Topical
anesthesia
of tracheal
receptors
E. M. CAMPORESI, J. P. MORTOLA, F. SANT’AMBROGIO, AND G. SANT’AMBROGIO Department of Physiology and Biophysics, University of Texas Medical Branch, Galveston, Texas 77550; and Department of Anesthesiology, Duke University, Durham, North Carolina 27710
CAMPORESI, E.M.,J.P. MORTOLA, F. SANT'AMBROGIO,AND G. SANT'AMBROGIO. Topical anesthesia of tracheaLreceptors. J. Appl. Physiol.: Respirat. Environ. Exercise Physiol. 47(s): 1123-1126,1979.-Two types of sensory receptors were individually identified in dogs on the exposed mucosa of the extrathoracic trachea: slowly adapting stretch receptors (SAR) and rapidly adapting receptors (RAR). Increasing concentrations of lidocaine (L), bupivacaine (B), and tetracaine (T) solutions were topically applied on the mucosa over the sensory fields of the receptors, while their neural activity in response to an appropriate stimulus was recorded. Action potentials from SARs were blocked by the anesthetic at a much higher concentration and with larger exposure times than potentials evoked from RARs (mean values: L = 51.1 mM for 7.4 min and 1.48 mM for 1.03 min, respectively; B = 9.4 mM for 9.4 min and 0.13 mM for 0.23 min; and T = 4.9 mM for 9.8 min and 0.06 mM for 2 min). The minimal blocking concentration of the topical anesthetics varied widely (L = 20 times, B = 5, and T = 5) among SARs and less (L = 3.2, B = 2, and T = 4) among RARs. These results could be explained by a different location in the mucosa of the two types of receptors and suggest the feasibility of a differential blockade by topical anesthetics. tracheal
intubation;
CARDIOVASCULAR
lidocaine;
AND
bupivacaine;
RESPIRATORY
tetracaine
reactions
to tra-
cheal intubation are often observed in clinical practice: transitory hypertension, tachycardia, arrhythmias, bronchoconstriction, and hyperventilation have been described, both after laryngoscopy and insertion of a tracheal tube. A few cases of sudden death immediately following intubation have been reported (11,lZ). Preventive measures recommended include deep anesthesia (lo), infusion of P-blockers (17)) topical anesthesia of the larynx and trachea (1, ZO), and intravenous infusion of local anesthetics (2). All these measures are only partially effective and may have undesirable side effects. Furthermore, the mechanism of action of local anesthetics is as yet controversial, because of their rapid systemic absorption after endotracheal instillation; indeed the blood levels of these agents have been found to be sufficient to depress medullary centers and the myocardium (16). The effect of local anesthesia of the airways by aerosol administration of anesthetic solutions has been tested in animals (7, 14) and man (6). In all species the cough reflex was blocked, whereas the Breuer-Hering reflex was only attenuated in dogs and man and blocked in a consistent proportion of rabbits, with administration of extremely high concentrations of these drugs. In these studies peak blood levels of anesthetics were measured 0161-7567/79/0000-0000$01.25
Copyright
0 1979 the American
and considered insufficient to cause central effects. To study the direct action of local anesthetics on tracheal receptors we obtained monofilament recordings of their activity before and after topical application of local anesthetics on circumscribed areas of the exposed surface of the extrathoracic trachea of dogs, thus minimizing their systemic absorption. Two different types of tracheal receptors have been identified, both transmitting through myelinated fibers, but differing in their properties, locations, and reflex effects. Slowly adapting stretch receptors (SAR) have been found to be uniquely located in the trachealis muscle of the posterior wall of the trachea (3) and are presumably involved in the regulation of depth and rate of breathing (13). The rapidly adapting receptors (RAR) have been located all around the tracheal circumference and seemingly branching both in the superficial and in the deeper structures of the tracheal wall (18). They are usually considered as “cough receptors” and their other reflex actions comprise bronchoconstriction and inspiratory augmentation (19). In this study we describe a different sensitivity of these two types of tracheal receptors to three local anesthetics suggesting the feasibility of a differential blockade.
Physiological
METHODS
Nine dogs (8-12 kg) were anesthetized by an intravenous injection of pentobarbital sodium (30 mg/kg), supplemented as needed. A midline incision exposed the entire cervical trachea; a tracheostomy was performed right below the larynx and a cannula inserted. Both the vagosympathetic trunks were isolated at the level of the larynx and sectioned. The peripheral cut end of the right vagus was dissected under mineral oil, using fine forceps, with the aid of a binocular microscope. Small nerve filaments were placed on a pair of platinum wire electrodes and single-unit activity was amplified, monitored on an oscilloscope and a loudspeaker, and recorded on an oscillograph (Visicorder). Sensory endings distributed to the extrathoracic trachea were identified as follows. During recording of the single-unit activity, the cervical trachea was mechanically stimulated by a large Foley catheter inserted through the tracheal cannula into the intrathoracic trachea. The cuff was inflated and the catheter withdrawn while spontaneous ventilation continued through the lumen of the Foley catheter. The mechanical deformation of the tracheal wall could stimulate both rapidly and Society
1123
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1124
CAMPOHESI,
slowly adapting stretch receptors. Neural filaments were tested and discarded until units were found whose sensory endings were localized in the cervical trachea. If the receptor located in the extrathoracic trachea was identified as an SAR for its slowly adapting response to tracheal distention, the anterior tracheal wall was cut longitudinally along its midline and the tracheal cannula inserted at the level of the thoracic inlet. The two cut ends of the cartilaginous ring adjacent to the site of the receptor were fixed to a clamp on the right side and attached to a movable arm on the left side. A transverse elongation was exerted on the portion of the membranous posterior wall, site of the ending, by a motor rotating a cam on which the moving arm was attached (Fig. 1). A train of at least six sinusoidal elongations at approximately 1 Hz constituted the constant testing pattern. The transversal displacement of the moving arm was recorded on a different channel of the Visicorder. If the receptor located in the extrathoracic trachea was rapidly adapting to the tracheal distension (RAR), its circumferential location was further defined by intraluminal probing. The anterior tracheal wall was then carefully sectioned longitudinally, most frequently along the left side of its anterior aspect. Intermittent mechanical probing tested the receptor’s activity after each exposure to the anesthetics: a Q-tip was used exerting pressure by its own weight while it was moved longitudinally on the tracheal mucosa. Stock solutions of local anesthetics were prepared just before each experiment from crystals of lidocaine HCl, bupivacaine HCl, and tetracaine HCl in normal saline, at a concentration of 148, 31, and 17 mM, respectively (Table 1). The pH of the stock solutions varied between 6.8 and 7.1. The stock solutions were then serially diluted, and the different dilutions obtained were tested on the receptors by progressively switching to more concentrated solutions until the ending was completely silenced. Topical application on the tracheal mucosa was accomplished with small cotton pledgets soaked in the test solutions, prewarmed at 30°C and changed every 2-3 min to avoid dilution from mucous secretions. The area of tracheal mucosa in contact with the local anesthetics was never in excess of 2 cm’), i.e., approximately 1.5% of the total surface of the trachea of the dog (130 cm’). In several trials the three anesthetics were used on the same receptor so as to compare the relative potencies of each drug while minimizing the differences due to location and intrinsic susceptibility of the receptor. With one exception, if complete blockade of the receptor was not achieved after 15 min of exposure to a given anesthetic concentration the mucosa was washed with saline and a new pledget applied at a higher concentration. This type of comparison assumes that residual effects of the previous drugs be dissipated when another drug was tested. In our study we waited until full recovery of the original firing pattern was observed prior to testing with the next anesthetic; only those receptors that resumed control firing after complete blockade were considered. Furthermore one SAR (Table 2, dog 3b) was retested with 14.8 mM lidocaine 30 min after resuming control firing, resulting in similar blocking times (11 and 9 min, respectively). In addition, throughout all recovery periods
MORTOLA,
’ :h
SANT’AMBKOGIO,
AND
SANT’AMBROGIO
l
--I-
Thorocic inlet
L
k
t
of experimental setup used for cyclic oscillation of membranous posterior wall of extrathoracic trachea at level of a SAR location (indicated by a fiZZed circle). SARs action potentials (AP) are recorded from a neural filament separated from right vagus nerve (HVN) while movement provided by a motor is also recorded (L). An example of two tracings is shown at bottom. FIG.
TABLE
1. Diagram
1. Physical and biological properties
ofzocaz anesthetics ~ -~ Lipid/H~Opartition coef Equieffective anesthetic concn Mel wt
Lidocaine
2.9 1 270
Bupivacaine 27.5 0.25 300
Tetracaine 80 0.25 324
(30 to 90 min) the mucosa was washed with warm saline. The order of application of the three anesthetics was not formally randomized but varied as follows: lidocaine was used first in three of five SARs and two of four RARs tested with more than one drug; bupivacaine was first in one of five SARs and one of four RARs; and tetracaine was first in one of five SAR and one of four RARs. In view of the precautions described we believe that the results obtained with repetitive tests on the same receptor can be interpreted independently of each other. RESULTS
AND
DISCUSSION
In 7 dogs, 10 different stretch receptors were subjected to 20 trials: 12 with lidocaine, 5 with bupivacaine, and 3 with tetracaine. In 3 dogs 6 different RARs were exposed
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ANESTHESIA
OF
TRACHEAL
1125
RECEPTORS
to 13 challenges: 6 with lidocaine, 3 with bupivacaine, and 4 with tetracaine. In addition to topical anesthetization, 3 of the RARs were retested 7 times (3 times for lidocaine and 2 times each for bupivacaine and tetraCaine) by exposing the mucosa to the various anesthetic solutions delivered by aerosol from a Bird nebulizer (mean droplet diam, 1 pm; carrier gas, 02). In general higher concentrations of the anesthetics were found to block the receptors in shorter times of application. This is apparent in Fig. 2, which shows the effective blocking time-concentration values for two different SARs challenged with lidocaine. The minimal anesthetic concentrations necessary to block receptors during topical applications within 15 min are reported in Tables 2 and 3 for all the receptors studied. RARs are blocked at concentrations and durations of exposure much lower than SARs for all the three anesthetic agents. The range of values is much narrower for RARs than for SARs, which behave as a more heterogeneous group. It is apparent the much greater susceptibility of RARs to each of the anesthetics both in
40
Lidocaine hM) 2.
IO
20
terms of duration of exposure (ca. 1 order of magnitude) and concentration (ca. 2 orders of magnitude). These results seem to be consistent with the different location of the two types of endings within the tracheal wall. RARs, as far as their response to local probing is concerned, are distributed in the more superficial layers of the mucosa (18), whereas SARs are distributed in the much deeper trachealis muscle (3). A greater perfusion of the trachealis muscle, as compared to the superficial layers of the tracheal wall, could provide an alternative and/or an additional explanation for the lesser accessibility of the SARs to the local anesthetization. Furthermore the two types of receptors could also be intrinsically different in their susceptibility. Direct application of the anesthetic solution to the mucosa is a very effective technique compared to aerosol inhalation. Table 4 indicates the enormous differences in concentration necessary for each anesthetic agent to block the same RAR within comparable exposure times. These results may explain the need for the very high anesthetic concentrations in aerosols required to block the cough reflex (6, 14), which presumably involves the RARs in the trachea (9). Furthermore the difference between the two modalities of application of local anesthetics might be due to the relatively impenetrable mucous layer, which is easily removed by the soaked pledgets but greatly hinders the action of the aerosolized droplets. The relative potencies of the different anesthetic agents tested on this preparation is in general agreement with their equieffective anesthetic concentrations on the sciatic nerves of the rat and frog (4,5). Tetracaine blocks RARs at the lowest observed values of concentration; moreover, it appears to have the widest difference in its blocking parameters (Tables 2 and 3) between RARs and SARs. For this reason it should be considered the drug 3. Minimal concentrations of topical anesthetic blocking RARs and duration of exposure
30
TABLE
min 2. Anesthetic concentrations (in mM) and durations of topical application (in min) sufficient to block two different SARs (0; x) in cervical trachea. More diluted solutions require a longer time of exposure to silence SARs. FIG.
TABLE 2. Minimal concentrations of topical anesthetics blocking SARs and duration of exposure -
m No. 1
2 3” 3h 4” 4h 5 6” 6h 6’ 9 Mean &SD
Bupivacaine
Lidocaine mM
37.0 37.0 7.4 14.8 37 148 7.4 148 37
min
-
mM
min
15.6 3.1 6.35 6.35
2 9 15 10
Tetracaine mM
min
7 1
6 9 6 16 11
6.7 1.3
0.5 14
Lidocaine
Bupivacaine
mM
7” 7”
1.48 3.70
8”
0.93
8’ 8” 9
0.46 0.46 1.85
Mean *SD
1.48 t1.22
min
2
15.6
11
51.1 k52.6
7.4 k5.6
9.4 t5.6
9.4 k4.7
4.9 k3.1
9.8 t8.1
mM
min
1 1
1 0.2
0.20 0.10
0.3 0.2
0.17
1
0.02 0.04 0.02
5 1 1
0.10
0.2
0.13 to.06
0.23 to.07
0.06 to.07
2 2
1
1.03 kO.57
4. Anesthetic concentrations necessary to block RAR activity and duration of exposure
6
37
min
2
Lidocaine
15
mM
TABLE
10
6.7
Tetracaine
Dog No.
T A A/T
T, topical;
Bupivacaine
Tetracaine
mM
min
mM
min
mM
min
0.62 123
1.03 1.00
0.13 7.8
1.1 2.5
0.03 3.3
1
198
60
1
110
A, aerosol.
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1126
CAMPORESI, -Control -c+ I % Lidocaine,30sec 100
.
impslsec 50 .
O/O
5.0 Elongotion
I 100
50 % Elongation
100
9 CYCLE I CYCLE FIG. 3. Relationships between elongation of tracheal back wall (in R of the app lied displacement) and frequency of discharge (imp/s) for a SAR. FiZZed circles (0) and continuous Lines, control; open circZes (0) and broken lines, partial block with lidocaine. Left, 1st cycle; right, 5th cycle. This unit was silenced after 2 min of exposure.
of choice for a selective topical anesthesia of RARs. The blocking action of the local anesthetic appears to be gradual, as shown in Fig. 3, where the transversal elongation of the back wall containing a SAR is plotted against its firing rate during the first (left) and the fifth applied oscillations (right). In both control (solid lines)
MORTOLA,
SANT’AMBROGIO,
AND
SANT’AMBROGIO
and partially blocked conditions (broken lines) the receptor’s firing leads the elongation and this is particularly apparent during the first elongation; the viscous properties of the trachealis muscle, far more pronounced during the first applied oscillation, have been found to account for this phenomenon (15). During partial anesthesia the receptors maintain their dynamic dl/dt sensitivity, but their maximum rate of firing becomes lower. This might be attributed to a frequency-dependent conduction block similarly observed in sciatic nerve fibers of the frog (4). Another possibility might be a direct relaxing action of the anesthetics on the trachealis muscle that could also explain the higher threshold of the receptor’s activation during the first cycle. At lower concentrations than those used in this study a decrease in smooth muscle tone has been observed in in vitro tracheal preparations from the guinea pig after treatment with lidocaine, bupivacaine, tetracaine, and other local anesthetics (8). The possibility to selectively block RARs may provide a useful tool to study their role in respiratory regulation. We thank Karen Rex and Sandra Travers for their assistance. This study was supported by National Institutes of Health Grant 5ROl-HL-20122. Lidocaine HCl was kindly supplied by Astra Pharmaceutical Products Inc. Bupivacaine HCl was kindly provided by Sterling-Winthorp Research Institute. Received
27 November
1978; accepted
in final
form
21 June
1979.
REFERENCES 1. ABOU-MADI, M. N., H. KESZLER, AND J. YACOUB. A method for prevention of cardiovascular reactions to laryngoscopy and intubation. Can. Anccesth. Sot. J. 22: 316-329, 1975. 2. ABOU-MADI, M. N., H. KESZLER, AND J. YACOUB. Cardiovascular reactions to laryngoscopy and tracheal intubation following small and large intravenous doses of lidocaine. Can. Anaesth. Sot. J. 24: 12-19, 1977. 3. BARTLETT, D., JR., P. JEFFERY, G. SANT’AMBROGIO, AND J. C. M. WISE. Location of stretch receptors in the trachea and bronchi of the dog. J. Physiol. London 258: 409-420, 1976. 4. COURTNEY, K. R., J. J. KENDIG, AND E. N. COHEN. Frequency dependent conduction block. Anesthesiology 48: 11 l-l 17, 1978. 5. COVINO, G. C., AND H. C. VASSALLO. Chemical aspects of local anesthetic agents. In: Local Anesthetics, Mechanism of Action and ClinicaL Use. New York: Grune & Stratton, 1976, p. 2-11. 6. CROSS, B. A., A. Guz, S. K. JAIN, S. ARCHER, J. STEVENS, AND F. REYNOLDS. The effect of anesthesia of the airway in dog and man: a study of respiratory reflexes, sensations and lung mechanics. CZin. Sci. 50: 439-454, 1979. 7. DAIN, D. S., H. A. BOUSHEY, AND W. M. GOLD. Inhibition of respiratory reflexes by local anesthetic aerosols in dogs and rabbits. J. AppZ. Physiol. 38: 1045-1050, 1975. 8. DOWNES, H., AND R. W. LOEHNING. Local anesthetic contracture and relaxation of airway smooth muscle. AnesthesioZogy 47: 430436, 1977. 9. FII,LENZ, M., AND J. G. WIDDICOMBE. Receptors of the lung and airways. In: Handbook of Sensory Physiology. Entroceptors. New York: Springer-Verlag, 1972, vol. III/l, p. 81-112. 10. FORBES, A. M., AND F. G. DALLY. Acute hypertension during induction of anesthesia and endotracheal intubation in normoten-
sive man. Br. J. Anaesth. 42: 849-857, 1970. 11. Fox, E. J., G. S. SKLAR, C. H. HILL, R. VILLANUEVA, AND B. KING. Complications related to the pressor response to endotracheal intubation. Anesthesiology 47: 72-74, 1977. 12. GIBBS, J. M. The effects of endothracheal intubation on cardiac rate and rhythm. NZ Med. J. 66: 465, 1968. 13. HAMMOUDA, M., AND W. H. WILSON. Reflex slowing of respiration accompanying changes in the intrapulmonary pressure. J. Physiol. London 88: 284-297, 1937. 14. JAIN, S. K., D. TRENCHARD, F. REYNOLDS, M. I. M. NOBLE, AND A. GUZ. The effect of local anesthesia of the airway on respiratory reflexes in the rabbit. CZin. Sci. 44: 519-538, 1973. 15. MORTOLA, J. P., AND G. SANT’AMBROGIO. Mechanics of the trachea and behaviour of its slowly adapting stretch receptors. J. PhysioZ. London 286: 577-590, 1979. 16. PELTON, D. A., M. DALY, P. D. COOPER, AND A. W. CONN. LidoCaine concentrations in plasma following aerosol application to trachea and bronchi. Can. Anaesth. Sot. J. 17: 250-255, 1970. 17. PRYS-ROBERTS, C., P. FOEX, G. P. BIRO, AND J. G. ROBERTS. Studies of anaesthesia in relation to hypertension. V. Adrenergic beta-receptor blockade. Br. J. Anaesth. 45: 671-680, 1973. 18. SANT’AMBROGIO, G., J. E. REMMERS, W. J. DEGROOT, G. CALLAS, AND J. P. MORTOLA. Localization of rapidly adapting receptors in the trachea and main stem bronchus of the dog. Respir. Physiol. 33: 359-366, 1978. 19. WIDDICOMBE, J. G. Respiratory reflexes. In: Handbook of PhysioZogy. Section 3. Respiration. Baltimore, MD: William & Wilkins, 1964, sect. 3, vol. I, chapt. 24, p. 585-630. 20. WYCOFF, C. C. Endotracheal intubation effects on blood pressure and pulse rate. Anesthesiology 21: 153-158, 1960.
Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (132.174.255.223) on October 10, 2018. Copyright © 1979 American Physiological Society. All rights reserved.
anesthesia
of tracheal
receptors
E. M. CAMPORESI, J. P. MORTOLA, F. SANT’AMBROGIO, AND G. SANT’AMBROGIO Department of Physiology and Biophysics, University of Texas Medical Branch, Galveston, Texas 77550; and Department of Anesthesiology, Duke University, Durham, North Carolina 27710
CAMPORESI, E.M.,J.P. MORTOLA, F. SANT'AMBROGIO,AND G. SANT'AMBROGIO. Topical anesthesia of tracheaLreceptors. J. Appl. Physiol.: Respirat. Environ. Exercise Physiol. 47(s): 1123-1126,1979.-Two types of sensory receptors were individually identified in dogs on the exposed mucosa of the extrathoracic trachea: slowly adapting stretch receptors (SAR) and rapidly adapting receptors (RAR). Increasing concentrations of lidocaine (L), bupivacaine (B), and tetracaine (T) solutions were topically applied on the mucosa over the sensory fields of the receptors, while their neural activity in response to an appropriate stimulus was recorded. Action potentials from SARs were blocked by the anesthetic at a much higher concentration and with larger exposure times than potentials evoked from RARs (mean values: L = 51.1 mM for 7.4 min and 1.48 mM for 1.03 min, respectively; B = 9.4 mM for 9.4 min and 0.13 mM for 0.23 min; and T = 4.9 mM for 9.8 min and 0.06 mM for 2 min). The minimal blocking concentration of the topical anesthetics varied widely (L = 20 times, B = 5, and T = 5) among SARs and less (L = 3.2, B = 2, and T = 4) among RARs. These results could be explained by a different location in the mucosa of the two types of receptors and suggest the feasibility of a differential blockade by topical anesthetics. tracheal
intubation;
CARDIOVASCULAR
lidocaine;
AND
bupivacaine;
RESPIRATORY
tetracaine
reactions
to tra-
cheal intubation are often observed in clinical practice: transitory hypertension, tachycardia, arrhythmias, bronchoconstriction, and hyperventilation have been described, both after laryngoscopy and insertion of a tracheal tube. A few cases of sudden death immediately following intubation have been reported (11,lZ). Preventive measures recommended include deep anesthesia (lo), infusion of P-blockers (17)) topical anesthesia of the larynx and trachea (1, ZO), and intravenous infusion of local anesthetics (2). All these measures are only partially effective and may have undesirable side effects. Furthermore, the mechanism of action of local anesthetics is as yet controversial, because of their rapid systemic absorption after endotracheal instillation; indeed the blood levels of these agents have been found to be sufficient to depress medullary centers and the myocardium (16). The effect of local anesthesia of the airways by aerosol administration of anesthetic solutions has been tested in animals (7, 14) and man (6). In all species the cough reflex was blocked, whereas the Breuer-Hering reflex was only attenuated in dogs and man and blocked in a consistent proportion of rabbits, with administration of extremely high concentrations of these drugs. In these studies peak blood levels of anesthetics were measured 0161-7567/79/0000-0000$01.25
Copyright
0 1979 the American
and considered insufficient to cause central effects. To study the direct action of local anesthetics on tracheal receptors we obtained monofilament recordings of their activity before and after topical application of local anesthetics on circumscribed areas of the exposed surface of the extrathoracic trachea of dogs, thus minimizing their systemic absorption. Two different types of tracheal receptors have been identified, both transmitting through myelinated fibers, but differing in their properties, locations, and reflex effects. Slowly adapting stretch receptors (SAR) have been found to be uniquely located in the trachealis muscle of the posterior wall of the trachea (3) and are presumably involved in the regulation of depth and rate of breathing (13). The rapidly adapting receptors (RAR) have been located all around the tracheal circumference and seemingly branching both in the superficial and in the deeper structures of the tracheal wall (18). They are usually considered as “cough receptors” and their other reflex actions comprise bronchoconstriction and inspiratory augmentation (19). In this study we describe a different sensitivity of these two types of tracheal receptors to three local anesthetics suggesting the feasibility of a differential blockade.
Physiological
METHODS
Nine dogs (8-12 kg) were anesthetized by an intravenous injection of pentobarbital sodium (30 mg/kg), supplemented as needed. A midline incision exposed the entire cervical trachea; a tracheostomy was performed right below the larynx and a cannula inserted. Both the vagosympathetic trunks were isolated at the level of the larynx and sectioned. The peripheral cut end of the right vagus was dissected under mineral oil, using fine forceps, with the aid of a binocular microscope. Small nerve filaments were placed on a pair of platinum wire electrodes and single-unit activity was amplified, monitored on an oscilloscope and a loudspeaker, and recorded on an oscillograph (Visicorder). Sensory endings distributed to the extrathoracic trachea were identified as follows. During recording of the single-unit activity, the cervical trachea was mechanically stimulated by a large Foley catheter inserted through the tracheal cannula into the intrathoracic trachea. The cuff was inflated and the catheter withdrawn while spontaneous ventilation continued through the lumen of the Foley catheter. The mechanical deformation of the tracheal wall could stimulate both rapidly and Society
1123
Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (132.174.255.223) on October 10, 2018. Copyright © 1979 American Physiological Society. All rights reserved.
1124
CAMPOHESI,
slowly adapting stretch receptors. Neural filaments were tested and discarded until units were found whose sensory endings were localized in the cervical trachea. If the receptor located in the extrathoracic trachea was identified as an SAR for its slowly adapting response to tracheal distention, the anterior tracheal wall was cut longitudinally along its midline and the tracheal cannula inserted at the level of the thoracic inlet. The two cut ends of the cartilaginous ring adjacent to the site of the receptor were fixed to a clamp on the right side and attached to a movable arm on the left side. A transverse elongation was exerted on the portion of the membranous posterior wall, site of the ending, by a motor rotating a cam on which the moving arm was attached (Fig. 1). A train of at least six sinusoidal elongations at approximately 1 Hz constituted the constant testing pattern. The transversal displacement of the moving arm was recorded on a different channel of the Visicorder. If the receptor located in the extrathoracic trachea was rapidly adapting to the tracheal distension (RAR), its circumferential location was further defined by intraluminal probing. The anterior tracheal wall was then carefully sectioned longitudinally, most frequently along the left side of its anterior aspect. Intermittent mechanical probing tested the receptor’s activity after each exposure to the anesthetics: a Q-tip was used exerting pressure by its own weight while it was moved longitudinally on the tracheal mucosa. Stock solutions of local anesthetics were prepared just before each experiment from crystals of lidocaine HCl, bupivacaine HCl, and tetracaine HCl in normal saline, at a concentration of 148, 31, and 17 mM, respectively (Table 1). The pH of the stock solutions varied between 6.8 and 7.1. The stock solutions were then serially diluted, and the different dilutions obtained were tested on the receptors by progressively switching to more concentrated solutions until the ending was completely silenced. Topical application on the tracheal mucosa was accomplished with small cotton pledgets soaked in the test solutions, prewarmed at 30°C and changed every 2-3 min to avoid dilution from mucous secretions. The area of tracheal mucosa in contact with the local anesthetics was never in excess of 2 cm’), i.e., approximately 1.5% of the total surface of the trachea of the dog (130 cm’). In several trials the three anesthetics were used on the same receptor so as to compare the relative potencies of each drug while minimizing the differences due to location and intrinsic susceptibility of the receptor. With one exception, if complete blockade of the receptor was not achieved after 15 min of exposure to a given anesthetic concentration the mucosa was washed with saline and a new pledget applied at a higher concentration. This type of comparison assumes that residual effects of the previous drugs be dissipated when another drug was tested. In our study we waited until full recovery of the original firing pattern was observed prior to testing with the next anesthetic; only those receptors that resumed control firing after complete blockade were considered. Furthermore one SAR (Table 2, dog 3b) was retested with 14.8 mM lidocaine 30 min after resuming control firing, resulting in similar blocking times (11 and 9 min, respectively). In addition, throughout all recovery periods
MORTOLA,
’ :h
SANT’AMBKOGIO,
AND
SANT’AMBROGIO
l
--I-
Thorocic inlet
L
k
t
of experimental setup used for cyclic oscillation of membranous posterior wall of extrathoracic trachea at level of a SAR location (indicated by a fiZZed circle). SARs action potentials (AP) are recorded from a neural filament separated from right vagus nerve (HVN) while movement provided by a motor is also recorded (L). An example of two tracings is shown at bottom. FIG.
TABLE
1. Diagram
1. Physical and biological properties
ofzocaz anesthetics ~ -~ Lipid/H~Opartition coef Equieffective anesthetic concn Mel wt
Lidocaine
2.9 1 270
Bupivacaine 27.5 0.25 300
Tetracaine 80 0.25 324
(30 to 90 min) the mucosa was washed with warm saline. The order of application of the three anesthetics was not formally randomized but varied as follows: lidocaine was used first in three of five SARs and two of four RARs tested with more than one drug; bupivacaine was first in one of five SARs and one of four RARs; and tetracaine was first in one of five SAR and one of four RARs. In view of the precautions described we believe that the results obtained with repetitive tests on the same receptor can be interpreted independently of each other. RESULTS
AND
DISCUSSION
In 7 dogs, 10 different stretch receptors were subjected to 20 trials: 12 with lidocaine, 5 with bupivacaine, and 3 with tetracaine. In 3 dogs 6 different RARs were exposed
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ANESTHESIA
OF
TRACHEAL
1125
RECEPTORS
to 13 challenges: 6 with lidocaine, 3 with bupivacaine, and 4 with tetracaine. In addition to topical anesthetization, 3 of the RARs were retested 7 times (3 times for lidocaine and 2 times each for bupivacaine and tetraCaine) by exposing the mucosa to the various anesthetic solutions delivered by aerosol from a Bird nebulizer (mean droplet diam, 1 pm; carrier gas, 02). In general higher concentrations of the anesthetics were found to block the receptors in shorter times of application. This is apparent in Fig. 2, which shows the effective blocking time-concentration values for two different SARs challenged with lidocaine. The minimal anesthetic concentrations necessary to block receptors during topical applications within 15 min are reported in Tables 2 and 3 for all the receptors studied. RARs are blocked at concentrations and durations of exposure much lower than SARs for all the three anesthetic agents. The range of values is much narrower for RARs than for SARs, which behave as a more heterogeneous group. It is apparent the much greater susceptibility of RARs to each of the anesthetics both in
40
Lidocaine hM) 2.
IO
20
terms of duration of exposure (ca. 1 order of magnitude) and concentration (ca. 2 orders of magnitude). These results seem to be consistent with the different location of the two types of endings within the tracheal wall. RARs, as far as their response to local probing is concerned, are distributed in the more superficial layers of the mucosa (18), whereas SARs are distributed in the much deeper trachealis muscle (3). A greater perfusion of the trachealis muscle, as compared to the superficial layers of the tracheal wall, could provide an alternative and/or an additional explanation for the lesser accessibility of the SARs to the local anesthetization. Furthermore the two types of receptors could also be intrinsically different in their susceptibility. Direct application of the anesthetic solution to the mucosa is a very effective technique compared to aerosol inhalation. Table 4 indicates the enormous differences in concentration necessary for each anesthetic agent to block the same RAR within comparable exposure times. These results may explain the need for the very high anesthetic concentrations in aerosols required to block the cough reflex (6, 14), which presumably involves the RARs in the trachea (9). Furthermore the difference between the two modalities of application of local anesthetics might be due to the relatively impenetrable mucous layer, which is easily removed by the soaked pledgets but greatly hinders the action of the aerosolized droplets. The relative potencies of the different anesthetic agents tested on this preparation is in general agreement with their equieffective anesthetic concentrations on the sciatic nerves of the rat and frog (4,5). Tetracaine blocks RARs at the lowest observed values of concentration; moreover, it appears to have the widest difference in its blocking parameters (Tables 2 and 3) between RARs and SARs. For this reason it should be considered the drug 3. Minimal concentrations of topical anesthetic blocking RARs and duration of exposure
30
TABLE
min 2. Anesthetic concentrations (in mM) and durations of topical application (in min) sufficient to block two different SARs (0; x) in cervical trachea. More diluted solutions require a longer time of exposure to silence SARs. FIG.
TABLE 2. Minimal concentrations of topical anesthetics blocking SARs and duration of exposure -
m No. 1
2 3” 3h 4” 4h 5 6” 6h 6’ 9 Mean &SD
Bupivacaine
Lidocaine mM
37.0 37.0 7.4 14.8 37 148 7.4 148 37
min
-
mM
min
15.6 3.1 6.35 6.35
2 9 15 10
Tetracaine mM
min
7 1
6 9 6 16 11
6.7 1.3
0.5 14
Lidocaine
Bupivacaine
mM
7” 7”
1.48 3.70
8”
0.93
8’ 8” 9
0.46 0.46 1.85
Mean *SD
1.48 t1.22
min
2
15.6
11
51.1 k52.6
7.4 k5.6
9.4 t5.6
9.4 k4.7
4.9 k3.1
9.8 t8.1
mM
min
1 1
1 0.2
0.20 0.10
0.3 0.2
0.17
1
0.02 0.04 0.02
5 1 1
0.10
0.2
0.13 to.06
0.23 to.07
0.06 to.07
2 2
1
1.03 kO.57
4. Anesthetic concentrations necessary to block RAR activity and duration of exposure
6
37
min
2
Lidocaine
15
mM
TABLE
10
6.7
Tetracaine
Dog No.
T A A/T
T, topical;
Bupivacaine
Tetracaine
mM
min
mM
min
mM
min
0.62 123
1.03 1.00
0.13 7.8
1.1 2.5
0.03 3.3
1
198
60
1
110
A, aerosol.
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1126
CAMPORESI, -Control -c+ I % Lidocaine,30sec 100
.
impslsec 50 .
O/O
5.0 Elongotion
I 100
50 % Elongation
100
9 CYCLE I CYCLE FIG. 3. Relationships between elongation of tracheal back wall (in R of the app lied displacement) and frequency of discharge (imp/s) for a SAR. FiZZed circles (0) and continuous Lines, control; open circZes (0) and broken lines, partial block with lidocaine. Left, 1st cycle; right, 5th cycle. This unit was silenced after 2 min of exposure.
of choice for a selective topical anesthesia of RARs. The blocking action of the local anesthetic appears to be gradual, as shown in Fig. 3, where the transversal elongation of the back wall containing a SAR is plotted against its firing rate during the first (left) and the fifth applied oscillations (right). In both control (solid lines)
MORTOLA,
SANT’AMBROGIO,
AND
SANT’AMBROGIO
and partially blocked conditions (broken lines) the receptor’s firing leads the elongation and this is particularly apparent during the first elongation; the viscous properties of the trachealis muscle, far more pronounced during the first applied oscillation, have been found to account for this phenomenon (15). During partial anesthesia the receptors maintain their dynamic dl/dt sensitivity, but their maximum rate of firing becomes lower. This might be attributed to a frequency-dependent conduction block similarly observed in sciatic nerve fibers of the frog (4). Another possibility might be a direct relaxing action of the anesthetics on the trachealis muscle that could also explain the higher threshold of the receptor’s activation during the first cycle. At lower concentrations than those used in this study a decrease in smooth muscle tone has been observed in in vitro tracheal preparations from the guinea pig after treatment with lidocaine, bupivacaine, tetracaine, and other local anesthetics (8). The possibility to selectively block RARs may provide a useful tool to study their role in respiratory regulation. We thank Karen Rex and Sandra Travers for their assistance. This study was supported by National Institutes of Health Grant 5ROl-HL-20122. Lidocaine HCl was kindly supplied by Astra Pharmaceutical Products Inc. Bupivacaine HCl was kindly provided by Sterling-Winthorp Research Institute. Received
27 November
1978; accepted
in final
form
21 June
1979.
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