CRYOBIOLOGY
Cardiac
15, 3543
(1978)
Output
of Marmots M.
Department
by the Thoracic
L. ZATZ~%fAN
Missotrri
Cardiac output of hibernating and hypothermic animals has been determined by dilution procedures (Z), Fick techniques (21), and electromagnetic flowmeters (12). The first two methods produce averaged cardiac output over several heartbeats, while measurements with flow probes result in instantaneous flow on a beat-by-beat basis. In an effort to find a method permitting the determination of stroke volume and cardiac output throughout a cycIe of hibernation, some of these methods were attempted and found to present severe difficulties. We isought a procedure involving a minimum of surgical invasion while providing maximal physiologic information. Chronic measurement of transthoracic impedance (1, 3, 6, 9, 11, 13, 14, 16, 19, 20, 22, 23) presented the best possibihty of achieving our aims, It was essential, however, to determine, by accepted procedures, the accuracy and reliability of the impedance method within the range of cardiac output expected to be encountered in a normothermic ,and hibernating animal. The animal model used for testing the procedure was that of experimental hypothermia with sufficient cooling to diminish heart rate from 300 to 40 beah/min and cardiac output from approximately 400 to 40 ml/min. Agreement with accepted procedures within this range of flow rates would, we felt, proReceived
Jmrmy
21,
1977.
Technique
ANL, W. J. RAY
of Physiology, Unirjersityof CoEumbia,
Impedance
Misx.wi 65201
Me&cd
Center,
vide sufficient verification of the suitability of the impedance procedure. Thus, the impedance method was compared simultaneously with either dye dilution or an electromagnetic flowmeter. Since some aspects of the impedance method are empirical, hinging in part on the estimation of the left ventricuIar ejection time ( T), we sought a consistent method of measuring the unique value of T which would produce resuhs in agreement with accepted methods of measuring cardiac output. IvlETHOIIS
Measurement of Trunsthoracic Impedance In previous reports metal bands or wires were placed on the skin surface of humans (1, 3, 6, 8, 11, 13, 14, 16, 19) and experimental animals (1, 9, 20, 22, 23) or were impIanted subcutaneously ( 15). Since a chronic preparation was ultimateIy desired we elected to implant the electrodes. Size 0 braided stainless steel sutures were implanted subcutaneously in six marmots. In one additional marmot the electrodes used were multistrand, siIver-plated 29 gauge wire. Wires were passed subcutaneously around the body of the animal in four regions: at the angle of the jaw, at the thoracic inlet, at the region of the xiphisternal joint, and in the abdominal area. The two anterior eIectrodes were nominally 3-4 cm apart. Distances were variable between the recording electrodes located at either end
OOll-Z240/78/0151-003~~~0~.~/0 Copyright @J 1978 by Academic Press., Inc. Ail rights of reproduction in any form reserved.
3G
ZATZMAN
of the thorax, but generahy varied between 7 and 9 cm, Small slits were made in the ventral and dorsal regions of the skin prior to implantation to permit electrode placement. Knots were tied in the wire at dorsal and ventral openings to facilitate measurement of the average interelectrode distance. All experiments were performed with the animaIs ventral side up. Wounds were closed and the emergent wires were trimmed to approximately 3 cm in length to permit connection by the alligator clamps supplied with the Minnesota Impedance Cardiograph (Model 304, Instrumentation for Medicine, Greenwich, Connecticut). The two outer eIectrodes (located at the angle of the jaw and the abdomen) supplied a sinusoidal current of 4 mA at a frequency of 100 KHz. Due to the low frequency response of the preamplifier, these electrodes aIso served as the connections for the eIectrocardiogram ( ECG). The inner electrodes at the anterior and posterior boundaries of the chest
AND
RAY
record potential variations reflecting the impedance (Z). The instrument supplies outputs of: impedance (Z), change in impcdance from the mean value (AZ), and the time derivative of the impedance change (AZ/&) or dZ/dt. The impedance, and therefore all functions of impedance, changed in association with alterations of thoracic volume produced by respiration and stroke volume of the left ventricle. The equation used to caIculate stroke volume was ( II):
dV = p ( L/Z,,) 2 ( dZ/dt ) tnin
T,
where dV is the stroke volume in milliliters, p is the resistivity of the blood in ohm-centimeters, L is the distance between the recording (inner) electrodes in centimeters, Z, is the baseline impedance, usually measured at end expiration, (dZ/dt),,,i, is the minimum rate of change in impedance in ohms per second, and T is the left ventricular ejection time in seconds, Figure X indicates some of the measurements
ECG+
150ARTERIAL PRESSURE IOOImm t9 50-
~
in---
_.-.-.-
;------
---”
,hJvw-L
oFIG. 1. A record obtained from a male marmot. Heart rate of 60/min, 21°C. From above, downward, the recordings represent: electrocardiogram (I-set marks), partially damped recording of flow from an electromagnetic of change of impedance (dZ/&) with calibration inscribed at low speed the tracing, and arterial pressure. The dill/& tracing shows the measurements lation of stroke volume. COB = 112 ml/min, COav = 100 ml/min.
recta1
temperatLlre (ECG), time flowmeter, rate on the Ieft side of made for calcu-
CARDlAC
OUTPUT
made for the calculation of dv. Cardiac output was calculated from: CO, = dV x HR, where CO,: is cardiac output in milliliters per minute, and HR is heart rate in beats per minute. Resistivity measurements were made on the blood of each animal at the various temperatures of the procedure (4, 17, 18), The resistivity ceil was similar in structure to those of Hill and Thompson (7) and Geddes and Sadler (5). The cell was constructed by cementing a 3-ml disposable syringe into the barrel of a 12-ml disposabIe syringe with a port between the two barrels for the introduction of blood. Betwecn measurements of rcsistivity the port was also used for monitoring the blood temperature with a copper-constantan thermocouple. Circular stainless steel electrodes (0.7,s cm’) were fastened to the ends of the barrel and plunger of the 3-ml syringe. The outer (12-ml syringe) barrel was fitted with two ports through which water of various temperatures was circulated to attain the desired blood tempernture prior to measurement of resistivity. Three measurements of resistivity were made at each temperature. The resistivity ccl1 formed one arm of a Schering bridge (7). Power to the bridge was supplied as a 100 KHz sinusoidal current by ma function generator (Model 30, Wavetek, San Diego, California). The output frequency of the generator was checked between mcasuremeld with II model 1900A multicounter (John Fhlkc Mfg. Co., Inc., SeattIe, Washington), Halance of the bridge was accomplished by observing its output on an oscilloscipe (Model 5103, Tektrouix, Benverton, Oregon), All measurements were made with the cell on a shaker (Yankee variable speed rotator, Clay Adams, Pnrsipl>any, New Jersey) agitating the blood sample at about 2 Hz to insure a homngenous distribution of the cellular components of the blood (5). Measurements were made at four different vo1ume.sof the 3-ml syringe, supplying data for three resistivity
OF MARMOTS
37
values at a temperature. Usually resistivity was determined at two temperatures, 3237 and 20-24”C, but when rectal temperature was depressed to below 2O”C, a measurement was made at the lowest temperature achieved. Blood resistivity was calculated from the equation given by Hi11 and Thompson (7): p = A( R,., - I&)/(
Z1- ZZ) = A AR/AZ
where p is the resistivity in ohm-centimeters, A is the cross-sectional area of the cell (circular electrodes at either end of the 3-ml syringe) in square centimeters, &I and R(,.2 are the resistive components of the electrode-electrolyte interface impedances at lengths (in centimeters) El and 12, respectively. The resistive components ( RcI and I&) were read directly from a lo-turn potentiometer at the bridge balance points. Resistance could be read to 0.5 ohm. The blood path length was changed by 0.64 cm (0.4 ml) between measurcmeuts. At final volume (1.8 ml), the minimum path length was 28.8 mm (7). An.imak Preparation Studies
for Dye Dilution
All studies were performed with anesthetized animals. Ketamine (10 mg/kg, intramuscularly) was first given to tranquilize the animal, and then pentobarbital (30 mg/kg, intraperitoneally ) was used to produce anesthesia, Cardiac output was determined by the dye dilution technique in conjunction with transthoracic impedance measurements in four marmots. After the electrodes were placed, the left carotid artery and left and right jugular veins were cannulated. The wound was cIosed with continuous sutures. Dye (0.2 to 0.5 ml of 5 mg/ml indocyanine green) was introduced into the right jugular venous catheter. Blood was drawn from the left carotid artery, through a densitometer cuvette (Model DC-140, Waters Instrument Co., Rochester, Minnesota), into a pump (Holtcr, Model 911, Extra-
ZATZMAN
38
AND RAY
corporeal Medical Specialties Co., Inc., ment of arterial blood pressure when dye Mount Laurel, New Jersey) to the Ieft jug- dilution studies were not being performed. Several measurements were made and ular venous catheter at 10 ml/min. The dye was quickly flushed into the right jugular then the animal was cooled by surrounding bags filled with vein with l-2 ml of isotonic saline. The it with poIyethylene crushed ice. To improve contact with the total volume of the external circuit (L ice the animal was soaked with cool water. carotid to L jugular) was approximately 3.5 ml. The cuvette was attached to a Additional anesthetic was administered as needed to prevent shivering. Recta1 temmodel TDI densitometer (Waters Instrument Co.); the densitometer output was perature was monitored during cooling recorded on a polygraph (Grass Instru(Yellow Springs Instrument Co., Yellow ments, Quincy, Massachusetts). The areas Springs, Ohio), and measurements of carunder the resuhing dye concentration diac output were made at Z-5°C intervals curves, excIusive of recirculation, were de- until rectal temperature reached 1%20°C. termined, and the cardiac output was calculated by dividing the amount of dye Animal Preparation for Flaw Probe Measurements injected by the minute-concentration of the dye in the arterial blood. The densitomAfter the impedance electrodes were eter-recorder system was calibrated at the placed, the left carotid artery was cannulated to monitor arterial pressure and perbeginning and end of the studies with bIood containing dye at a concentration mit injection of auesthetic when necessary. of 25 pg/ml. Clotting of blood during the The thorax was opened between the third procedure was prevented by giving the and fourth ribs and the sternum was trananimal 500 U of heparin/kg intravenously. sected at this level to permit easy access A three-way stopcock was connected to the to the ascending aorta. A previously cabcarotid artery catheter permitting measure- brated 5-mm diameter electromagnetic TABLE Measurement
of Resistivity of Marmot
1 Blood at Various Temperatures*
Tet11pepwe 0
33
m Y”;;le
~;~Wl~
-
1tcsistance (If, ohm) -_---_.
AR (ohms)
Al (-1
3.0 2.6 2.2 1.x
4.80 4.16 3.52 2.88
1281 I 090 902 765
141 188 1x7
0.64 0.64 0.64
223.8 220.3 160.6 201.6
3.0 2.6 2.2 1.8
4.80 4.16 3.52 2.88
1465 1243 1055 I< 830
212 1X8 225
o.ti4 0.64 0.64
248.4 220.3 263.7 244.1
3.0 2.6 2.2 1.8
4.80 4.16 3.52 2.88
1730 1482 1248 IO.10
248 234 218
0.64 0.64 0.64
290.6 274.2 255.0 273.3
Average 26
Average 19
P = 0.75AR/AI (ohm-cm)
~.l-l---
Average * .4uimal # 222 ; Female, HCT, 43 “j&
---
-
_-
CARDIAC flow
lpb”
( Statllaln
IlHtrulnellts,
OUTPUT OXlm-d,
California) was placed around the root of the aorta while the marmot was vent&ted by a positive pressure respirator (Model 607, Harvard Apparatus Co., Dover, Massachusetts) at 75 ml/breath and 22 breaths/ min. Positive pressure ventilation was maintained throughout the procedure unless otherwise noted. The chest wall and overlying skin were closed with continuous sutures. Chess closure was necessary since it was found that the impedance waveforms were distorted in auimals with the thorax open (23). The Bow probe was connected to an SP 2202 blood Aowmeter (Statham Instrumenta ) and the output of the flowmeter was recorded on the polygraph. Cooling of animals was performed as indicated in the dye dilution studies. Impedance and flow measurements were made with the respiratory pump off in the expired phase. After each series of measurements the flow probe was calibrated in situ using blood from the animal involved in the study. Due to the extreme surgical procedures required for the flow probe
OF
MARMOTS
39
studies and the necessity for IloI);Lrinization for the dye diIution procedure, it was not possible to perform the three methods simultaneously. RESULTS
The average resistivity of marmot blood at 33-36”C, and hematocrit of 42-46s was 212 -t 9.0 ohm-cm. Resistivity of marmot blood, as has been found with other animals, tended to increase with hematocrit (4, 5, 7, 8) and decreasing temperatures (4, 17, 18). Table 1 contains the data obtained from the bIood of one animal at three different temperatures, and indicates the method of calculating resistivity. The variability of calculated resistivity indicated by the table is not unusual and has been observed by other investigators (7). The baseline transthoracic impedance measured at end expiration (2,) prior to cooling was 45.1 5 0.60 ohm for the seven animals studied. Baseline impedance increased with cooling, but was influenced by surface temperature as well as body (rectal) temperature; removal of the ice.
/
250 CARDIAC OUTPUT (ml/mid
t ZEW
t
150
100 I-
/
50 1
/
/
/
/
/’ I 50
FIG. ( COS) dashed ml/min.
1 100
I 150
I I I 250 300 200 CARDIAC OUTPUT
I 350 D (mlhnin)
I 400
1 450
I 500
I 550
I 600
2. Rdation between cardiac output by the dilution (CO, ) and transthoracic impedance procedures. Thick solid Iine is the regression for the relation at COS > 120 ml/min, line (- - - -) is the line of identity, thin s&d line is the regression for the COe < 120 COZ = 120 ml/min is shown by the alternate long and short dashed lines (- - -).
40
ZATZMAN
AND RAY
FIG. 3. Dye dilution curves obtained at 20°C (curve A) and 36°C (curve R). Curve A: CO.5, 38 ml/min; CO=, 158 ml/min. Curve l3: CO+, 274 ml/min; COD, 294 ml/min.
containing pIastic bags led to a gradual decrease of 2” in the face of a constant rectal temperature. Since no attempt at steadystate temperature was made in this study, we are unable to accurately relate 20 with rectal temperature. Stroke volumes observed in these studies varied from 0.22 ml/beat to 3.3 ml/beat, Figure 2 demonstrates the reIation between cardiac output measured by dye dilution (CO,) and that determined by the impedance method ( COi!), As cooling progressed, cardiac output measured by both methods diminished. However, dye dilution curves gradually changed in character from that shown in Fig. 33 to the flattened and extended form in which recirculation was difficult to observe as shown in Fig. 3A. At CO, of 120 ml/min or less the relationship between output determined by the two methods demonstrated a negative correlation (Fig. 2). Figure 2 is composed of 79 pairs of data of which 39 corresponded to COa greater than 120 ml/min. Statistical analysis of the paired data above COs of 120 ml/min demonstrates a mean difference ( COzCOO) of 5.20 ml/min (P > 0.7). The average ratio of CO,/CO, for the 39 measuremeuts was 1,045 3- 0.047. Unlike the results obtained with dye dilution, the correlation remained high at all flows between cardiac output measured by the flowmeter (CO,,) and COX (Fig. 4). A total of 81 paired estimates of cardiac output was made by flowmeter and im-
pedance methods. The mean difference ( COE-COj,P) was 10.3 -t- 6.8 ml/min (P < .05). The average ratio of CO,/CO,, was 1.12 -+ 0.05. DISCUSSION
Marmots demonstrate a higher baseline transthoracic impedance than that observed in humans (3) and dogs ( 15). This high impedance may be due to the relatively thick subcutaneous layer of fat found in this species, The resistivity of marmot blood is higher than that reported for other animals studied (4, 5, 7). These animals demonstrate transient periods of hyperIipemia. As a result, any attempts at a systematic study of alteration of transthoracic impedance with temperature must take into account interanimal variation of bIood resistivity. Variation of resistivity could be due to differences in blood lipid concentration as well as dissimilarities in hcmatocrit ( 15). Initial cardiac output studies were performed comparing dye dilution with the impedance method. It was found that 2’ measured between the points at which dZ/& crosses the basehne produced the best relation between the two methods, Indeed, the same estimate of T was used with relation to measurements with flow probes. However, the correlation between the dye and impedance methods was low (T = 0.566). Correlations of similar magnitude between the averaging methods (Fick and dilution) and the instantaneous
CARDIAC
OUTPUT
mctld of transthoracic imped~mce wcrc reported by other investigators (1, 6, 11, 16, 22). Comparison of the impedance procedure with the instantaneous methods of measurement of cardiac output usually led to higher correlation coefficients jn the order of magnitude (0.905) found in our studies (1, 20, 23). The significant mean difference, although small, between COe and COpI’ may be accounted for by the very small standard error making the paired test very sensitive to small differences (20). The greatest decrease in cardiac output observed by the methods used in this study was 90% ( 400 to 40 mI/min ) . Other investigators working with hibernating animaIs demonstrated a 9599’$ decrease in cardiac output during hibernation (2, 12, 21). Since i.ransthoracic impedance produces a beat-by-beat estimate of stroke volume, it is more meaningful to compare our data with those of other investigators
CARDIAC OUTPUT hVmink
I 100
I I50
CARDIAC
between cardiac output impedance (CO,). Solid line (-)
4. Relation
41
MARMOTS
ill terms of thr: widrast rangr of’ stroke volun~s that can bc determined. It may bc assumed that: measurement of heart rate poses no problem. Kirkebii (12) found that the stroke volume of hibernating hedgehogs was 37% Iower than that of the aroused animal (0.22 ml/beat as compared to 0.35 ml/beat). Rullard and Funkhouser (2), on the other hand, found no difference in stroke volume of ground squirreIs during arousal. Assuming 300 beats/min and 4 beats/min for the heart rate of the normothermic and hibernating ground scluirrel, respectively ( lo), one can calculate stroke volume from Popovic’s (21) data. The result of the calculation indicates that stroke volume was 13% higher in the hibernator as compared to the normothcrmic animals (0.26 ml/beat in hibernating and 0.23 ml/beat in normothermic animals). Since the range of stroke volumes encountered in our studies (0.22-3.3 ml) was Is-fold, it is obvious that the imped-
2
50
FE. thoracic identity.
OF
by
I 200 OUTPUT
1 250
I 300
I 350
I 400
FP hlhin)
flowmeter (CO,,) and transis regression, dashed line (- - - -) is line of
electromagnetic
ZATZMAN
42
a11ce mt:thod
is at IKIS~
iis mlsitive
AND RAY REF!IRENCES
I-O
variation in stroke vohune as those of the previous investigators. In addition, the slope of the regression line for the relation between COYP and COz was 0.984. Taken together, these facts indicate that the transthoracic impedance method is sufficiently accurate to permit chronic, continuous measurement of cardiac output in normothermic and hibernating marmots. We cannot explain the failure of the dye dilution method at low cardiac outputs and heart rates since Bullard and Funkhouser (2) successfully measured cardiac output of hibernating animaIs at low heart rates and outputs by means of dye dilution, SUMMARY
Baseline impedance (Z,,) and resistivity of blood were higher for marmots than reported for other species. The transthoracic impedance method was compared to dye dilution and electromagnetic flowmeter procedures for estimation of cardiac output of seven marmots at a range of flows from 40 to 400 ml/min. There was a law, positive, but significant corrcIation (r = 0.566) found in comparison to dye dilution at outputs measured by the impedance method exceeding 120 ml/min. Correlation was better (T = 0.905) in the comparison between impedance and Aowmeter methods. It was concluded that transthoracic impedance provides data that are sufficiently accurate for chronic measurements of stroke volume and cardiac output of this species. The method has the additional advantage of supplying ECG and respiratory data without supplemental connections to the anima1 preparations. ACKNOWLEDGMENTS The authors wish to express their sincere appreciation to Ms. Jeannctte Kilfnil for her technical assistance. We wish to thank Dr. J, Alan Johnson for permitting us to use his Slraters densitometer and Statham Flowmeter. This work was supported by Public Health Service Grant HL 17847.
1. H&r, L. E., Judy, W. V., Geddes, L. E., Langley, L. M., and Hill, D. W. The measurement of cardiac output by means of electrical impedance. Cardiouasc. Res. Cent. BuEl. 9, m-145 ( 1971). 2. Bulk&I, R. W., and Funkhouscr, G. E. Estimated regiona blood flaw by rubidium 86 distribution during arousal from hibernation. Amer. J. Pliysiol. 203, 268-270 ( 196.2 ). 3. Denniston, J. C., Maher, J. T., Reeves, J. T., Cruz, J. C., Cymerman, A., and Grover, R. F. Measurement of cardiac output by electrical impedance at rest and during exercise. j. A&. PhysioE. 40, 91-95 ( 1978). 4. Geddes, L. A., and Baker, L. E. The specific resistance of biological material-a compendium of data for the biomedical engineer and physiologist. Med. Biol. Eng. 5, 271-293 ( 1967). 5. Geddes, I,. A., and Sadler, C. The specific resistance of blood at body temperature.
hlecl. Bid. Eng. 11, 336-339
( 1973).
8. Harley, A., and Greenfield, J. C. Determination of cardiac output in man by means of impedance plethysmography. Aerospace Med. 39, 248-252 ( 1968). 7. Hill, D. W., and Thompson, I?. D. The effect of hematocrit on the resistivity of human blood at 37°C and 100 KHz. Med. Biok
Eng. 13, 182-186
( 1975).
8. Hill, D. IV., and Thompson, portance of blood resistivity surement of cardiac output impedance method. hfed.
187-190 9. Ito,
F.
D. The imin the meaby the thoracic
BioE. Eng.
13,
(1975).
H., Yamakoshi, K., and Yamada, A. PhysioIogica1 and fluid-dynamic investigations of the transthoracic impedance plethysmography method for measuring cardiac output. Part Il. Analysis of the transthoracic impedance wave by perfusing dogs. Med. Biol. Eng. 14, 373-378 ( 1976 ). 10. Jtrhansson, B. W. Heart and circulation in hibernators. Ira “Mammalian Hibernation III (K. C. Fisher, A. R. Dawe, C. P. Lyman, E. Sohtinhanm, and F. E. South, Jr., Eds.), pp. 200-218. American Elsevier, New York, 1967. II. Judy, W. V., Langley, F. M., MuCowan, K. D., Stinnett, D. M., Baker, L. E., and Johnson, P. C. Comparative evaluation of the thoracic impedance and isotope dilution methods for measuring cardiac output. Aerospace Med. 40, 532-536 ( 1909). 12. Kirkebij, A. Cardiovascular investigations on hedgehogs during arousal from the hiber-
CARDIAC
OUTPUT
13. Kubicek, W. G., Kuncgis, J, N., Yattcrson, R. P., Whitsoe, D. A., and Mattson, R. H. Development and evaluation of an impedance cardiac output system. Aerospace Med. 37, 1208-1212 (1966). 14. Lahabidi, Z., Ehmke, D. A., Durnin, R. E., Leaverton, P. E., and Lauer, R. M. Evaluation of impedance cardiac output in children. Pediatrics 47, 870-879 ( 1971) L5, Luepker, A. V., Mi&ael, J. R., and Warbasse, J. R. Transthoracic electrical impedance: Quantitative evaluation of a non-invasive measure of thoracic fluid volume. Amer.
Hewt J. 85, 83-93
( 1973).
OF
20.
21.
22.
16. Nechwatal, W., Bier, P., Eversmann, A., and Kiinig, li. Die unblutige Bestimmung des Herzzeitvolumens mit der Impedanz karkiographie. Basic Res. Cadiol. 71, 542-552
(1976).
23.
17. Nyboer, J. “Electrical Impedance Plethysmography.” Charles Thomas, Springileld, Illinois, 1959. X8. Nyboer, J. Electrorheometric properties of tissues and fluids, Ann. N.Y. Acad. Sci. 170,
410-420 19.
riyboer,
( 1970).
J., Bagno,
S., Barnett,
A.,
and Halsey,
MARMOTS
43
R. II. IIIIIX&IIICC cartliograuts ;ttd cliffmx~tiated-impedance cardiograms-the electricaI impedance changes of the heart in relation to electrocardiograms and heart sounds. Ann. N.Y. Acad. Sci. 170, 421436 (1970). Pate, T. D., Baker, L. E., and Rosborough, J. P. The simultaneous comparison of the eJectrica1 impedance method for measuring stroke volume and cardiac output with four other methods. CarrlioGasc. Res. Cent. Bd. 14, 39-52 (1975). Popovic, V. Cardiac output in hibernating ground squirrels. Amer. J. Physiol. 207, 1345-1348 ( 1964). Rasmussen, J. P., Sorensen, B., and Kann, T. Evaluation of impedance cardiography as a non-invasive means of measuring systolic time intervals and cardiac output. A&Q Anaesth. Stand. 19, 210-218 (1975). Yamakoshi, K., Ito, H., Yamada, A., Miura, S., and Tomino, T. Physiological and Auiddynamic investigations of the transthoracic impedance plethysmography method fur measuring cardiac output: Part I. A fluid dynamic approach to the theory using an expansible tube model. Med. BioE. Eng. 14, 365-372 ( 1967).
Cardiac
15, 3543
(1978)
Output
of Marmots M.
Department
by the Thoracic
L. ZATZ~%fAN
Missotrri
Cardiac output of hibernating and hypothermic animals has been determined by dilution procedures (Z), Fick techniques (21), and electromagnetic flowmeters (12). The first two methods produce averaged cardiac output over several heartbeats, while measurements with flow probes result in instantaneous flow on a beat-by-beat basis. In an effort to find a method permitting the determination of stroke volume and cardiac output throughout a cycIe of hibernation, some of these methods were attempted and found to present severe difficulties. We isought a procedure involving a minimum of surgical invasion while providing maximal physiologic information. Chronic measurement of transthoracic impedance (1, 3, 6, 9, 11, 13, 14, 16, 19, 20, 22, 23) presented the best possibihty of achieving our aims, It was essential, however, to determine, by accepted procedures, the accuracy and reliability of the impedance method within the range of cardiac output expected to be encountered in a normothermic ,and hibernating animal. The animal model used for testing the procedure was that of experimental hypothermia with sufficient cooling to diminish heart rate from 300 to 40 beah/min and cardiac output from approximately 400 to 40 ml/min. Agreement with accepted procedures within this range of flow rates would, we felt, proReceived
Jmrmy
21,
1977.
Technique
ANL, W. J. RAY
of Physiology, Unirjersityof CoEumbia,
Impedance
Misx.wi 65201
Me&cd
Center,
vide sufficient verification of the suitability of the impedance procedure. Thus, the impedance method was compared simultaneously with either dye dilution or an electromagnetic flowmeter. Since some aspects of the impedance method are empirical, hinging in part on the estimation of the left ventricuIar ejection time ( T), we sought a consistent method of measuring the unique value of T which would produce resuhs in agreement with accepted methods of measuring cardiac output. IvlETHOIIS
Measurement of Trunsthoracic Impedance In previous reports metal bands or wires were placed on the skin surface of humans (1, 3, 6, 8, 11, 13, 14, 16, 19) and experimental animals (1, 9, 20, 22, 23) or were impIanted subcutaneously ( 15). Since a chronic preparation was ultimateIy desired we elected to implant the electrodes. Size 0 braided stainless steel sutures were implanted subcutaneously in six marmots. In one additional marmot the electrodes used were multistrand, siIver-plated 29 gauge wire. Wires were passed subcutaneously around the body of the animal in four regions: at the angle of the jaw, at the thoracic inlet, at the region of the xiphisternal joint, and in the abdominal area. The two anterior eIectrodes were nominally 3-4 cm apart. Distances were variable between the recording electrodes located at either end
OOll-Z240/78/0151-003~~~0~.~/0 Copyright @J 1978 by Academic Press., Inc. Ail rights of reproduction in any form reserved.
3G
ZATZMAN
of the thorax, but generahy varied between 7 and 9 cm, Small slits were made in the ventral and dorsal regions of the skin prior to implantation to permit electrode placement. Knots were tied in the wire at dorsal and ventral openings to facilitate measurement of the average interelectrode distance. All experiments were performed with the animaIs ventral side up. Wounds were closed and the emergent wires were trimmed to approximately 3 cm in length to permit connection by the alligator clamps supplied with the Minnesota Impedance Cardiograph (Model 304, Instrumentation for Medicine, Greenwich, Connecticut). The two outer eIectrodes (located at the angle of the jaw and the abdomen) supplied a sinusoidal current of 4 mA at a frequency of 100 KHz. Due to the low frequency response of the preamplifier, these electrodes aIso served as the connections for the eIectrocardiogram ( ECG). The inner electrodes at the anterior and posterior boundaries of the chest
AND
RAY
record potential variations reflecting the impedance (Z). The instrument supplies outputs of: impedance (Z), change in impcdance from the mean value (AZ), and the time derivative of the impedance change (AZ/&) or dZ/dt. The impedance, and therefore all functions of impedance, changed in association with alterations of thoracic volume produced by respiration and stroke volume of the left ventricle. The equation used to caIculate stroke volume was ( II):
dV = p ( L/Z,,) 2 ( dZ/dt ) tnin
T,
where dV is the stroke volume in milliliters, p is the resistivity of the blood in ohm-centimeters, L is the distance between the recording (inner) electrodes in centimeters, Z, is the baseline impedance, usually measured at end expiration, (dZ/dt),,,i, is the minimum rate of change in impedance in ohms per second, and T is the left ventricular ejection time in seconds, Figure X indicates some of the measurements
ECG+
150ARTERIAL PRESSURE IOOImm t9 50-
~
in---
_.-.-.-
;------
---”
,hJvw-L
oFIG. 1. A record obtained from a male marmot. Heart rate of 60/min, 21°C. From above, downward, the recordings represent: electrocardiogram (I-set marks), partially damped recording of flow from an electromagnetic of change of impedance (dZ/&) with calibration inscribed at low speed the tracing, and arterial pressure. The dill/& tracing shows the measurements lation of stroke volume. COB = 112 ml/min, COav = 100 ml/min.
recta1
temperatLlre (ECG), time flowmeter, rate on the Ieft side of made for calcu-
CARDlAC
OUTPUT
made for the calculation of dv. Cardiac output was calculated from: CO, = dV x HR, where CO,: is cardiac output in milliliters per minute, and HR is heart rate in beats per minute. Resistivity measurements were made on the blood of each animal at the various temperatures of the procedure (4, 17, 18), The resistivity ceil was similar in structure to those of Hill and Thompson (7) and Geddes and Sadler (5). The cell was constructed by cementing a 3-ml disposable syringe into the barrel of a 12-ml disposabIe syringe with a port between the two barrels for the introduction of blood. Betwecn measurements of rcsistivity the port was also used for monitoring the blood temperature with a copper-constantan thermocouple. Circular stainless steel electrodes (0.7,s cm’) were fastened to the ends of the barrel and plunger of the 3-ml syringe. The outer (12-ml syringe) barrel was fitted with two ports through which water of various temperatures was circulated to attain the desired blood tempernture prior to measurement of resistivity. Three measurements of resistivity were made at each temperature. The resistivity ccl1 formed one arm of a Schering bridge (7). Power to the bridge was supplied as a 100 KHz sinusoidal current by ma function generator (Model 30, Wavetek, San Diego, California). The output frequency of the generator was checked between mcasuremeld with II model 1900A multicounter (John Fhlkc Mfg. Co., Inc., SeattIe, Washington), Halance of the bridge was accomplished by observing its output on an oscilloscipe (Model 5103, Tektrouix, Benverton, Oregon), All measurements were made with the cell on a shaker (Yankee variable speed rotator, Clay Adams, Pnrsipl>any, New Jersey) agitating the blood sample at about 2 Hz to insure a homngenous distribution of the cellular components of the blood (5). Measurements were made at four different vo1ume.sof the 3-ml syringe, supplying data for three resistivity
OF MARMOTS
37
values at a temperature. Usually resistivity was determined at two temperatures, 3237 and 20-24”C, but when rectal temperature was depressed to below 2O”C, a measurement was made at the lowest temperature achieved. Blood resistivity was calculated from the equation given by Hi11 and Thompson (7): p = A( R,., - I&)/(
Z1- ZZ) = A AR/AZ
where p is the resistivity in ohm-centimeters, A is the cross-sectional area of the cell (circular electrodes at either end of the 3-ml syringe) in square centimeters, &I and R(,.2 are the resistive components of the electrode-electrolyte interface impedances at lengths (in centimeters) El and 12, respectively. The resistive components ( RcI and I&) were read directly from a lo-turn potentiometer at the bridge balance points. Resistance could be read to 0.5 ohm. The blood path length was changed by 0.64 cm (0.4 ml) between measurcmeuts. At final volume (1.8 ml), the minimum path length was 28.8 mm (7). An.imak Preparation Studies
for Dye Dilution
All studies were performed with anesthetized animals. Ketamine (10 mg/kg, intramuscularly) was first given to tranquilize the animal, and then pentobarbital (30 mg/kg, intraperitoneally ) was used to produce anesthesia, Cardiac output was determined by the dye dilution technique in conjunction with transthoracic impedance measurements in four marmots. After the electrodes were placed, the left carotid artery and left and right jugular veins were cannulated. The wound was cIosed with continuous sutures. Dye (0.2 to 0.5 ml of 5 mg/ml indocyanine green) was introduced into the right jugular venous catheter. Blood was drawn from the left carotid artery, through a densitometer cuvette (Model DC-140, Waters Instrument Co., Rochester, Minnesota), into a pump (Holtcr, Model 911, Extra-
ZATZMAN
38
AND RAY
corporeal Medical Specialties Co., Inc., ment of arterial blood pressure when dye Mount Laurel, New Jersey) to the Ieft jug- dilution studies were not being performed. Several measurements were made and ular venous catheter at 10 ml/min. The dye was quickly flushed into the right jugular then the animal was cooled by surrounding bags filled with vein with l-2 ml of isotonic saline. The it with poIyethylene crushed ice. To improve contact with the total volume of the external circuit (L ice the animal was soaked with cool water. carotid to L jugular) was approximately 3.5 ml. The cuvette was attached to a Additional anesthetic was administered as needed to prevent shivering. Recta1 temmodel TDI densitometer (Waters Instrument Co.); the densitometer output was perature was monitored during cooling recorded on a polygraph (Grass Instru(Yellow Springs Instrument Co., Yellow ments, Quincy, Massachusetts). The areas Springs, Ohio), and measurements of carunder the resuhing dye concentration diac output were made at Z-5°C intervals curves, excIusive of recirculation, were de- until rectal temperature reached 1%20°C. termined, and the cardiac output was calculated by dividing the amount of dye Animal Preparation for Flaw Probe Measurements injected by the minute-concentration of the dye in the arterial blood. The densitomAfter the impedance electrodes were eter-recorder system was calibrated at the placed, the left carotid artery was cannulated to monitor arterial pressure and perbeginning and end of the studies with bIood containing dye at a concentration mit injection of auesthetic when necessary. of 25 pg/ml. Clotting of blood during the The thorax was opened between the third procedure was prevented by giving the and fourth ribs and the sternum was trananimal 500 U of heparin/kg intravenously. sected at this level to permit easy access A three-way stopcock was connected to the to the ascending aorta. A previously cabcarotid artery catheter permitting measure- brated 5-mm diameter electromagnetic TABLE Measurement
of Resistivity of Marmot
1 Blood at Various Temperatures*
Tet11pepwe 0
33
m Y”;;le
~;~Wl~
-
1tcsistance (If, ohm) -_---_.
AR (ohms)
Al (-1
3.0 2.6 2.2 1.x
4.80 4.16 3.52 2.88
1281 I 090 902 765
141 188 1x7
0.64 0.64 0.64
223.8 220.3 160.6 201.6
3.0 2.6 2.2 1.8
4.80 4.16 3.52 2.88
1465 1243 1055 I< 830
212 1X8 225
o.ti4 0.64 0.64
248.4 220.3 263.7 244.1
3.0 2.6 2.2 1.8
4.80 4.16 3.52 2.88
1730 1482 1248 IO.10
248 234 218
0.64 0.64 0.64
290.6 274.2 255.0 273.3
Average 26
Average 19
P = 0.75AR/AI (ohm-cm)
~.l-l---
Average * .4uimal # 222 ; Female, HCT, 43 “j&
---
-
_-
CARDIAC flow
lpb”
( Statllaln
IlHtrulnellts,
OUTPUT OXlm-d,
California) was placed around the root of the aorta while the marmot was vent&ted by a positive pressure respirator (Model 607, Harvard Apparatus Co., Dover, Massachusetts) at 75 ml/breath and 22 breaths/ min. Positive pressure ventilation was maintained throughout the procedure unless otherwise noted. The chest wall and overlying skin were closed with continuous sutures. Chess closure was necessary since it was found that the impedance waveforms were distorted in auimals with the thorax open (23). The Bow probe was connected to an SP 2202 blood Aowmeter (Statham Instrumenta ) and the output of the flowmeter was recorded on the polygraph. Cooling of animals was performed as indicated in the dye dilution studies. Impedance and flow measurements were made with the respiratory pump off in the expired phase. After each series of measurements the flow probe was calibrated in situ using blood from the animal involved in the study. Due to the extreme surgical procedures required for the flow probe
OF
MARMOTS
39
studies and the necessity for IloI);Lrinization for the dye diIution procedure, it was not possible to perform the three methods simultaneously. RESULTS
The average resistivity of marmot blood at 33-36”C, and hematocrit of 42-46s was 212 -t 9.0 ohm-cm. Resistivity of marmot blood, as has been found with other animals, tended to increase with hematocrit (4, 5, 7, 8) and decreasing temperatures (4, 17, 18). Table 1 contains the data obtained from the bIood of one animal at three different temperatures, and indicates the method of calculating resistivity. The variability of calculated resistivity indicated by the table is not unusual and has been observed by other investigators (7). The baseline transthoracic impedance measured at end expiration (2,) prior to cooling was 45.1 5 0.60 ohm for the seven animals studied. Baseline impedance increased with cooling, but was influenced by surface temperature as well as body (rectal) temperature; removal of the ice.
/
250 CARDIAC OUTPUT (ml/mid
t ZEW
t
150
100 I-
/
50 1
/
/
/
/
/’ I 50
FIG. ( COS) dashed ml/min.
1 100
I 150
I I I 250 300 200 CARDIAC OUTPUT
I 350 D (mlhnin)
I 400
1 450
I 500
I 550
I 600
2. Rdation between cardiac output by the dilution (CO, ) and transthoracic impedance procedures. Thick solid Iine is the regression for the relation at COS > 120 ml/min, line (- - - -) is the line of identity, thin s&d line is the regression for the COe < 120 COZ = 120 ml/min is shown by the alternate long and short dashed lines (- - -).
40
ZATZMAN
AND RAY
FIG. 3. Dye dilution curves obtained at 20°C (curve A) and 36°C (curve R). Curve A: CO.5, 38 ml/min; CO=, 158 ml/min. Curve l3: CO+, 274 ml/min; COD, 294 ml/min.
containing pIastic bags led to a gradual decrease of 2” in the face of a constant rectal temperature. Since no attempt at steadystate temperature was made in this study, we are unable to accurately relate 20 with rectal temperature. Stroke volumes observed in these studies varied from 0.22 ml/beat to 3.3 ml/beat, Figure 2 demonstrates the reIation between cardiac output measured by dye dilution (CO,) and that determined by the impedance method ( COi!), As cooling progressed, cardiac output measured by both methods diminished. However, dye dilution curves gradually changed in character from that shown in Fig. 33 to the flattened and extended form in which recirculation was difficult to observe as shown in Fig. 3A. At CO, of 120 ml/min or less the relationship between output determined by the two methods demonstrated a negative correlation (Fig. 2). Figure 2 is composed of 79 pairs of data of which 39 corresponded to COa greater than 120 ml/min. Statistical analysis of the paired data above COs of 120 ml/min demonstrates a mean difference ( COzCOO) of 5.20 ml/min (P > 0.7). The average ratio of CO,/CO, for the 39 measuremeuts was 1,045 3- 0.047. Unlike the results obtained with dye dilution, the correlation remained high at all flows between cardiac output measured by the flowmeter (CO,,) and COX (Fig. 4). A total of 81 paired estimates of cardiac output was made by flowmeter and im-
pedance methods. The mean difference ( COE-COj,P) was 10.3 -t- 6.8 ml/min (P < .05). The average ratio of CO,/CO,, was 1.12 -+ 0.05. DISCUSSION
Marmots demonstrate a higher baseline transthoracic impedance than that observed in humans (3) and dogs ( 15). This high impedance may be due to the relatively thick subcutaneous layer of fat found in this species, The resistivity of marmot blood is higher than that reported for other animals studied (4, 5, 7). These animals demonstrate transient periods of hyperIipemia. As a result, any attempts at a systematic study of alteration of transthoracic impedance with temperature must take into account interanimal variation of bIood resistivity. Variation of resistivity could be due to differences in blood lipid concentration as well as dissimilarities in hcmatocrit ( 15). Initial cardiac output studies were performed comparing dye dilution with the impedance method. It was found that 2’ measured between the points at which dZ/& crosses the basehne produced the best relation between the two methods, Indeed, the same estimate of T was used with relation to measurements with flow probes. However, the correlation between the dye and impedance methods was low (T = 0.566). Correlations of similar magnitude between the averaging methods (Fick and dilution) and the instantaneous
CARDIAC
OUTPUT
mctld of transthoracic imped~mce wcrc reported by other investigators (1, 6, 11, 16, 22). Comparison of the impedance procedure with the instantaneous methods of measurement of cardiac output usually led to higher correlation coefficients jn the order of magnitude (0.905) found in our studies (1, 20, 23). The significant mean difference, although small, between COe and COpI’ may be accounted for by the very small standard error making the paired test very sensitive to small differences (20). The greatest decrease in cardiac output observed by the methods used in this study was 90% ( 400 to 40 mI/min ) . Other investigators working with hibernating animaIs demonstrated a 9599’$ decrease in cardiac output during hibernation (2, 12, 21). Since i.ransthoracic impedance produces a beat-by-beat estimate of stroke volume, it is more meaningful to compare our data with those of other investigators
CARDIAC OUTPUT hVmink
I 100
I I50
CARDIAC
between cardiac output impedance (CO,). Solid line (-)
4. Relation
41
MARMOTS
ill terms of thr: widrast rangr of’ stroke volun~s that can bc determined. It may bc assumed that: measurement of heart rate poses no problem. Kirkebii (12) found that the stroke volume of hibernating hedgehogs was 37% Iower than that of the aroused animal (0.22 ml/beat as compared to 0.35 ml/beat). Rullard and Funkhouser (2), on the other hand, found no difference in stroke volume of ground squirreIs during arousal. Assuming 300 beats/min and 4 beats/min for the heart rate of the normothermic and hibernating ground scluirrel, respectively ( lo), one can calculate stroke volume from Popovic’s (21) data. The result of the calculation indicates that stroke volume was 13% higher in the hibernator as compared to the normothcrmic animals (0.26 ml/beat in hibernating and 0.23 ml/beat in normothermic animals). Since the range of stroke volumes encountered in our studies (0.22-3.3 ml) was Is-fold, it is obvious that the imped-
2
50
FE. thoracic identity.
OF
by
I 200 OUTPUT
1 250
I 300
I 350
I 400
FP hlhin)
flowmeter (CO,,) and transis regression, dashed line (- - - -) is line of
electromagnetic
ZATZMAN
42
a11ce mt:thod
is at IKIS~
iis mlsitive
AND RAY REF!IRENCES
I-O
variation in stroke vohune as those of the previous investigators. In addition, the slope of the regression line for the relation between COYP and COz was 0.984. Taken together, these facts indicate that the transthoracic impedance method is sufficiently accurate to permit chronic, continuous measurement of cardiac output in normothermic and hibernating marmots. We cannot explain the failure of the dye dilution method at low cardiac outputs and heart rates since Bullard and Funkhouser (2) successfully measured cardiac output of hibernating animaIs at low heart rates and outputs by means of dye dilution, SUMMARY
Baseline impedance (Z,,) and resistivity of blood were higher for marmots than reported for other species. The transthoracic impedance method was compared to dye dilution and electromagnetic flowmeter procedures for estimation of cardiac output of seven marmots at a range of flows from 40 to 400 ml/min. There was a law, positive, but significant corrcIation (r = 0.566) found in comparison to dye dilution at outputs measured by the impedance method exceeding 120 ml/min. Correlation was better (T = 0.905) in the comparison between impedance and Aowmeter methods. It was concluded that transthoracic impedance provides data that are sufficiently accurate for chronic measurements of stroke volume and cardiac output of this species. The method has the additional advantage of supplying ECG and respiratory data without supplemental connections to the anima1 preparations. ACKNOWLEDGMENTS The authors wish to express their sincere appreciation to Ms. Jeannctte Kilfnil for her technical assistance. We wish to thank Dr. J, Alan Johnson for permitting us to use his Slraters densitometer and Statham Flowmeter. This work was supported by Public Health Service Grant HL 17847.
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hlecl. Bid. Eng. 11, 336-339
( 1973).
8. Harley, A., and Greenfield, J. C. Determination of cardiac output in man by means of impedance plethysmography. Aerospace Med. 39, 248-252 ( 1968). 7. Hill, D. W., and Thompson, I?. D. The effect of hematocrit on the resistivity of human blood at 37°C and 100 KHz. Med. Biok
Eng. 13, 182-186
( 1975).
8. Hill, D. IV., and Thompson, portance of blood resistivity surement of cardiac output impedance method. hfed.
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F.
D. The imin the meaby the thoracic
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13,
(1975).
H., Yamakoshi, K., and Yamada, A. PhysioIogica1 and fluid-dynamic investigations of the transthoracic impedance plethysmography method for measuring cardiac output. Part Il. Analysis of the transthoracic impedance wave by perfusing dogs. Med. Biol. Eng. 14, 373-378 ( 1976 ). 10. Jtrhansson, B. W. Heart and circulation in hibernators. Ira “Mammalian Hibernation III (K. C. Fisher, A. R. Dawe, C. P. Lyman, E. Sohtinhanm, and F. E. South, Jr., Eds.), pp. 200-218. American Elsevier, New York, 1967. II. Judy, W. V., Langley, F. M., MuCowan, K. D., Stinnett, D. M., Baker, L. E., and Johnson, P. C. Comparative evaluation of the thoracic impedance and isotope dilution methods for measuring cardiac output. Aerospace Med. 40, 532-536 ( 1909). 12. Kirkebij, A. Cardiovascular investigations on hedgehogs during arousal from the hiber-
CARDIAC
OUTPUT
13. Kubicek, W. G., Kuncgis, J, N., Yattcrson, R. P., Whitsoe, D. A., and Mattson, R. H. Development and evaluation of an impedance cardiac output system. Aerospace Med. 37, 1208-1212 (1966). 14. Lahabidi, Z., Ehmke, D. A., Durnin, R. E., Leaverton, P. E., and Lauer, R. M. Evaluation of impedance cardiac output in children. Pediatrics 47, 870-879 ( 1971) L5, Luepker, A. V., Mi&ael, J. R., and Warbasse, J. R. Transthoracic electrical impedance: Quantitative evaluation of a non-invasive measure of thoracic fluid volume. Amer.
Hewt J. 85, 83-93
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23.
17. Nyboer, J. “Electrical Impedance Plethysmography.” Charles Thomas, Springileld, Illinois, 1959. X8. Nyboer, J. Electrorheometric properties of tissues and fluids, Ann. N.Y. Acad. Sci. 170,
410-420 19.
riyboer,
( 1970).
J., Bagno,
S., Barnett,
A.,
and Halsey,
MARMOTS
43
R. II. IIIIIX&IIICC cartliograuts ;ttd cliffmx~tiated-impedance cardiograms-the electricaI impedance changes of the heart in relation to electrocardiograms and heart sounds. Ann. N.Y. Acad. Sci. 170, 421436 (1970). Pate, T. D., Baker, L. E., and Rosborough, J. P. The simultaneous comparison of the eJectrica1 impedance method for measuring stroke volume and cardiac output with four other methods. CarrlioGasc. Res. Cent. Bd. 14, 39-52 (1975). Popovic, V. Cardiac output in hibernating ground squirrels. Amer. J. Physiol. 207, 1345-1348 ( 1964). Rasmussen, J. P., Sorensen, B., and Kann, T. Evaluation of impedance cardiography as a non-invasive means of measuring systolic time intervals and cardiac output. A&Q Anaesth. Stand. 19, 210-218 (1975). Yamakoshi, K., Ito, H., Yamada, A., Miura, S., and Tomino, T. Physiological and Auiddynamic investigations of the transthoracic impedance plethysmography method fur measuring cardiac output: Part I. A fluid dynamic approach to the theory using an expansible tube model. Med. BioE. Eng. 14, 365-372 ( 1967).
