables were alveolar carbon dioxide and oxygen partial pressure (PACT, and PA+), ventilatory gas movement, ,pH,, and arterial blood pressure. These were recorded continuously on a Brush six-channel rectilinear oscillograph. PACES, ventilation, and pH were also recorded on electromagnetic tape for later playback. End-tidal CO, and 0, tensions were obtained by drawing tracheal (PETco2 and PET& gas through a Beckman infrared CO, analyzer in series with a Westinghouse electrochemical 0, analyzer. The amplitude of ventilation was recorded by integration of the tracheal gas flow-pressure signal across a Fleisch no. 0 pneumotachometer. The integrator was operated in the plus-minus mode offset to balance the intratracheal gas sampling flow for the CO, and 0, determinations. Amplitude measurements were made on the strip chart with micrometer calipers to the nearest 0.05 mm. pH, was measured uninterruptedly by means of a rapidly responding (time constant = 0.1 s) indwelling pH electrode obtained through the courtesy of R. Lawton and 0. LeBlanc of the General Electric Company (15). The hydrogen ion-sensing electrode was inserted into the femoral artery and was advanced into the abdominal aorta to reach just above iliac bifurcation* The reference electrode was placed alongside the abdominal aorta in the track made by a blunt seeker inserted retroperitoneally through the femoral triangle. This extravascular arrangement of the reference electrode dispensed with the need for heparin since the active intra-arterial electrode was fabricated from an elastomeric polymer that does not by itself cause blood coagulation. The pH, signal was amplified through a Radiometer pH meter, followed by an intermediate differential amplifier with a low-frequency filter to reduce noise in the recording. Correction of electrode drift, if any, was accomplished in situ by administering CO, to stabilize PA co, and by measuring pH directly from an arterial blood sample taken periodically for comparison with and adjustment of the electrode signal. Two methods were employed for forcing of the pH,. In either case, PACT, was controlled isocapnically by breath-to-breath adjustment of inhaled CO, concentration. In the first four experiments, NaHCO, (up to 1.0 N) or HCl (up to 0.5 N) was infused into the inferior vena cava at constant rates provided by a Braun syringe pump. In all subsequent experiments, NaHCO, or HCl were infused by means of a variable-speed roller pump that permitted the delivery of a ramp-step form of pH displacement for evaluation of ventilatory response dynamics. Our technique for isocapnic “step” forcing of pHa is schematized in Fig. 1. The first maneuver was
ventilatory stabilization at an elevated level of ~~~~~ to permit subsequent control of variations induced by the acid or base infusion, both of which generate CO, in the venous return to the lung. At tiPne 0, the roller pump was switched on to a high flow (up to 8 mllmin) by one operator while the second operator rapidly adjusted inhaled CO, to maintain a constant PACES. The counterreduction in CO, administration had to be started prior to the arrival of the pH wave front at the arterial sensor since CO, was evolved from the blood during the first pass of the infused acid or alkali through the lung. After the initial pH, displacement, the pump speed was quickly backed off and then adjusted continuously to hold pHa steady. Carotid sinus nerve section or vagotomy or both was performed in every experiment. The carotid sinus nerve was identified and severed at its point of connection with the glossopharyngeal nerve. The vagus nerve was cut at the midcervical level. In those instances after combined buffer nerve section where the resulting neurogenic hypertension was considered excessive, hexamethonium chloride was administered intravenously at 1 .O mg/kg per dose to reduce the blood pressure to a reasonable level. Oxygen was administered occasionally after the peripheral denervations to forestall circulatory and respiratory instability. Otherwise, PAN, was maintained around 100 Torr to avoid interactive effects of hypoxia or hyperoxia with CO, or H,+ chemosensitivity or both (10, 18). RESULTS
Isocapnic
?‘Ramp” Forcing
of pHa
The ventilatory effects of constant rate infusions of HCl and NaHCO, for 5-min periods in a decerebrate vagotomized cat are shown in Fig. 2. Inspired Pcoz was adjusted during and after the infusions to maintain a demonstrated fixed ~~~~~ of 22 Torr. This experiment that the tidal volume is affected promptly and continuously in a direction specific manner by long ramp shifts (more nearly hyperbolic) in arterial blood pH. The total shifts in pH, amounted to 0.2 and 0.3 of a pH unit, but tidal volume started to change with barely detectable pH, displacement. The lack of influence of the pH, alterations on frequency of breathing in the vagotomized preparation deserves particular emphasis (2). Blood pressure was not remarkably affected during the acid and base infusions, although a delayed decrease of approximately 25 Torr followed the change in pH, steady state from 7.3 to 7.2 The effects of acid and base infusions at constant PACT, after combined vagotomy and carotid sinus neurotomy in another decerebrate cat are shown in Fig. 3, In this case, the pHa changes were recorded at two levels of amplification. Stimulus arrival at the pH sensor and respiratdry related oscillations are more evident at the full scale amplitude of 0,2 pH unit. Acid shifts developed more slowly than did shifts in the alkaline direction owing to the logarithmic nature of the pH scale and because infused bicarbonate was tolerated at twice the concentration of HCl, i.e., 1.0 N NaHCO, versus 0.5 N HCl. This experiment demon-
Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (129.237.035.237) on January 1, 2019.
504
BORISON,
GONSALVES,
MONTGOMERY,
AND
MCCARTHY
FIG. 1. Schematic of technique for ApH, forcing at a constant ~~~~~~ Vertical line in recording on right marks starting time of roller pump infusion cava. Interval into inferior vena marker is spaced every 10 s. Initial correction of inspired Pco2 was needed before the ApH stimulus arrived at hydrogen ion sensor in the abdominal aorta. Note extravascular placement of pH reference electrode, Sample physiological recording, taken from a cat with intact peripheral nerves, shows a typical fast -A% response to an alkaline step shift in pH,.
-_. -----~-I:::-._
&rated that the onset of the respiratory response e0 a change in pHa was not remarkably delayed by peripheral chemodenervation. However, owing to the slow rate of change in the pH shift, it was not possible to establish the effect of carotid sinus neurotomy on the response half time. Isocapnic
‘Step”
Forcing
of pHEL
Figure 4 shows the recorded physiological variables in relation to rapid and sustained alkaline shifts in an anesthetized cat (above) compared with a decerebrate cat (below) before and after section of the vagus and carotid sinus nerves. The tracings of tracheal Pco2 and the blood pressure are illustrated for the anesthetized cat only. Hexamethonium was administered after the debuffering procedure in this cat to counteract the resultant neurogenic hypertension; the effect of the drug treatment is manifested in the reduced blood pressure in the panel on the right. Hexamethonium, tried previously, did not noticeably affect the ventilatory response dynamics. Owing to the close similarity in stimulus-response behavior of anesthetized and decerebrate preparations, no distinction was made between these experimental conditions in our data analysis.
.-
Most of the timing measurements were made on expanded writeouts of tape recordings. Figure 5 illustrates the front-end transients of acid and base displacements recorded from the arterial pH sensor (below) in association with respective ventilatory responses (above). Signal amplification was adjusted bet-ween panels to provide approximately equal excursions, and so the amnlitudes have no quantitative significance. The two &right parallel line; in each paner mark the transit time from the start of infusion into the inferior vena cava to the start of the pH change sensed in the abdominal aorta, The arrows placed below the timer indicate the estimated onset latency of the respiratory response from the start of the pH, shift. The middle panel illustrates the technique of back extrapolation of inspiratory peaks to find the time of onset between breaths in the slow breathing vagotomized cat. In Table 1 are summarized the time and amplitude characteristics of all satisfactory step pH, forcing tests separated for acid and base shifts that were performed under the different conditions of peripheral innervation. Test data were selected on the basis of acceptable control of PA co2 and of pHa levels between initial and final steady states as judged by plateau attainment. The overall range of pH, examined was from 6.87 to 7.70. Among all innervation categories, average step
Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (129.237.035.237) on January 1, 2019.
ARTERIAL
HYDROGEN
ION
AND
TIDAL
VOLUME
CONTROL
505
DECEREBRATE,VAGX 40 paco2 mmHg 20
vT 2OOmI
200 I3.P 100 mmHg 0 FIG. 2. Ventilatory response to slow ramp constant PA co, after vagotomy in a decerebrate mark start and end of acid (lefi) and base (right)
ApH, forcing with cat. Vertical lines infusions yielding
slope varied from 0.018 to 0.031 U/s for an average wave-front duration between 5.5 and 9.9 s, which produced average pH, shifts between 0.10 and 0.22 unit. These pH, shifts amounted to average H,+ concentration changes of from 15 to 22 nmol/l. Effects of Peripheral Denervations on Tidal Volume Response Dynamics (Table 2) Transit time. The average elapsed time from the point of infusion into the inferior vena cava to the sensing point at the iliac bifurcation of the aorta was slightly less than 6 s for all tests in the nerve-intact condition. Section of the vagus and carotid sinus nerves performed independently did not significantly affect the transit time. However, combined denervation resulted in an increase in average transit time of approximately 2 s over the nerve-intact condition, constituting a statistically significant difference in durations. Onset latency. The average delay of response onset following the start of the pH perturbation at the sensor amounted to less than 5 s in the nerve-intact condition.
long pH, displacements shown directly above blood pressure tracing. Note continuous progression of change in tidal volume appropriate to the direction of ApH, forcing.
It should be kept in mind that this latency is within the rise time of the pH, step forcing stimulus (Table 1). After vagotomy (VAGX) alone, the average response onset latency was reduced to 3.3 s, just significantly less than in the nerve-intact condition. After carotid sinus neurotomy (CSNX) alone, the average onset latency was about doubled. Despite the rather large standard deviation of the measurement after CSNX, the difference in latency from the nerve-intact condition was highly significant. After the sinus nerve section, the onset latency approximately matched the pH, step rise time (Table 1). After combined denervation (Both), the onset latency was no different from that following CSNX alone. The large standard deviations obtained in the CSNX and Both conditions can be attributed in part to the less acute break that occurred at the start of the respiratory response after chemodenervation (Fig. 6). Respunse half time. Before any peripheral nerve section was done, the average time taken to reach half the final change in tidal volume was 8.4 s. Inasmuch as the average rise time of the step forcing stimulus for all tests in the nerve-intact condition amounted to some 7
Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (129.237.035.237) on January 1, 2019.
506
BORISON,
GONSALVES,
MONTGOMERY,
AND
MCCARTHY
VAGX+CSNX 40r tco2 mmHg 20
vT IOOml
7.8 r 73 l
200 BP l
l
100
mmHg
F
FIG. 3. Ventilatory constant PA co, after omy in a decerebrate (left) and acid ( right)
response to slow ramp ApH, forcing with combined vagotomy and carotid sinus neurotcat. Vertical Lines mark start and end of base infusions, yielding progressive pH, displace-
s, the response half time exceeded the stimulus transient by little more than a second which is certainly within the error of measurement. VAGX by itself did not affect the tidal volume response half time, By contrast, carotid sinus neurotomy alone prolonged the response half time about eightfold to an average duration of 64.6 s, a highly significant difference from the Intact and VAGX conditions. Combined denervation produced no further significant prolongation in response half time, suggesting thereby that carotid chemodenervation fully accounted for the observed effect. The tidal volume response behavior before and after carotid sinus neurotomy in a vagotomized cat is illustrated graphically in Fig. 7. Response decay fraction is plotted semilogarithmically against time. For the acid infusions, tidal volume growth has been plotted as an inverted function. In this graph, zero time was taken at the start of infusion which means that the time to onset includes the transit time in addition to the response latency. The average total time to onset was approximately 10 s. After VAGX alone, the response half time
ments change vation.
shown at two in tidal volume
levels even
of amplification. after complete
Note peripheral
early onset chemodener-
of
for acid and base infusions was between 2 and 8 s, contrast, following CSNX the half time amounts for tests to about 40 s, representing a prolongation by least five times in tidal volume control lag produced the carotid chemodenervation.
In all at by
DISCUSSION
Estimation of a biological response time hinges crucially on the nature of the forcing transient. Figure 8 illustrates how a ramp-step with a total rise time of 12.5 s should affect the time course of a first-order response by comparison with the theoretical response to a square step. Prechemodenervation data points are shown in relation to the curve calculated for a time constant of 6 s when forced by the given ramp-step according to the following equations. These equations were derived by adding equal and opposite ramp-forced first-order response functions (17) displaced from each other by the parameter that specifies the rise time for the step shift. When t 5 a,
Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (129.237.035.237) on January 1, 2019.
ARTERIAL
HYDROGEN
ION
AND
TIDAL
VOLUME
PENTOBARBITAL
CONTROL
507
DEBUFFERED
40 pAco2 mmHg
INS? 50 ml I
.-
DECEREBRATE DEBWFERED
100 ml
V’ ’
r-----
0.5u
J
--_ _ - / I’ 7.42 /
7.48
1-I
4. Comparison of tidal volume responses to ramp-step alkaline pH, forcing in a pentobarbital-anesthetized cat (aboue) and in an unanesthetized decerebrate cat (below) before and after combined section of carotid sinus and vagus nerves. PAN% and blood pressure tracings are not shown for decerebrate cat. Vertical lines mark start FIG.
y(t) =
t - ~(1 - e@‘) a
When t 2 a, a + T[e-‘/7 - e’“-““] y(t) =
a
where y = FAVT; and a is the ramp-step rise time, t the time from start of response, and T the response time constant. The data points in Fig. 8 correspond to the base forcing test before CSNX shown in Fig. 7. The measured half time of the biological response matches that of the derived curve for the ramp-step, but is approximately double the theoretical minimum determined for a square step. On the other hand, for a time constant of
of infusion for each test. Arrow heads mark end of infusion, which after denervations was sustained at constant pH for over 10 min. Note prolonged response half time with early onset of effect in both cats after debuffering procedure.
50 s as might be obtained after CSNX, the error of measurement introduced by a ramp-step stimulus becomes a small fraction of the response time. Our measurements of ApH,-AVT half time (uncorrected) before chemodenervation are practically identical to those reported by Gray (12) for discharges recorded from Hering’s nerve in response to pH forcing of the carotid body perfused in situ. He found a median half time of 11 s, which is reduced to 8 s after subtracting stimulus transit time. Gray delivered the test solutions to the common carotid artery in the cat at the rate of 0.1 ml/s for 30-60 s. Rise time of the pH stimulus wave front at the receptor site cannot be ascertained in the method employed by him, thus leaving open the possibility of blunted forcing of the carotid body with its consequence of error in the observed response time.
Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (129.237.035.237) on January 1, 2019.
508
BORISON,
VAGX ACIQ
GONSALVES,
MONTGOMERY,
CSNX BASE
BASE
AND
MCCARTHY
WSEC.
f--
FIG. 5. Expanded tracings from a tape recording of VT and pH, alterations associated with acid and base infusions at constant PAN%. Amplitudes have no absolute significance. Upper trueing is a pneumotachogram in plus-minus integration mode. Lower tracing shows wave front of ramp-step change in pH,. Vertical lines bracket stimulus transit time in each test. Estimated response onset is
TABLE Peripheral Innervation
1. Characteristics n (Cats)
of step pHa forcing tests
ApHJs
At, s
4%
AHa+,
nM
EL:;Intact Acid (7.47-7.10) Base (7.13-7.67) VAGX Acid (7.56-6.97) Base (7.15-7.66) CSNX Acid (7.66-7.15) Base (7.11-7.49) Both * Acid (7.43-7.15) Base (7.13-7.70)
3 (2) 13 (9)
7 (4) 6 (4)
3 (1) 6 (3)
0.021 (0.011-0.026) 0.024 (0.008-O. 048)
5.5 (3.5-8.0) 8.6 (4.5-16.6)
0.10 (0.09-O. 13) 0.18 (0.08-0.26)
15 (8-20) 17 (6-34)
0.031 (0.011-0.08) 0.036 (0.008-0.08)
8.1 (2.5-15.5) 9.2 (3.0-20.0)
0.15 (0.1 l-0.20) 0.20 (0.14-k 28)
19 (10-35) 19 (12-26)
0.029 (0.011-0.056) 0.036 (0.027-0.06)
8.2 (2.7-14.5) 8.2 (3.5-20.0)
0.15 (0.15-o. 16) 0.21 (0.15-0.30)
16 (10-21) 25 (20-39)
marked by arrows below timer. Spike on final pH trace after CSNX is pump-off artifact indicating that infusion was maintained for longer than 3 min to achieve a VT stea’dy state in contrast to rapid response completion resulting after vagotomy alone, which was practically simultaneous with rise time of the ApH forcing stimulus.
central chemoreceptors. The transit time from the inferior vena cava to the iliac bifurcation of the aorta, which varied between 5 and 10 s, is accurately measured by the present method; and uniform blood pH in all branches of the aorta is well assured. Any difference between stimulus arrival time at the carotid and iliac bifurcation, however, introduces an error in the response latency that follows stimulus delivery. According to Machella (16), the average linear velocity of blood flows in the carotid and femoral arteries of the dog are in the neighborhood of 10 cm/s, the carotid flow being somewhat faster. For our purposes, we are assuming that linear blood flow velocities in the cat are not TABLE 2. Stimulus-VT to pHa step forcing Peripheral Innervation
3 (3) 8 (5)
0.018 (O.OlS-0.021) 0.027 (0.019-0.047)
9.9 (8.0-12.2) 8.5 (6.4-13.5)
0.18 (0.15-o. 20) 0.22 (0.17-0.30)
22 (20-26) 19 (10-24)
Values are averages with ranges in parentheses. n, No. of tests performed in no. of cats given in parentheses; ApHJs, best eye-fitted slope of the stimulus transient from time of arrival at the pH sensor to maximum pHa change achieved; At, time required for excursion of pI& between initial and final levels; ApI&, magnitude of pH, shift between initial and final levels; AH;+, corresponding change in concentration of hydrogen ion in arterial blood. * Both signifies VAGX and CSNX without regard to sequence of denervation.
Response onset. Interpretation of response onset latency in the present work is complicated by the fact that the pH sensor was placed at a site in the arterial tree different from the location of the peripheral as well as
Transit
response dynamics
Time,
s
Onset
Latency,
VT-Response Half Time, s
Intact
(n = 16)
5.79 k 1.26
VAGX
(n = 13)
6.08 2 0.77
3.32 2 1.94 P = 0.05 vs. Intact
8.15 f 4.2
CSNX
(n = 9)
5.39 k 0.70
10.72 f 6.56 P < 0.05 vs. Intact
64.56 + 27.64 P K 0.05 vs. Intact
Both*
(n = 11)
12.11 * 4.26 P < 0.05 vs. Intact P = 0.05 vs. CSNX
76.64 f 50.34 P c 0.05 vs. Intact P = 0.05 vs. CSNX
8.11 2 1.04 P K 0.05 vs. Intact
4.74 5 1.69
s
8.39 k 3.92
Values are means 2 SD. Measurements include acid and base tests tabulated without regard to whether the cat was anesthetized or decerebrated. n, No. of tests; no fewer than 3 cats are represented for each condition. Student’s t test was used to determine probability (P) of significant difference between groups. * Both signifies VAGX and CSNX without regard to sequence of denervation.
Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (129.237.035.237) on January 1, 2019.
ARTERIAL
HYDROGEN
ION
AND
TIDAL
VOLUME
CONTROL
7.48
CSNX
FIG.
profiles (above) ogram
6. Apposition of ApH, forcing and AVT response time-related of acid and base tests at constant PA~~,, after VAGX alone and after combined VAGX and CSNX (below). Pneumotachshows expiratory excursions only. Vertical line marks infu-
materially different from relative distances of the electrode from the heart mate that the magnitude
those in the dog. Given the chemoreceptors and the pH in our experiments, we estiof possible error in the deter-
sion pump starting time for all tests. Arrow termination time. Note close approximation sponse contours before CSNX.
heads mark of stimulus
stimulus and re-
mination of response latency is no greater than one second. Eldridge (7) recorded carotid sinus nerve discharge within one second after bolus injection of a variety of agents into the aortic root of anesthetized
Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (129.237.035.237) on January 1, 2019.
510
BORISON,
-e ---- - ~~~~~~-L- - _-------_---l
.4
x
fAVT
* _--___
0
l
0
i
o0ASE ACID
i- ax
P
VAGX +CSNX O* M?& A ACID
A
0 00
bA
A
A0
.3 i 0
0
0
A 0 0A
0
0
A0
0
0 A
0 0
l
A
0
A
A
A
A
A 0 a
I
4
d A
0 uo 11
30
II
40
50
I11
60
70
80
90
A 4
I
I1
100
110
120
SECONDS FIG. 7. Semilogarithmic breath-by-breath plot of fractional AVT response decay (for base step-forcing tests) and response growth (inverted for acid step-forcing tests) before and after carotid sinus neurotomy in a vagotomized cat. Dashed line crosses responses in progress at their respective half times including latency of onset and transit time from start of the infusion pump at 0 s. Each symbol represents a separate test.
FAV,
0
IO
20
40
50 SECONDS
60
70
80
90
100
FIG. 8. Comparison of first-order response curves calculated nondimensionally for square-step (instantaneous) and ramp-step (dashed line) forcing of systems operating with time constants of 6 and 50 s. For each time constant (T), fine-line curue represents response to square-step stimulus and bold-line curue represents response to ramp-step stimulus. Ramp-step forcing time was set at 12.5 s to simulate stimulus used for VAGX-base test shown in Fig. 7. Actual data points corresponding to said ramp-step test were found to be best fitted by theoretical curve calculated with time constant of 6 s. Ramp-step forced response curves were obtained with use of equations given in text. Square-step forced response curves were obtained with use of standard equation for a first-order response to an instantaneous displacement. Difference in half times between square-step and ramp-step forced response curves is indicated by bracketed lines.
cats. By comparison, our measured average latency (posttransit) varied from a minimum of 3.3 s after VAGX alone to 12.1 s after CSNX plus VAGX. Part of the response onset delay must be attributed to the time taken for the forcing stimulus to reach chemoreceptor activation threshold. As the response half time before chemodenervation was in many cases less than the ApH rise time, it follows that the activation threshold was reached at a small fraction of the pH change. The average forcing amplitude varied from 0.1 to 0.2 pH unit delivered over a time span from 2.5 to 20 s. Taken together with response onset latency, these values indicate that the carotid body stimulus threshold may be as small as 0.01 pH unit. Fluctuations of this magnitude occur in connection with the normal breathing cycle (I) and could account for the respiratory
GONSALVES,
MONTGOMERY,
AND
MCCARTHY
modulation of discharges in the carotid sinus nerve observed during air breathing (13). However, respiratory-induced pH, oscillations are much reduced during CO, breathing, as was the case in the present experiments. Thus, an influence of natural pH, oscillations on stimulus-response dynamics must be considered inconsequential in this study. On the other hand, arrival time of the ApH, wave front at the peripheral chemoreceptors in relation to the breathing phase of the respiratory cycle could possibly affect the latency if the controller is indeed blind to chemoreceptor stimulation during expiration (1, 7). Curiously, Band et al. (1) were unable to obtain prompt ventilatory stimulation by intracarotid injections of lactic acid and hydrochloric acid timed to coincide with early inspiration as they did with injections of saline equilibrated with 100% CO*, which yielded comparable changes in pH,. In contrast, Eldridge (7) obtained inspiration-linked decreases in phrenic nerve discharge in response to intra-arterial bolus injections of NH,OH. Response half time. The primary problem that is faced in resolving the stimulus-response dynamics of the chemoreceptor-respiratory effector loop is where signal lag is introduced, whether in the chemoreceptor transduction stage or in the central controlling stage or both. Except for Gray (12), other workers have largely restricted their studies on pH transients to latency of response onset while tending to neglect time-course characteristics. Since the pH forcing wave form was not measured by Gray (12), the response half time he reported may simply reflect an imperfect step-forcing function. Eldridge (6) showed that carotid sinus nerve-tophrenic nerve reflex latency amounts to 30 ms in the cat. Additionally he found that the ventilatory off-response time course following carotid sinus nerve electrical stimulation under isocapnic conditions consisted of two phases, a fast decay completed in 10 s followed by a slow decay with a time constant of 87 s in cats with vagus nerves intact and 134 s after vagotomy (8). These response times following whole carotid sinus nerve stimulation were necessarily attributed to controller processing lag. Unfortunately for us, Eldridge (8) did not analyze the ‘ton” time-course characteristics of the phrenic nerve response to carotid sinus nerve stimulation; but it is apparent from his published figures that, for approximately one minute of stimulation, the onresponse consisted solely of a fast time course, reaching its peak within a few seconds. On this basis it would seem reasonable to attribute the nerve-intact response half time observed in the present work to controller processing lag rather than to receptor activation kinetics. ControLler dynamics. The main objective of this work was to determine the direct reactivity of the respiratory controller to arterial pH perturbation. With currently available techniques, isohydric CO, forcing is less feasible than is isocapnic pH forcing of the respiratory control system. Archival data on CO, stimulus-response dynamics are invariably contaminated with a pH component of influence. On the other hand, since ACO, is considerabl-y more effective as a respiratory stimulus
Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (129.237.035.237) on January 1, 2019.
ARTERIAL
HYDROGEN
ION
AND
TIDAL
VOLUME
511
CONTROL
than is ApH (Z), differences in response dynamics reported for CO, forcing (pH uncontrolled) and isocapnic valid for the most part. In PH forcing are undoubtedly an earlier communication on CO, response dynamics (3), we advocated the contention that tidal volume controller response time was governed actively by neurophysiological processes which is in consonance with the position taken by Eldridge (8). Our results with pH, forcing after carotid sinus nerve section show that central responding time course as with CO, forcing was greatly prolonged over the nerveintact condition. On the other hand, we found that the average onset latency to pH, forcing was significantly increased by carotid sinus nerve section, whereas no change was reported for the CO, response latency (3, 4). It is, however, remarkable that, in spite of the much slower development of the respiratory response to pH, forcing following CSNX, the onset of change in tidal volume was in some instances detectable within 5 s after stimulus arrival at the pH sensor. We must conclude, therefore, that the central chemoreceptors are for all practical purposes promptly accessible to hydrogen ion and bicarbonate from the arterial blood. It follows that the long time course of the respiratory controller response after CSNX, i.e., 65 s, cannot be explained by diffusion time required for central chemo-
stimulusreceptor activation, but that the dynamic response behavior of the controller is actively adjusted bY a neural mechanism. A number of differences are evident in the ventilatory response dynamics obtained with pH, and PACT, forcing in this laboratory. 1) When the peripheral ner+es were intact, the ApH, response half time was considerably shorter than the APA c0z response half time. 2) Whereas vagotomy alone reduced the APATHY half time, it produced no change in the ApH, half time. 3) After carotid sinus neurotomy, which by itself yielded near equal response half times for A ~~~~~ and ApH,, subsequent vagotomy shortened the APATHY half time, whereas it further prolonged the ApH, half time, if anything. These differences in dynamic behavior suggest that independent mechanisms operate in the control of tidal volume by ~~~~~ and pH,. Rosaline Borison contributed the mathematical derivations and curve-fitting operations to this study. This study was supported in part by Public Health Service Grant NS-04456. A preliminary report was presented at the 1976 meeting of the American Physiological Society in Philadelphia. S. F. Gonsalves is a predoctoral fellow of the Albert J. Ryan Foundation. Received
19 October
1977; accepted
in final
form
2 May
1978.
REFERENCES 1. BAND, D. M., I. R. CAMERON, AND S. J. G. SEMPLE. The effect on respiration of abrupt changes in carotid artery pH and Pco2 in the cat. J. PhysioZ. London 211: 479-494, 1970. 2. BORISON, H. L., J. H. HURST, L. E. MCCARTHY, AND R. ROSENSTEIN. Arterial hydrogen ion versus CO, on depth and rate of breathing in decerebrate cats. Respiration PhysioZ. 30: 311-325, 1977. 3. BORISON, H. It., AND L. E, MCCARTHY. COP ventilatory response time obtained by inhalation step forcing in decerebrate cats. J. AppZ. Physiol. 34: l-7, 1973. 4. DAUBENSPECK, J. A. Frequency analysis of CO, regulation: afferent influences on tidal volume control. J. AppZ. PhysioZ. 35: 662-672, 1973. 5. DOMIZI, D. B,, AND J. F. PERKINS, JR. Response of carotid chemoreceptors to transient perfusion of acidified blood (Abstract). Physiologist 7: 118, 1964. 6, ELDRIDGE, F. L. The importance of timing on the respiratory effects of intermittent carotid sinus nerve stimulation. J. Physiol. London. 222: 297-318, 1972. 7. ELDRIDGE, F. L. The importance of timing on the respiratory effects of intermittent carotid body chemoreceptor stimulation. J. Physiol. London 222: 319-333,1972. 8. ELDRIDGE, F. L. Central neural stimulation of respiration in unanesthetized decerebrate cats. J. AppZ. Physiol. 40: 23-28, 1976. 9. FITZGERALD, R. S., N. GROSS, AND R. E. DUTTON. Ventilatory responses to transient acidic and hypercapnic vertebral artery
infusions. Respiration Physiol. 4: 387-395, 1968. 10. FITZGERALD, R. S., AND D. C. PARKS. Effect of hypoxia on carotid chemoreceptor response to carbon dioxide in cats. Respiration PhysioZ. 12: 218-229, 1971. 11. FITZGERALD, R. S., J. T. ZAJTCHUK, R. W, B. PENMAN, AND J. F+ PERKINS, JR. Ventilatory response to transient perfusion of carotid chemoreceptors. Am. J. Physiol. 207: 1305-1313, 1964. 12. GRAY, B. A. Response of the perfused carotid body to changes in pH and Pco2. Respiration Physiol. 4: 229-245, 1968. 13. HORNBEIN, T. F., 2. J. GRIFFO, AND A. Roos. Quantitation of chemoreceptor activity: inter-relation of hypoxia and hypercapnia. J. NeurophysioZ. 24: 561-568, 1961. 14. KATSAROS, B. Die Rolle der Chemoreceptoren des Carotisgebiets der narkotisierten Katze fiir die Antwort der Atmung auf isolierte Anderung der Wasserstofionen-Konzentration und des COz-Drucks des Blutes. Pfluegers Arch. 282: 157-178, 1965. 15. LEBLANC, 0. H., JR., J. F. BROWN, JR,, J. F. KLEBE, L. W, NIEDRACH, G. M. J. SLUSARCZUK, AND W. H. STODDARD, JR. Polymer membrane sensors for continuous intravascular monitoring of blood pH. J. Appl. P/xysioZ. 40: 644-647, 1976. 16. MACHELLA, T. E. The velocity of blood flow in arteries in animals. Am. J. Physiol. 115: 632-644, 1936. 17. OGATA, K. Modern Control Engineering. Englewood Cliffs, N.J.: Prentice-Hall, 1970. 18. ROSENSTEIN, R., L. E. MCCARTHY, AND H. L. BORISON. Slow respiratory stimulant effect of hyperoxia in chemodenervated decerebrate cats. J. Appl. Physiol. 39: 767-772, 1975.
Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (129.237.035.237) on January 1, 2019.
of respiratory VT response to pH, forcing in chemodenervated
H. L. BORISON, S. F. GONSALVES, S. P. MONTGOMERY, AND L. E. MCCARTHY Department uf Pharmacology and Toxicology, Dartmouth Medical Hanover, New Hampshire 03755
B~RISON, H. L., S. F, GONSALVES, S. P. MONTGOMERY, AND L. E. MCCARTHY. Dynamics of respiratory VT response to isocapnic pH,, forcing in chemodenervated cats. J. Appl. Physiol.: Respirat. Environ. Exercise Physiol, 45(4): 502-511, 1978. -Arterial blood pH (pH,) was continuously monitored in decerebrate or pentobarbital-anesthetized cats with a rapidly responding hydrogen ion sensor inserted into the aorta. Alveolar carbon dioxide partial pressure and pH, were controlled independently during infusions of 1 N NaHCO:, or 0.5 N HCl into the inferior vena cava. Shifts in pH, up to 0.3 unit were effected isocapnically within 2.5-20 s over a working pH, range of 6.9-7.7. Before carotid sinus neurotomy, average onset latency of the tidal volume (VT) response to acid and alkaline pH, shifts was less than 5 s and the average VT response half time was less than 8.5 s regardless of whether the vagus nerves had been interrupted. After carotid sinus neurotomy, the AVT onset latency was approximately doubled, whereas the response half time was prolonged about eightfold on the average. Subsequent vagotomy tended further to increase the responding time lag. Nevertheless, the minimum response latency after peripheral chemodenervation was less than the ApH, forcing rise time. It is concluded that the central chemoreceptors promptly sense pH change in the arterial blood and that neural processes adjust the time course of the respiratory response through the VT controller.
indwelling pH sensor; ramp-step pH, forcing; carotid neurotomy; vagotomy; central hydrogen ion receptors; volume controller
sinus tidal
ATTEMPTS IN THE PAST to study ventilation response dynamics to systemic forcing of arterial blood carbon dioxide partial pressure (Pa& and pH (pH,) have been hampered by inadequate means of monitoring pH, in situ. To circumvent this problem, Fitzgerald et al. (11) devised the technique of administering intra-arterially a transient infusion of tonometered blood with known gas and hydrogen ion composition on the assumption that the chemical stimulus was thereby transmitted abruptly to the intended target. Their first effort was concerned with intracarotid infusions of hypercapnic and hypocapnic blood uncorrected for associated change in pH. They found that the carotid body reacted to PacOz (and pH,) displacements as promptly as to cyanide. Employing the same technique, Domizi and Perkins (5) then showed that acidified normocapnic blood infused into the carotid arteries elicited respiratory stimulation as fast as did hypercapnic blood; moreover, 502
cats
School,
alkaline blood depressed the respiration. By contrast, when Fitzgerald et al. (9) made infusions into the vertebral arteries to bypass the peripheral chemoreceptars, they found that acidified normocapnic blood caused no significant change in ventilation during a 100-s delivery period, whereas infusions of hypercapnic blood produced an increase in ventilation but only after a delay of 20-30 s. They concluded that increased acidity of the arterial blood does not stimulate the central chemoreceptors in the absence of an increase in C02. In the present work, we have attempted to exercise more strict control over pH, forcing transients with the use of a recently developed fast responding pH electrode suitable for intravascular measurements (15). We found in apparent, conflict with the observations of Fitzgerald et al. (9) that, after peripheral chemodenervation, the respiratory controller responds with minimal delay in a direction-specific manner to isocapnic acid and alkaline shifts in the arterial blood pH. Nevertheless, the time course of the ventilatory response is markedly prolonged by inactivation of the peripheral chemoreceptors. It has been our practice to use tidal volume (VT) as the sufficient criterion of respiratory chemosensitivity to CO, (3). We have reported that pH,, like Pacq , has no direct influence on respiratory frequency, which is affected reflexly through the vagus nerves as a function of tidal volume (2). Accordingly, all chemogenic ventilatory responses presently described are expressed solely as changes in tidal volume. Furthermore, since this paper deals only with the dynamics of chemorespiratory behavior, no attention is given to actual respiratory amplitude. In this regard, the influence of arterial blood pH change on breathing amplitude under steady-state conditions remains largely unaffected after section of the vagus and carotid sinus nerves in cats (2, 14). Hence, the changes in response dynamics that resulted from the peripheral denervations, as reported herein, were not accompanied by any notable loss in response gain. METHODS
Experiments were performed on 17 well-nourished, fully grown cats unselected for sex. All of the cats were initially anesthetized with pentobarbital sodium, 40 mg/kg, given intraperitoneally. In six of the animals,
0021-8987/78/0000-0000$01.25
Copyright
0 1978
the American
Physiological
Society
Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (129.237.035.237) on January 1, 2019.
ARTERIAL
HYDROGEN
ION
AND
TIDAL
VOLUME
503
CONTROL
anesthesia was maintained to completion of the experiment in the same work day. The remaining animals were decerebrated supratentorially and were kept overnight at constant body temperature, regulated by rectal probe and heating lamp through an on-off controller. The decerebration procedure consisted of ligation of the external carotid arteries followed by suction removal of the forebrain rostra1 to a precollicular midbrain transection. The acute experiment was performed on the following day when for all practical purposes the cat was unanesthetized. The monitored experimental variables were alveolar carbon dioxide and oxygen partial pressure (PACT, and PA+), ventilatory gas movement, ,pH,, and arterial blood pressure. These were recorded continuously on a Brush six-channel rectilinear oscillograph. PACES, ventilation, and pH were also recorded on electromagnetic tape for later playback. End-tidal CO, and 0, tensions were obtained by drawing tracheal (PETco2 and PET& gas through a Beckman infrared CO, analyzer in series with a Westinghouse electrochemical 0, analyzer. The amplitude of ventilation was recorded by integration of the tracheal gas flow-pressure signal across a Fleisch no. 0 pneumotachometer. The integrator was operated in the plus-minus mode offset to balance the intratracheal gas sampling flow for the CO, and 0, determinations. Amplitude measurements were made on the strip chart with micrometer calipers to the nearest 0.05 mm. pH, was measured uninterruptedly by means of a rapidly responding (time constant = 0.1 s) indwelling pH electrode obtained through the courtesy of R. Lawton and 0. LeBlanc of the General Electric Company (15). The hydrogen ion-sensing electrode was inserted into the femoral artery and was advanced into the abdominal aorta to reach just above iliac bifurcation* The reference electrode was placed alongside the abdominal aorta in the track made by a blunt seeker inserted retroperitoneally through the femoral triangle. This extravascular arrangement of the reference electrode dispensed with the need for heparin since the active intra-arterial electrode was fabricated from an elastomeric polymer that does not by itself cause blood coagulation. The pH, signal was amplified through a Radiometer pH meter, followed by an intermediate differential amplifier with a low-frequency filter to reduce noise in the recording. Correction of electrode drift, if any, was accomplished in situ by administering CO, to stabilize PA co, and by measuring pH directly from an arterial blood sample taken periodically for comparison with and adjustment of the electrode signal. Two methods were employed for forcing of the pH,. In either case, PACT, was controlled isocapnically by breath-to-breath adjustment of inhaled CO, concentration. In the first four experiments, NaHCO, (up to 1.0 N) or HCl (up to 0.5 N) was infused into the inferior vena cava at constant rates provided by a Braun syringe pump. In all subsequent experiments, NaHCO, or HCl were infused by means of a variable-speed roller pump that permitted the delivery of a ramp-step form of pH displacement for evaluation of ventilatory response dynamics. Our technique for isocapnic “step” forcing of pHa is schematized in Fig. 1. The first maneuver was
ventilatory stabilization at an elevated level of ~~~~~ to permit subsequent control of variations induced by the acid or base infusion, both of which generate CO, in the venous return to the lung. At tiPne 0, the roller pump was switched on to a high flow (up to 8 mllmin) by one operator while the second operator rapidly adjusted inhaled CO, to maintain a constant PACES. The counterreduction in CO, administration had to be started prior to the arrival of the pH wave front at the arterial sensor since CO, was evolved from the blood during the first pass of the infused acid or alkali through the lung. After the initial pH, displacement, the pump speed was quickly backed off and then adjusted continuously to hold pHa steady. Carotid sinus nerve section or vagotomy or both was performed in every experiment. The carotid sinus nerve was identified and severed at its point of connection with the glossopharyngeal nerve. The vagus nerve was cut at the midcervical level. In those instances after combined buffer nerve section where the resulting neurogenic hypertension was considered excessive, hexamethonium chloride was administered intravenously at 1 .O mg/kg per dose to reduce the blood pressure to a reasonable level. Oxygen was administered occasionally after the peripheral denervations to forestall circulatory and respiratory instability. Otherwise, PAN, was maintained around 100 Torr to avoid interactive effects of hypoxia or hyperoxia with CO, or H,+ chemosensitivity or both (10, 18). RESULTS
Isocapnic
?‘Ramp” Forcing
of pHa
The ventilatory effects of constant rate infusions of HCl and NaHCO, for 5-min periods in a decerebrate vagotomized cat are shown in Fig. 2. Inspired Pcoz was adjusted during and after the infusions to maintain a demonstrated fixed ~~~~~ of 22 Torr. This experiment that the tidal volume is affected promptly and continuously in a direction specific manner by long ramp shifts (more nearly hyperbolic) in arterial blood pH. The total shifts in pH, amounted to 0.2 and 0.3 of a pH unit, but tidal volume started to change with barely detectable pH, displacement. The lack of influence of the pH, alterations on frequency of breathing in the vagotomized preparation deserves particular emphasis (2). Blood pressure was not remarkably affected during the acid and base infusions, although a delayed decrease of approximately 25 Torr followed the change in pH, steady state from 7.3 to 7.2 The effects of acid and base infusions at constant PACT, after combined vagotomy and carotid sinus neurotomy in another decerebrate cat are shown in Fig. 3, In this case, the pHa changes were recorded at two levels of amplification. Stimulus arrival at the pH sensor and respiratdry related oscillations are more evident at the full scale amplitude of 0,2 pH unit. Acid shifts developed more slowly than did shifts in the alkaline direction owing to the logarithmic nature of the pH scale and because infused bicarbonate was tolerated at twice the concentration of HCl, i.e., 1.0 N NaHCO, versus 0.5 N HCl. This experiment demon-
Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (129.237.035.237) on January 1, 2019.
504
BORISON,
GONSALVES,
MONTGOMERY,
AND
MCCARTHY
FIG. 1. Schematic of technique for ApH, forcing at a constant ~~~~~~ Vertical line in recording on right marks starting time of roller pump infusion cava. Interval into inferior vena marker is spaced every 10 s. Initial correction of inspired Pco2 was needed before the ApH stimulus arrived at hydrogen ion sensor in the abdominal aorta. Note extravascular placement of pH reference electrode, Sample physiological recording, taken from a cat with intact peripheral nerves, shows a typical fast -A% response to an alkaline step shift in pH,.
-_. -----~-I:::-._
&rated that the onset of the respiratory response e0 a change in pHa was not remarkably delayed by peripheral chemodenervation. However, owing to the slow rate of change in the pH shift, it was not possible to establish the effect of carotid sinus neurotomy on the response half time. Isocapnic
‘Step”
Forcing
of pHEL
Figure 4 shows the recorded physiological variables in relation to rapid and sustained alkaline shifts in an anesthetized cat (above) compared with a decerebrate cat (below) before and after section of the vagus and carotid sinus nerves. The tracings of tracheal Pco2 and the blood pressure are illustrated for the anesthetized cat only. Hexamethonium was administered after the debuffering procedure in this cat to counteract the resultant neurogenic hypertension; the effect of the drug treatment is manifested in the reduced blood pressure in the panel on the right. Hexamethonium, tried previously, did not noticeably affect the ventilatory response dynamics. Owing to the close similarity in stimulus-response behavior of anesthetized and decerebrate preparations, no distinction was made between these experimental conditions in our data analysis.
.-
Most of the timing measurements were made on expanded writeouts of tape recordings. Figure 5 illustrates the front-end transients of acid and base displacements recorded from the arterial pH sensor (below) in association with respective ventilatory responses (above). Signal amplification was adjusted bet-ween panels to provide approximately equal excursions, and so the amnlitudes have no quantitative significance. The two &right parallel line; in each paner mark the transit time from the start of infusion into the inferior vena cava to the start of the pH change sensed in the abdominal aorta, The arrows placed below the timer indicate the estimated onset latency of the respiratory response from the start of the pH, shift. The middle panel illustrates the technique of back extrapolation of inspiratory peaks to find the time of onset between breaths in the slow breathing vagotomized cat. In Table 1 are summarized the time and amplitude characteristics of all satisfactory step pH, forcing tests separated for acid and base shifts that were performed under the different conditions of peripheral innervation. Test data were selected on the basis of acceptable control of PA co2 and of pHa levels between initial and final steady states as judged by plateau attainment. The overall range of pH, examined was from 6.87 to 7.70. Among all innervation categories, average step
Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (129.237.035.237) on January 1, 2019.
ARTERIAL
HYDROGEN
ION
AND
TIDAL
VOLUME
CONTROL
505
DECEREBRATE,VAGX 40 paco2 mmHg 20
vT 2OOmI
200 I3.P 100 mmHg 0 FIG. 2. Ventilatory response to slow ramp constant PA co, after vagotomy in a decerebrate mark start and end of acid (lefi) and base (right)
ApH, forcing with cat. Vertical lines infusions yielding
slope varied from 0.018 to 0.031 U/s for an average wave-front duration between 5.5 and 9.9 s, which produced average pH, shifts between 0.10 and 0.22 unit. These pH, shifts amounted to average H,+ concentration changes of from 15 to 22 nmol/l. Effects of Peripheral Denervations on Tidal Volume Response Dynamics (Table 2) Transit time. The average elapsed time from the point of infusion into the inferior vena cava to the sensing point at the iliac bifurcation of the aorta was slightly less than 6 s for all tests in the nerve-intact condition. Section of the vagus and carotid sinus nerves performed independently did not significantly affect the transit time. However, combined denervation resulted in an increase in average transit time of approximately 2 s over the nerve-intact condition, constituting a statistically significant difference in durations. Onset latency. The average delay of response onset following the start of the pH perturbation at the sensor amounted to less than 5 s in the nerve-intact condition.
long pH, displacements shown directly above blood pressure tracing. Note continuous progression of change in tidal volume appropriate to the direction of ApH, forcing.
It should be kept in mind that this latency is within the rise time of the pH, step forcing stimulus (Table 1). After vagotomy (VAGX) alone, the average response onset latency was reduced to 3.3 s, just significantly less than in the nerve-intact condition. After carotid sinus neurotomy (CSNX) alone, the average onset latency was about doubled. Despite the rather large standard deviation of the measurement after CSNX, the difference in latency from the nerve-intact condition was highly significant. After the sinus nerve section, the onset latency approximately matched the pH, step rise time (Table 1). After combined denervation (Both), the onset latency was no different from that following CSNX alone. The large standard deviations obtained in the CSNX and Both conditions can be attributed in part to the less acute break that occurred at the start of the respiratory response after chemodenervation (Fig. 6). Respunse half time. Before any peripheral nerve section was done, the average time taken to reach half the final change in tidal volume was 8.4 s. Inasmuch as the average rise time of the step forcing stimulus for all tests in the nerve-intact condition amounted to some 7
Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (129.237.035.237) on January 1, 2019.
506
BORISON,
GONSALVES,
MONTGOMERY,
AND
MCCARTHY
VAGX+CSNX 40r tco2 mmHg 20
vT IOOml
7.8 r 73 l
200 BP l
l
100
mmHg
F
FIG. 3. Ventilatory constant PA co, after omy in a decerebrate (left) and acid ( right)
response to slow ramp ApH, forcing with combined vagotomy and carotid sinus neurotcat. Vertical Lines mark start and end of base infusions, yielding progressive pH, displace-
s, the response half time exceeded the stimulus transient by little more than a second which is certainly within the error of measurement. VAGX by itself did not affect the tidal volume response half time, By contrast, carotid sinus neurotomy alone prolonged the response half time about eightfold to an average duration of 64.6 s, a highly significant difference from the Intact and VAGX conditions. Combined denervation produced no further significant prolongation in response half time, suggesting thereby that carotid chemodenervation fully accounted for the observed effect. The tidal volume response behavior before and after carotid sinus neurotomy in a vagotomized cat is illustrated graphically in Fig. 7. Response decay fraction is plotted semilogarithmically against time. For the acid infusions, tidal volume growth has been plotted as an inverted function. In this graph, zero time was taken at the start of infusion which means that the time to onset includes the transit time in addition to the response latency. The average total time to onset was approximately 10 s. After VAGX alone, the response half time
ments change vation.
shown at two in tidal volume
levels even
of amplification. after complete
Note peripheral
early onset chemodener-
of
for acid and base infusions was between 2 and 8 s, contrast, following CSNX the half time amounts for tests to about 40 s, representing a prolongation by least five times in tidal volume control lag produced the carotid chemodenervation.
In all at by
DISCUSSION
Estimation of a biological response time hinges crucially on the nature of the forcing transient. Figure 8 illustrates how a ramp-step with a total rise time of 12.5 s should affect the time course of a first-order response by comparison with the theoretical response to a square step. Prechemodenervation data points are shown in relation to the curve calculated for a time constant of 6 s when forced by the given ramp-step according to the following equations. These equations were derived by adding equal and opposite ramp-forced first-order response functions (17) displaced from each other by the parameter that specifies the rise time for the step shift. When t 5 a,
Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (129.237.035.237) on January 1, 2019.
ARTERIAL
HYDROGEN
ION
AND
TIDAL
VOLUME
PENTOBARBITAL
CONTROL
507
DEBUFFERED
40 pAco2 mmHg
INS? 50 ml I
.-
DECEREBRATE DEBWFERED
100 ml
V’ ’
r-----
0.5u
J
--_ _ - / I’ 7.42 /
7.48
1-I
4. Comparison of tidal volume responses to ramp-step alkaline pH, forcing in a pentobarbital-anesthetized cat (aboue) and in an unanesthetized decerebrate cat (below) before and after combined section of carotid sinus and vagus nerves. PAN% and blood pressure tracings are not shown for decerebrate cat. Vertical lines mark start FIG.
y(t) =
t - ~(1 - e@‘) a
When t 2 a, a + T[e-‘/7 - e’“-““] y(t) =
a
where y = FAVT; and a is the ramp-step rise time, t the time from start of response, and T the response time constant. The data points in Fig. 8 correspond to the base forcing test before CSNX shown in Fig. 7. The measured half time of the biological response matches that of the derived curve for the ramp-step, but is approximately double the theoretical minimum determined for a square step. On the other hand, for a time constant of
of infusion for each test. Arrow heads mark end of infusion, which after denervations was sustained at constant pH for over 10 min. Note prolonged response half time with early onset of effect in both cats after debuffering procedure.
50 s as might be obtained after CSNX, the error of measurement introduced by a ramp-step stimulus becomes a small fraction of the response time. Our measurements of ApH,-AVT half time (uncorrected) before chemodenervation are practically identical to those reported by Gray (12) for discharges recorded from Hering’s nerve in response to pH forcing of the carotid body perfused in situ. He found a median half time of 11 s, which is reduced to 8 s after subtracting stimulus transit time. Gray delivered the test solutions to the common carotid artery in the cat at the rate of 0.1 ml/s for 30-60 s. Rise time of the pH stimulus wave front at the receptor site cannot be ascertained in the method employed by him, thus leaving open the possibility of blunted forcing of the carotid body with its consequence of error in the observed response time.
Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (129.237.035.237) on January 1, 2019.
508
BORISON,
VAGX ACIQ
GONSALVES,
MONTGOMERY,
CSNX BASE
BASE
AND
MCCARTHY
WSEC.
f--
FIG. 5. Expanded tracings from a tape recording of VT and pH, alterations associated with acid and base infusions at constant PAN%. Amplitudes have no absolute significance. Upper trueing is a pneumotachogram in plus-minus integration mode. Lower tracing shows wave front of ramp-step change in pH,. Vertical lines bracket stimulus transit time in each test. Estimated response onset is
TABLE Peripheral Innervation
1. Characteristics n (Cats)
of step pHa forcing tests
ApHJs
At, s
4%
AHa+,
nM
EL:;Intact Acid (7.47-7.10) Base (7.13-7.67) VAGX Acid (7.56-6.97) Base (7.15-7.66) CSNX Acid (7.66-7.15) Base (7.11-7.49) Both * Acid (7.43-7.15) Base (7.13-7.70)
3 (2) 13 (9)
7 (4) 6 (4)
3 (1) 6 (3)
0.021 (0.011-0.026) 0.024 (0.008-O. 048)
5.5 (3.5-8.0) 8.6 (4.5-16.6)
0.10 (0.09-O. 13) 0.18 (0.08-0.26)
15 (8-20) 17 (6-34)
0.031 (0.011-0.08) 0.036 (0.008-0.08)
8.1 (2.5-15.5) 9.2 (3.0-20.0)
0.15 (0.1 l-0.20) 0.20 (0.14-k 28)
19 (10-35) 19 (12-26)
0.029 (0.011-0.056) 0.036 (0.027-0.06)
8.2 (2.7-14.5) 8.2 (3.5-20.0)
0.15 (0.15-o. 16) 0.21 (0.15-0.30)
16 (10-21) 25 (20-39)
marked by arrows below timer. Spike on final pH trace after CSNX is pump-off artifact indicating that infusion was maintained for longer than 3 min to achieve a VT stea’dy state in contrast to rapid response completion resulting after vagotomy alone, which was practically simultaneous with rise time of the ApH forcing stimulus.
central chemoreceptors. The transit time from the inferior vena cava to the iliac bifurcation of the aorta, which varied between 5 and 10 s, is accurately measured by the present method; and uniform blood pH in all branches of the aorta is well assured. Any difference between stimulus arrival time at the carotid and iliac bifurcation, however, introduces an error in the response latency that follows stimulus delivery. According to Machella (16), the average linear velocity of blood flows in the carotid and femoral arteries of the dog are in the neighborhood of 10 cm/s, the carotid flow being somewhat faster. For our purposes, we are assuming that linear blood flow velocities in the cat are not TABLE 2. Stimulus-VT to pHa step forcing Peripheral Innervation
3 (3) 8 (5)
0.018 (O.OlS-0.021) 0.027 (0.019-0.047)
9.9 (8.0-12.2) 8.5 (6.4-13.5)
0.18 (0.15-o. 20) 0.22 (0.17-0.30)
22 (20-26) 19 (10-24)
Values are averages with ranges in parentheses. n, No. of tests performed in no. of cats given in parentheses; ApHJs, best eye-fitted slope of the stimulus transient from time of arrival at the pH sensor to maximum pHa change achieved; At, time required for excursion of pI& between initial and final levels; ApI&, magnitude of pH, shift between initial and final levels; AH;+, corresponding change in concentration of hydrogen ion in arterial blood. * Both signifies VAGX and CSNX without regard to sequence of denervation.
Response onset. Interpretation of response onset latency in the present work is complicated by the fact that the pH sensor was placed at a site in the arterial tree different from the location of the peripheral as well as
Transit
response dynamics
Time,
s
Onset
Latency,
VT-Response Half Time, s
Intact
(n = 16)
5.79 k 1.26
VAGX
(n = 13)
6.08 2 0.77
3.32 2 1.94 P = 0.05 vs. Intact
8.15 f 4.2
CSNX
(n = 9)
5.39 k 0.70
10.72 f 6.56 P < 0.05 vs. Intact
64.56 + 27.64 P K 0.05 vs. Intact
Both*
(n = 11)
12.11 * 4.26 P < 0.05 vs. Intact P = 0.05 vs. CSNX
76.64 f 50.34 P c 0.05 vs. Intact P = 0.05 vs. CSNX
8.11 2 1.04 P K 0.05 vs. Intact
4.74 5 1.69
s
8.39 k 3.92
Values are means 2 SD. Measurements include acid and base tests tabulated without regard to whether the cat was anesthetized or decerebrated. n, No. of tests; no fewer than 3 cats are represented for each condition. Student’s t test was used to determine probability (P) of significant difference between groups. * Both signifies VAGX and CSNX without regard to sequence of denervation.
Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (129.237.035.237) on January 1, 2019.
ARTERIAL
HYDROGEN
ION
AND
TIDAL
VOLUME
CONTROL
7.48
CSNX
FIG.
profiles (above) ogram
6. Apposition of ApH, forcing and AVT response time-related of acid and base tests at constant PA~~,, after VAGX alone and after combined VAGX and CSNX (below). Pneumotachshows expiratory excursions only. Vertical line marks infu-
materially different from relative distances of the electrode from the heart mate that the magnitude
those in the dog. Given the chemoreceptors and the pH in our experiments, we estiof possible error in the deter-
sion pump starting time for all tests. Arrow termination time. Note close approximation sponse contours before CSNX.
heads mark of stimulus
stimulus and re-
mination of response latency is no greater than one second. Eldridge (7) recorded carotid sinus nerve discharge within one second after bolus injection of a variety of agents into the aortic root of anesthetized
Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (129.237.035.237) on January 1, 2019.
510
BORISON,
-e ---- - ~~~~~~-L- - _-------_---l
.4
x
fAVT
* _--___
0
l
0
i
o0ASE ACID
i- ax
P
VAGX +CSNX O* M?& A ACID
A
0 00
bA
A
A0
.3 i 0
0
0
A 0 0A
0
0
A0
0
0 A
0 0
l
A
0
A
A
A
A
A 0 a
I
4
d A
0 uo 11
30
II
40
50
I11
60
70
80
90
A 4
I
I1
100
110
120
SECONDS FIG. 7. Semilogarithmic breath-by-breath plot of fractional AVT response decay (for base step-forcing tests) and response growth (inverted for acid step-forcing tests) before and after carotid sinus neurotomy in a vagotomized cat. Dashed line crosses responses in progress at their respective half times including latency of onset and transit time from start of the infusion pump at 0 s. Each symbol represents a separate test.
FAV,
0
IO
20
40
50 SECONDS
60
70
80
90
100
FIG. 8. Comparison of first-order response curves calculated nondimensionally for square-step (instantaneous) and ramp-step (dashed line) forcing of systems operating with time constants of 6 and 50 s. For each time constant (T), fine-line curue represents response to square-step stimulus and bold-line curue represents response to ramp-step stimulus. Ramp-step forcing time was set at 12.5 s to simulate stimulus used for VAGX-base test shown in Fig. 7. Actual data points corresponding to said ramp-step test were found to be best fitted by theoretical curve calculated with time constant of 6 s. Ramp-step forced response curves were obtained with use of equations given in text. Square-step forced response curves were obtained with use of standard equation for a first-order response to an instantaneous displacement. Difference in half times between square-step and ramp-step forced response curves is indicated by bracketed lines.
cats. By comparison, our measured average latency (posttransit) varied from a minimum of 3.3 s after VAGX alone to 12.1 s after CSNX plus VAGX. Part of the response onset delay must be attributed to the time taken for the forcing stimulus to reach chemoreceptor activation threshold. As the response half time before chemodenervation was in many cases less than the ApH rise time, it follows that the activation threshold was reached at a small fraction of the pH change. The average forcing amplitude varied from 0.1 to 0.2 pH unit delivered over a time span from 2.5 to 20 s. Taken together with response onset latency, these values indicate that the carotid body stimulus threshold may be as small as 0.01 pH unit. Fluctuations of this magnitude occur in connection with the normal breathing cycle (I) and could account for the respiratory
GONSALVES,
MONTGOMERY,
AND
MCCARTHY
modulation of discharges in the carotid sinus nerve observed during air breathing (13). However, respiratory-induced pH, oscillations are much reduced during CO, breathing, as was the case in the present experiments. Thus, an influence of natural pH, oscillations on stimulus-response dynamics must be considered inconsequential in this study. On the other hand, arrival time of the ApH, wave front at the peripheral chemoreceptors in relation to the breathing phase of the respiratory cycle could possibly affect the latency if the controller is indeed blind to chemoreceptor stimulation during expiration (1, 7). Curiously, Band et al. (1) were unable to obtain prompt ventilatory stimulation by intracarotid injections of lactic acid and hydrochloric acid timed to coincide with early inspiration as they did with injections of saline equilibrated with 100% CO*, which yielded comparable changes in pH,. In contrast, Eldridge (7) obtained inspiration-linked decreases in phrenic nerve discharge in response to intra-arterial bolus injections of NH,OH. Response half time. The primary problem that is faced in resolving the stimulus-response dynamics of the chemoreceptor-respiratory effector loop is where signal lag is introduced, whether in the chemoreceptor transduction stage or in the central controlling stage or both. Except for Gray (12), other workers have largely restricted their studies on pH transients to latency of response onset while tending to neglect time-course characteristics. Since the pH forcing wave form was not measured by Gray (12), the response half time he reported may simply reflect an imperfect step-forcing function. Eldridge (6) showed that carotid sinus nerve-tophrenic nerve reflex latency amounts to 30 ms in the cat. Additionally he found that the ventilatory off-response time course following carotid sinus nerve electrical stimulation under isocapnic conditions consisted of two phases, a fast decay completed in 10 s followed by a slow decay with a time constant of 87 s in cats with vagus nerves intact and 134 s after vagotomy (8). These response times following whole carotid sinus nerve stimulation were necessarily attributed to controller processing lag. Unfortunately for us, Eldridge (8) did not analyze the ‘ton” time-course characteristics of the phrenic nerve response to carotid sinus nerve stimulation; but it is apparent from his published figures that, for approximately one minute of stimulation, the onresponse consisted solely of a fast time course, reaching its peak within a few seconds. On this basis it would seem reasonable to attribute the nerve-intact response half time observed in the present work to controller processing lag rather than to receptor activation kinetics. ControLler dynamics. The main objective of this work was to determine the direct reactivity of the respiratory controller to arterial pH perturbation. With currently available techniques, isohydric CO, forcing is less feasible than is isocapnic pH forcing of the respiratory control system. Archival data on CO, stimulus-response dynamics are invariably contaminated with a pH component of influence. On the other hand, since ACO, is considerabl-y more effective as a respiratory stimulus
Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (129.237.035.237) on January 1, 2019.
ARTERIAL
HYDROGEN
ION
AND
TIDAL
VOLUME
511
CONTROL
than is ApH (Z), differences in response dynamics reported for CO, forcing (pH uncontrolled) and isocapnic valid for the most part. In PH forcing are undoubtedly an earlier communication on CO, response dynamics (3), we advocated the contention that tidal volume controller response time was governed actively by neurophysiological processes which is in consonance with the position taken by Eldridge (8). Our results with pH, forcing after carotid sinus nerve section show that central responding time course as with CO, forcing was greatly prolonged over the nerveintact condition. On the other hand, we found that the average onset latency to pH, forcing was significantly increased by carotid sinus nerve section, whereas no change was reported for the CO, response latency (3, 4). It is, however, remarkable that, in spite of the much slower development of the respiratory response to pH, forcing following CSNX, the onset of change in tidal volume was in some instances detectable within 5 s after stimulus arrival at the pH sensor. We must conclude, therefore, that the central chemoreceptors are for all practical purposes promptly accessible to hydrogen ion and bicarbonate from the arterial blood. It follows that the long time course of the respiratory controller response after CSNX, i.e., 65 s, cannot be explained by diffusion time required for central chemo-
stimulusreceptor activation, but that the dynamic response behavior of the controller is actively adjusted bY a neural mechanism. A number of differences are evident in the ventilatory response dynamics obtained with pH, and PACT, forcing in this laboratory. 1) When the peripheral ner+es were intact, the ApH, response half time was considerably shorter than the APA c0z response half time. 2) Whereas vagotomy alone reduced the APATHY half time, it produced no change in the ApH, half time. 3) After carotid sinus neurotomy, which by itself yielded near equal response half times for A ~~~~~ and ApH,, subsequent vagotomy shortened the APATHY half time, whereas it further prolonged the ApH, half time, if anything. These differences in dynamic behavior suggest that independent mechanisms operate in the control of tidal volume by ~~~~~ and pH,. Rosaline Borison contributed the mathematical derivations and curve-fitting operations to this study. This study was supported in part by Public Health Service Grant NS-04456. A preliminary report was presented at the 1976 meeting of the American Physiological Society in Philadelphia. S. F. Gonsalves is a predoctoral fellow of the Albert J. Ryan Foundation. Received
19 October
1977; accepted
in final
form
2 May
1978.
REFERENCES 1. BAND, D. M., I. R. CAMERON, AND S. J. G. SEMPLE. The effect on respiration of abrupt changes in carotid artery pH and Pco2 in the cat. J. PhysioZ. London 211: 479-494, 1970. 2. BORISON, H. L., J. H. HURST, L. E. MCCARTHY, AND R. ROSENSTEIN. Arterial hydrogen ion versus CO, on depth and rate of breathing in decerebrate cats. Respiration PhysioZ. 30: 311-325, 1977. 3. BORISON, H. It., AND L. E, MCCARTHY. COP ventilatory response time obtained by inhalation step forcing in decerebrate cats. J. AppZ. Physiol. 34: l-7, 1973. 4. DAUBENSPECK, J. A. Frequency analysis of CO, regulation: afferent influences on tidal volume control. J. AppZ. PhysioZ. 35: 662-672, 1973. 5. DOMIZI, D. B,, AND J. F. PERKINS, JR. Response of carotid chemoreceptors to transient perfusion of acidified blood (Abstract). Physiologist 7: 118, 1964. 6, ELDRIDGE, F. L. The importance of timing on the respiratory effects of intermittent carotid sinus nerve stimulation. J. Physiol. London. 222: 297-318, 1972. 7. ELDRIDGE, F. L. The importance of timing on the respiratory effects of intermittent carotid body chemoreceptor stimulation. J. Physiol. London 222: 319-333,1972. 8. ELDRIDGE, F. L. Central neural stimulation of respiration in unanesthetized decerebrate cats. J. AppZ. Physiol. 40: 23-28, 1976. 9. FITZGERALD, R. S., N. GROSS, AND R. E. DUTTON. Ventilatory responses to transient acidic and hypercapnic vertebral artery
infusions. Respiration Physiol. 4: 387-395, 1968. 10. FITZGERALD, R. S., AND D. C. PARKS. Effect of hypoxia on carotid chemoreceptor response to carbon dioxide in cats. Respiration PhysioZ. 12: 218-229, 1971. 11. FITZGERALD, R. S., J. T. ZAJTCHUK, R. W, B. PENMAN, AND J. F+ PERKINS, JR. Ventilatory response to transient perfusion of carotid chemoreceptors. Am. J. Physiol. 207: 1305-1313, 1964. 12. GRAY, B. A. Response of the perfused carotid body to changes in pH and Pco2. Respiration Physiol. 4: 229-245, 1968. 13. HORNBEIN, T. F., 2. J. GRIFFO, AND A. Roos. Quantitation of chemoreceptor activity: inter-relation of hypoxia and hypercapnia. J. NeurophysioZ. 24: 561-568, 1961. 14. KATSAROS, B. Die Rolle der Chemoreceptoren des Carotisgebiets der narkotisierten Katze fiir die Antwort der Atmung auf isolierte Anderung der Wasserstofionen-Konzentration und des COz-Drucks des Blutes. Pfluegers Arch. 282: 157-178, 1965. 15. LEBLANC, 0. H., JR., J. F. BROWN, JR,, J. F. KLEBE, L. W, NIEDRACH, G. M. J. SLUSARCZUK, AND W. H. STODDARD, JR. Polymer membrane sensors for continuous intravascular monitoring of blood pH. J. Appl. P/xysioZ. 40: 644-647, 1976. 16. MACHELLA, T. E. The velocity of blood flow in arteries in animals. Am. J. Physiol. 115: 632-644, 1936. 17. OGATA, K. Modern Control Engineering. Englewood Cliffs, N.J.: Prentice-Hall, 1970. 18. ROSENSTEIN, R., L. E. MCCARTHY, AND H. L. BORISON. Slow respiratory stimulant effect of hyperoxia in chemodenervated decerebrate cats. J. Appl. Physiol. 39: 767-772, 1975.
Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (129.237.035.237) on January 1, 2019.
