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Electrocardiographic observations on the tuatara, Sphenodon punctatus
Cmnp. Biochem. Phytial., 1971, VoZ. #A, pp. 881 to 892. Peridot
Press. Printed in Great Britain
ELECTROCARDIOGRAPHIC OBSERVATIONS ON THE T~ATA~~ ~~~~~0~0~ ~U~C~A~U~ HARRY
S. McDONALDl
and JAMES
E. HEATH2
‘Department of Biology, Stephen F. Austin State University, Nacogdoches, Texas 75961; and aDepartment of Physiology and Biophysics, University of Illinois, Urbana, Illinois 61803 (received 28 April 1971) AbafrpLct-1. Electrocardiograms were obtained from two unrestrained tuataras at body temperatures from 7 to 26°C. 2. Heart rates were lower than previously reported for resting Sphenodon, but were similar to resting rates of large lizards. 3. ECG intervals (P-R, R-T, P,-T,) did not differ greatly from those of Iguana iguana, except that duration of ventricular depolarization was more sensitive to temperature in Sphenodon. 4. In general, the electrocardiology of Sphenodon closely resembles that of squamate reptiles. 5. An unusual feature was an occasional increase in the absolute duration of R-T when heart rate increased slightly in response to disturbance. INTRODUCTION
IN ITS general morphology, the heart of the tuatara, Sphenodrmpmctutus,
resembles that of lizards (White, 1959; Simons, 1965), and largely on the basis of the internal structure of the ventricle, White (1959) has suggested that this heart probably functions like a squamate heart as well. However, the recent paper of Wilson & Lee (1970) seems to be the only substantial contribution to the cardiovascular physiology of this interesting and unique animal. A single electrocardiographic record from a tuatara appeared in a report on recording methodology (Wallis et al., 1968). Since there is apparently no other published information on the electrocardiology of S#rf~~~don, the present observations, though limited in scope, may be of interest. MATERIALS
AND METHODS
Through the efforts of Dr. H. M. Smith and one of us (J. E, H.) two tuataras, a 470-g male (Specimen L) and a 197-g female (Specimen S), were obtained from the government of New Zealand. After arrival in Urbana they were maintained at 23-24°C for approximately 2 months before study. Water was provided ad lib., and they were periodically fed young rat pups. Both specimens appeared to be vigorous and in good health until they were killed in connection with other studies several months after the electrocardiographic observations were made. Electrocardiograms were recorded on a Beckman Dynograph with a Model 9853 input coupler. Though this D.C. coupler has a cut-off frequency of 180 Hz, to minimize movement artifacts and interference, the bandwidth was limited to 0.16-60 Hz for most records. 881
882
HARRY S. MCDONALD AND JAMES E. HEATH
Preliminary tests and periodic checks showed that reducing the bandwidth caused no significant changes in waveform or amplitude. Almost all records were made at a sensitivity of 0.2 mV/cm of pen deflection, and a chart speed of 2.5 cm/set. To verify the accuracy of measurements made on routine records, occasional samples were recorded at 10.0 cmjsec and 0.1 mV/cm. Several lengthy records were made at 0.1 cm/set to check for arrhythmias or to monitor heart rate when specimens were unattended. Self-adhering silver surface electrodes (Narco Biosystems, Inc.) were used in various lead configurations. For the routine display of all waveforms, one electrode on the left side of the neck close to the shoulder and one on the ventral midline at the posterior end of the rib cage proved satisfactory. In terms of electrode positions relative to the heart, this combination resembles most closely a Lead III arrangement, and for convenience it will be referred to as Lead III* in this report. Body temperature was measured with a Yellow Springs Telethermometer and a YSI small animal thermistor probe. The probe was inserted through the cloaca to a depth of 5 cm in the large intestine. In the majority of records at room temperature (23-25°C) specimens were in thermal equilibrium; however, other records were made while body temperature was changing. To vary body temperature, specimens were outfitted with ECG electrodes and thermistor probes, placed in large circular plastic containers, and transferred from one constant temperature room to another. The maximum rate of rise in body temperaThe maximum rate of cooling was 0.25 ture in either specimen was 0.14 degree/min. degree/min, and this rate was maintained for only a few minutes after transfer from room temperature to a 5°C environment. RESULTS
Heart rate Heart rates recorded at various body temperatures between 7 and 26°C are illustrated in Fig. 1. Except when specimens were moving spontaneously or disturbed by handling, rates fell between 6 and 31 beats/min. All rates above 35 beats/min were recorded within 10 min of electrode attachment. The restraint necessary to affix the leads was apparently a significant factor, because after the initial tachycardia subsided, intentional disturbance (even to the extent of poking the specimen with a pencil) failed to raise heart rate above 32 beats/min. The pattern of decline in heart rate after handling can be seen in Fig. 2, which is a composite graph of data from five different episodes. Usually only minor variations in R-R’ interval occurred within samples ; however, occasional arrhythmic periods were observed in both specimens. The most extreme case subject to accurate measurement was in a sample from Specimen I, at room temperature where two consecutive R-R’ intervals were 4.56 and 2.56 sec. In a segment recorded at O-1 cm/set on another occasion, an arrhythmia comprised of alternate long and short R-R’ intervals appeared to wax and wane, and at the peak of the arrhythmia the ratio of consecutive long and short intervals approximated 2 : 1. Neither in this case nor in any other was there evidence of dissociation due to an A-V block. Electrocardiographic
waoes
Examples of actual electrocardiograms recorded from Sphenodon are presented in Fig. 3. Under the recording conditions that we used, the ECG’s generally
El..ECTROCARDIOGRAPHY
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Specimen S
l
Specimen L
883
OF SPHENODON
I
I
I
I
10
15
20
25
BODY
TEMPERATURE
(“C)
FIG. 1. Influence of body temperature on heart rate in two unrestrained tuataras. Over most of the temperature range heart rates were recorded while body temperature was slowly changing. The proportionally greater dispersion of rates in the vicinity of 25°C is due in part to (a) samples recorded while specimens were in thermal equilibrium and undisturbed for some time, and (b) samples reflecting excitement attending attachment of electrodes. 60 -
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FIG. 2 A composite graph of five episodes illustrating the pattern of decline in heart rate after animals were restrained for attachment of electrodes. consisted
only of
P, R and T waves.
No regular deflection that could be identified ECG’s,
as an SV wave was apparent in any of the records. As in other vertebrate the ventricular depolarization wave was the strongest and most definitive, is therefore
described
first.
and it
884
HARRY S. MCDONALDANDJAMESE. HEATH
With but one exception no distinct Q or S waves appeared in any of the records. In some samples the descending portion of the R wave overshot the baseline slightly. However, in these cases the return to the baseline was very gradual over a period many times longer than the R duration. Maximum R amplitude was 0.34 and 0.20 mV for Specimens L and S, respectively, and these values were obtained with Lead III *. Although sets of “standard” leads were not
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FIG. 3. Selected ECG records from Sphenodon. A, B, C. Records from Specimen L at three temperatures. Waves are designated in C because the T wave is shallow in that sample. D. Record from Specimen S at 25”C, illustrating lengthening of the P-R and shortening of the R-T. Note that the Pz-T, does not differ greatly from that in A. E, F, G, H, I. Samples recorded from Specimen L at 25°C illustrating lengthening of the R-T interval during disturbance despite increased heart rate. In E, specimen was resting quietly with a heart rate of 18.5 beats/min. F, G and H were recorded after progressively greater disturbance. Sample I was recorded after the specimen had been allowed to rest for several minutes. Note that the P-R shortened with tachycardia as would be expected. All samples were recorded with Lead III* arrangement (see text). The time and amplitude scale is applicable to all samples.
taken, transverse leads (from electrodes located on the right and left sides of the neck, or in right and left axillary regions) indicated a small right to left component in the mean axis of ventricular depolarization. The duration of the R wave was approximately 0.10 set at 23-25°C and 0.28 set at 7°C. The only record that exhibited anything more than a simple R wave in the ventricular depolarization complex was taken from the small specimen at room temperature. Electrodes were located on the left side of the neck and in the right axillary region. The initial record was biphasic with an S wave about half the amplitude of the R wave. The specimen was then induced to bend its body to the right, whereupon the R and S deflections became equal and the overall amplitude of the complex increased slightly to 0.08 mV.
El.ECTROCARDIOGRAPHY
OF SPHENODON
885
The P wave was low, monophasic and upright. We did not attempt to measure its duration because of the gently sloping leading and trailing edges and the low amplitude, but P wave duration was always clearly less than that of the accompanying R wave. The waveform did not always seem to be symmetrical, and in some records at low temperature the P seemed to be bimodal. As in other reptiles, the T wave was variable in amplitude and configuration. Most commonly, it was inverted and moderate in amplitude (30-60 per cent of R amplitude), but in both specimens it was occasionally upright, and in Specimen S there were a few instances when an upright T wave exceeded the R wave in amplitude by as much as 25 per cent. In several samples transitional biphasic T waves appeared ; these were always moderate in amplitude with the initial deflection opposing the R wave polarity. We did not attempt to measure T wave duration because frequently either the leading or trailing edge (or both) merged with the baseline very gradually. Electrocardiographic
intervals
Graphs illustrating the effect of temperature on atrioventricular conduction time (P-R interval), duration of ventricular depolarization (R-T interval) and total length of the active electrical cycle of the heart (P,T, interval) are presented in Fig. 4. Estimated mean absolute and relative values at selected temperatures are given in Table 1. Measurements of P-R and R-T intervals were made from peak to peak of the respective waves because of the aforementioned difficulty in identifying the beginning and end of some waveforms. The P,Tt interval, on the other hand, was measured from the beginning of the P wave to the end of the T wave, and in cases of doubt the longest conceivable interval was recorded to avoid underestimating the active duty cycle of the heart. A similar approach has previously been used in analyzing other reptilian ECG’s (see, for example, Dawson, 1960; Dawson & Templeton, 1963; Moberly, 1968). The P-R values recorded ranged from 0.32 to 1.98 sec. As can be seen in Fig. 4, at a given temperature level the P-R was proportionally more variable than either R-T or P,T,. A straight line fitted by eye to mean values for one-degree intervals yielded a Q10 of 2.14 for the reciprocal of P-R over the entire temperature range. Since the semilogarithmic plots did not seem to indicate true linear relationships between the logarithms of the intervals and temperature, a line was also fitted to the most linear portion of the graph, between 7 and lS”C, and a Q10 of 2.40 was obtained for this narrower range. The relative P-R interval, P-R : R-R, ranged from 0.15 to O-38, and tended to be greater for Specimen S than Specimen L. It never exceeded 0.30 for Specimen L under any condition, nor for either specimen at heart rates of 20 beats/min or less (regardless of temperature). The overall range of R-T was 0.50-3.64 sec. Though less variable (on a proportional basis) than P-R at any given temperature level, it was more strongly influenced by temperature than either P-R or P,T,. Estimates of Q1a’s for the reciprocal of R-T were 2.46 for the entire temperature range, and 2.80 for the region between 7 and 18°C. The relative R-T value, R-T : R-R’, ranged from
886
HARRY S. MCDONALD AND JAMES E. HEATH
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FIG. 4. Influence of body temperature on electrocardiographic intervals in two tuataras. Each scatter-graph includes values from both specimens. The P-R and R-T intervals were measured from peak to peak of the indicated waves, and are intended as estimates of atrio-ventricular conduction time and duration of ventricular depolarization, respectively. The Pi-T, interval was measured from the beginning of the P wave to the end of the T wave, and represents the active portion of the electrical cycle of the heart.
0.21 to 0.58, and both of these extreme values were from samples at room temperature. A plot of R-T vs. R-R’ at room temperature (Fig. 5) reveals that this relationship is quite variable. The duration of the active phase of the cardiac electrical cycle, the Pi--T, interval, varied from 1.0 to 5.8 sec. There was a trend towards greater variability in relative P,-T, at higher temperatures; at 7-8°C it ranged only from 0.64 to 0.80, while at 25-26°C minimum and maximum values were 0.42 and 0.95. Estimated Q1,,‘s for the reciprocal of Pi-T1 were 2.16 and 2.37 for temperature ranges of 7-26°C and 7-18°C respectively. At all temperatures Specimen S tended to have higher relative P,--T, values than Specimen L. This is due to the fact that temperature influenced interval durations in the two specimens about equally,
ELE~R~~I~~P~
OF
887
SPHENODON
while the smaller specimen exhibited higher heart rates throughout the temperature range used. A few specific observations relating to ECG intervals bear comment. The first concerns an elongated P-R interval that appeared on two separate occasions in Specimen S (see Fig. 3). In both cases the specimen was at room temperature and the P-R exceeded 0.70 set while the accompanying R-T values were relatively low (less than 0.60 set). One sample in which this phenomenon was noted was preceded by a lengthy record at 0.1 cm/set, and although precise measurements could not be made from the low-speed record, it was clear that the relative duration of the P-R and R-T intervals had been maintained for over 90 min prior to .
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. I
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2. R-R’
1NTERVAL
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(seconds)
FIG. 5. Duration of R-T plotted vs. the beat-to-beat interval in two tuataras at room temperature (23-25”(J), showing the great variability of this relationship. The dashed lines represent the boundaries of the area within which R-T accounted for 20 to 60 per cent of the beat-to-beat interval.
recording the 25 cm/set sample. It is also interesting, though perhaps not significant, that the changes in P-R and R-T intervals so balanced one another that the Pa-T, was near the mean value for that interval at room temperature. The long P-R intervals were quite regular, and there was no indication of abnormality in the ECG or distress in the specimen. The other observation involved both specimens. In several episodes disturbance led to increased R-T intervals while the heart rate was also increasing. In two cases the elongation of R-T could be traced through several consecutive samples as the specimen was intentionally disturbed. In all cases the accompanying P-R intervals decreased with rising heart rate, as would be anticipated. This phenomenon is illustrated in Fig. 3.
888
HARRY S. MCDONALD AND JAMES E. HEATH
DISCUSSION
In a laboratory study utilizing a thermal gradient chamber, Stebbins (1958) found that Sphenodon would voluntarily tolerate body temperatures from 3.5 to 27*9”C. The mean body temperature, based on over 200 measurements from each of two specimens, was 18*3”C, and fewer than 3 per cent of all measurements were outside of the 7 to 26°C range used in our own study. Thus, at least as far as temperature is concerned, our data probably reflect the electrocardiology of Sphenodon under normal resting conditions. To qualify the temperature sensitivity of heart rate in our specimens, the relationship between log heart rate and temperature was evaluated. Though some of our samples were recorded while specimens were known to be spontaneously moving or disturbed, we did not feel that adequate data were on hand to justify separation of “resting” and “active” rates. The least-squares regression equation based on all samples is log H. R. = 0.760 + 0.027 Tb. The regression coefficient corresponds to a Qr,, of 1.86. Between 7 and 26”C, our mean heart rate values fall below those reported by Wilson & Lee (1970) for “resting” conditions, although the extrapolated regression lines would intersect at 2.6”C. The difference is striking at higher temperatures because the Qn, for their heart rate curve is 2.34, and at 25°C even the ranges of rates recorded in the two studies barely overlap. In view of the marked effect of restraint on heart rate in our specimens and the fact that the tuataras studied by Wilson & Lee were restrained for all recording, we are inclined to attribute the difference to that factor. A parallel situation in a lizard was reported by Licht (1965) who noted that heart rates in untethered Dipsosaurus dorsalis were lower than “resting” rates for restrained specimens of the same species (Dawson & Bartholomew, 1958). Although our heart rate results for Sphenodon differ strikingly from those of Wilson & Lee, they resemble the results obtained on several species of large lizards: Tiliqua scincoides (Bartholomew et al., 1965), T. rugosa (Licht, 1965), Amphibolurus barbatus (Bartholomew & Tucker, 1963), Varanus spp. (Bartholomew & Tucker, 1964) and Iguana iguana (Moberly, 1968). Within the common temperature range our data are closest to the resting rates for I. iguana and T. scincoides. Our regression coefficient corresponds to a lower Qr,,, however, and the regression line intersects that for I. iguana between 10 and 15°C and that for T. scincoides between 20 and 25°C. In those reports wherein data for both resting and active conditions were presented (Bartholomew & Tucker, 1963, 1964; Bartholomew et al., 1965; Moberly, 1968) the jQrs’s for resting heart rates were above 2.0, while those for active rates were below 2.0. The unusual combination of low absolute values and low &,, that we obtained may be a sampling artifact in that most of the rates between 15 and 23°C were recorded during change in body temperature, while many of the values between 23 and 25”C, on the other hand, were from specimens in thermal equilibrium. If changing body temperature acted as a minor stress, this combination of circumstances may have led to underestimation of the
QlW
ELECTROCARDIOGRAPHY
OF SPHENODON
889
On the basis of their work in Australian varanid lizards, Bartholomew & Tucker (1964) developed an equation for predicting resting heart rate at 30°C in lizards of various sizes. Using the mean weight of our specimens, a predicted rate of 35.5 beats/mm was calculated. Although we did not warm our specimens beyond 26”C, extrapolation of the regression line to 30°C yields a heart rate of 37 beats/min. Since Sphenodon apparently will not voluntarily allow its body temperature to reach 30°C even when adequate heat sources are available (Stebbins, 1958), it is unlikely that a true “resting” rate could be recorded at that temperature, the close agreement of the two predictions notwithstanding. Although our specimens did not appear to be easily disturbed by the presence of observers, or even by threatening actions (other than restraint), the gradual decline in heart rate after handling indicates that at least 1 hr must be allowed after attachment of electrodes before “resting” rates can be expected. A similar pattern of heart rate change after disturbance has been reported in crocodilians (Huggins et aZ., 1969). The absolute values of the ECG intervals in Sphenodon do not differ greatly from those of resting I. iguana at corresponding temperatures (Moberly, 1968), and the P-R intervals are almost identical. The Q1s’s for the reciprocals of P-R and P,T, are also similar in these two species. However, the effect of temperature on the duration of ventricular depolarization is much more pronounced in Sphenodon (QIO = 2.46 for l/R-T) than in I. t&ma (QIO = 1.5 for l/S-T). For two smaller lizards, Eumeces obsoletus (Dawson, 1960) and Crotuphytus collaris (Dawson & Templeton, 1963) the QiO’s for l/S-T were 2.1 and 2.2 respectively, and it is possible that the marked difference between Sphenodon and Iguana in this respect results from less striking deviations in opposite directions from a more typical intermediate value. Because the QlO’s for the reciprocals of ECG intervals were all greater than the Q10 for heart rate in Sphenodon, the mean active fraction of the heart’s electrical cycle decreased with rising temperature (see Table 1). Without additional information, the combined effects of changes in filling time and duration of ventricular systole (to the extent that it is influenced by R-T duration) on heart performance cannot be estimated. It should also be recalled that while mean relative P,-Tf is inversely related to temperature, the variability of this quantity increases with temperature and higher individual P,-T,/R-R’ values occur at 25°C than at lower temperatures (see Fig. 6). The great variability of relative R-T values is surprising and seems related in part at least to the unexpected lengthening of R-T with rising heart rate seen in some instances following disturbances. It is not unusual for the relative R-T to increase after disturbance, but an increase in absolute value of R-T under such circumstances has not been observed in a number of species of snakes (H. S. McDonald, unpublished observations), nor has it, to our knowledge, been reported in other reptiles. An increase in sympathetic activity would be expected to accompany excitement, and endogenous or exogenous catecholamines sometimes lengthen the ventricular action potential (Hoffman & Cranefield, 1960; Schaefer & Haas,
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IO
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P-R/P,-T,
AND RELATIVEECG
P-R/R-R’
TABLE I-ABSOLUTE
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2.60
(set)
R-T
0.30
0.35
040
0.46
R-T/R-R’
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INTERVALSOF Sphenodon AT SELECTED TEMPERATURES
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ELECTFtOCARDIOGRAPHY
891
OF SPHENODON
1962), but if this is the basis of the response in Sphenodon the sensitivity of its cardiac tissue to catecholamines must differ from that of other reptiles. The lengthening of R-T was noted only when increases in heart rate were moderate; it did not accompany the greatly elevated rates that occurred after handling. In Specimen L it was associated with a shift in form of the T wave from inverted to upright. ,.o-
0 Specimen
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L
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FIG. 6. Influence of body temperature on the active fraction of the cardiac electrical cycle in two tuataras. The solid line connecting the x ‘s represents the trend of mean relative P,-T, values derived from Table 1.
The general orientation of the ventricular depolarization vector, predominately from anterior to posterior with a small right to left component, agrees with that of squamate reptiles (Mullen, 1967; Valentinuzzi et aE., 1969). We did not attempt to evaluate the P or T wave vectors. Although an SV wave was not identifiable in our records, its occurrence in squamate ECG’s suggests that more extensive electrocardiographic study of Sphenodon will reveal it in that species also. Attempts to record ECG’s from our specimens using standard limb leads were quickly abandoned, because they generally yielded weak signals which were sometimes completely masked by noise from skeletal muscles. This result is not surprising since the Sphenodon heart is located within the region of the pectoral girdle (Simons, 1965) and, as previously mentioned, the mean electrical axis is almost coincident with the longitudinal axis of the animal, having only a small lateral component. With the accesssion wave moving almost at right angles to electrodes located on the forelimbs, a very small Lead I signal would be expected. Lead II or Lead III, having one electrode more nearly in line with the electrical axis would produce a stronger signal, but the forelimb electrode would still contribute little to the amplitude. An actual Lead II record yielded a 0.1 mV R wave. By locating one electrode anterior to the heart, and the other posterior to but close to the heart (Lead III*), a consistently stronger signal was obtained. Given the present lack of knowledge of reptilian electrocardiology, and the variability in 30
892
HARRY S. MCDONALDANDJAMESE. HEATH
heart position and thoracic structure among reptiles, we feel that electrode placements yielding strong signals are preferable to standard leads, as long as the former are identified well enough to permit replication. REFERENCES BARTHOLOMEW G. A. & TUCKER V. A. (1963) Control of changes in body temperature, metabolism, and circulation by the agamid lizard, Amphibolurus barbatus. Physiol. 2051. 36, 199-218. BARTHOLOMEW G. A. & TUCKERV. A. (1964) Size, body temperature, thermal conductance, oxygen consumption, and heart rate in Australian varanid lizards. Physiol. Zodl. 37, 341-354. BARTHOLOMEW G. A., TUCKER V. A. & LEE A. K. (1965) Oxygen consumption, thermal conductance, and heart rate in the Australian skink Tiliqua scincoides. Copeia 169-173. LAWSON W. R. (1960) Physiological responses to temperature in the lizard Eumeces obsoletus. Physiol. ZoBl. 33, 87-103. DAWSONW. R. & BARTHOLOMEW G. A. (1958) Metabolic and cardiac responses to temperature in the lizard Dipsosaurus dorsalis. Physiol. Zoiil.31, 100-l 11. DAWSONW. R. & TEMPLETONJ. R. (1963) Physiological responses to temperature in the lizard Crotaphytus collaris. Physiol. Zoiil. 36, 219-236. HOFFMANB. F. & CRANEFIELDP. F. (1960) Electrophysiology of the Heart. McGraw-Hill, New York. HUGGINSS. E., HOFF H. E. & PENA R. V. (1969) Heart and respiratory rates in crocodilian reptiles under conditions of minimal stimulation. Physiol. Zoiil. 42, 320-333. LICHT P. (1965) Effects of temperature on heart rates of lizards during rest and activity. Physiol. Zoiil. 38, 129-137. MOBERLYW. R. (1968) The metabolic responses of the common iguana, Iguana iguana, to activity under restraint. Camp. Biochem. Physiol. 27, l-20. MULLEN R. K. (1967) Comparative electrocardiography of the Squamata. Physiol. Zoiil. 40, 114-126. SCHAEFERH. & HAAS H. G. (1962) Electrocardiography. In Handbook of Physiology, Section 2, Circulation (Edited by HAMILTONW. F. & Dow P.), Vol. I, pp. 323-415. American Physiological Sot., Washington, D.C. SIMONSJ. R. (1965) The heart of the tuatara Sphenodon punctutus. J. 2001. 146, 451-466. STEBBINSR. C. (1958) An experimental study of the “third eye” of the tuatara. Copeiu 183190. VALENTINUZZI M. E., HOFF H. E. & GEDDESL. A. (1969) Electrocardiogram of the snake: effect of the location of the electrodes and cardiac vectors. r. Electrocardiol. 2, 245-252. WALLIS A. T., MEEK A. P. & Nc J, (1968) Clinical and experimental applications of a central recording system. N. 2. Med. J. 67, 356-361. WHITE F. N. (1959) Circulation in the reptilian heart (Squamata). Anut. Rec. 135, 129-134. WILSON K. J. & LEE A. K. (1970) Changes in oxygen consumption and heart rate with activity and body temperature in the tuatara, Sphenodon punctatum. Camp. Biochem. Physiol. 33, 311-322. Key Word Index-Electrocardiology; Iguana iguana; reptile heart; Tuatara; Sphenodon punctatus.
heart rate; temperature-heart;
Press. Printed in Great Britain
ELECTROCARDIOGRAPHIC OBSERVATIONS ON THE T~ATA~~ ~~~~~0~0~ ~U~C~A~U~ HARRY
S. McDONALDl
and JAMES
E. HEATH2
‘Department of Biology, Stephen F. Austin State University, Nacogdoches, Texas 75961; and aDepartment of Physiology and Biophysics, University of Illinois, Urbana, Illinois 61803 (received 28 April 1971) AbafrpLct-1. Electrocardiograms were obtained from two unrestrained tuataras at body temperatures from 7 to 26°C. 2. Heart rates were lower than previously reported for resting Sphenodon, but were similar to resting rates of large lizards. 3. ECG intervals (P-R, R-T, P,-T,) did not differ greatly from those of Iguana iguana, except that duration of ventricular depolarization was more sensitive to temperature in Sphenodon. 4. In general, the electrocardiology of Sphenodon closely resembles that of squamate reptiles. 5. An unusual feature was an occasional increase in the absolute duration of R-T when heart rate increased slightly in response to disturbance. INTRODUCTION
IN ITS general morphology, the heart of the tuatara, Sphenodrmpmctutus,
resembles that of lizards (White, 1959; Simons, 1965), and largely on the basis of the internal structure of the ventricle, White (1959) has suggested that this heart probably functions like a squamate heart as well. However, the recent paper of Wilson & Lee (1970) seems to be the only substantial contribution to the cardiovascular physiology of this interesting and unique animal. A single electrocardiographic record from a tuatara appeared in a report on recording methodology (Wallis et al., 1968). Since there is apparently no other published information on the electrocardiology of S#rf~~~don, the present observations, though limited in scope, may be of interest. MATERIALS
AND METHODS
Through the efforts of Dr. H. M. Smith and one of us (J. E, H.) two tuataras, a 470-g male (Specimen L) and a 197-g female (Specimen S), were obtained from the government of New Zealand. After arrival in Urbana they were maintained at 23-24°C for approximately 2 months before study. Water was provided ad lib., and they were periodically fed young rat pups. Both specimens appeared to be vigorous and in good health until they were killed in connection with other studies several months after the electrocardiographic observations were made. Electrocardiograms were recorded on a Beckman Dynograph with a Model 9853 input coupler. Though this D.C. coupler has a cut-off frequency of 180 Hz, to minimize movement artifacts and interference, the bandwidth was limited to 0.16-60 Hz for most records. 881
882
HARRY S. MCDONALD AND JAMES E. HEATH
Preliminary tests and periodic checks showed that reducing the bandwidth caused no significant changes in waveform or amplitude. Almost all records were made at a sensitivity of 0.2 mV/cm of pen deflection, and a chart speed of 2.5 cm/set. To verify the accuracy of measurements made on routine records, occasional samples were recorded at 10.0 cmjsec and 0.1 mV/cm. Several lengthy records were made at 0.1 cm/set to check for arrhythmias or to monitor heart rate when specimens were unattended. Self-adhering silver surface electrodes (Narco Biosystems, Inc.) were used in various lead configurations. For the routine display of all waveforms, one electrode on the left side of the neck close to the shoulder and one on the ventral midline at the posterior end of the rib cage proved satisfactory. In terms of electrode positions relative to the heart, this combination resembles most closely a Lead III arrangement, and for convenience it will be referred to as Lead III* in this report. Body temperature was measured with a Yellow Springs Telethermometer and a YSI small animal thermistor probe. The probe was inserted through the cloaca to a depth of 5 cm in the large intestine. In the majority of records at room temperature (23-25°C) specimens were in thermal equilibrium; however, other records were made while body temperature was changing. To vary body temperature, specimens were outfitted with ECG electrodes and thermistor probes, placed in large circular plastic containers, and transferred from one constant temperature room to another. The maximum rate of rise in body temperaThe maximum rate of cooling was 0.25 ture in either specimen was 0.14 degree/min. degree/min, and this rate was maintained for only a few minutes after transfer from room temperature to a 5°C environment. RESULTS
Heart rate Heart rates recorded at various body temperatures between 7 and 26°C are illustrated in Fig. 1. Except when specimens were moving spontaneously or disturbed by handling, rates fell between 6 and 31 beats/min. All rates above 35 beats/min were recorded within 10 min of electrode attachment. The restraint necessary to affix the leads was apparently a significant factor, because after the initial tachycardia subsided, intentional disturbance (even to the extent of poking the specimen with a pencil) failed to raise heart rate above 32 beats/min. The pattern of decline in heart rate after handling can be seen in Fig. 2, which is a composite graph of data from five different episodes. Usually only minor variations in R-R’ interval occurred within samples ; however, occasional arrhythmic periods were observed in both specimens. The most extreme case subject to accurate measurement was in a sample from Specimen I, at room temperature where two consecutive R-R’ intervals were 4.56 and 2.56 sec. In a segment recorded at O-1 cm/set on another occasion, an arrhythmia comprised of alternate long and short R-R’ intervals appeared to wax and wane, and at the peak of the arrhythmia the ratio of consecutive long and short intervals approximated 2 : 1. Neither in this case nor in any other was there evidence of dissociation due to an A-V block. Electrocardiographic
waoes
Examples of actual electrocardiograms recorded from Sphenodon are presented in Fig. 3. Under the recording conditions that we used, the ECG’s generally
El..ECTROCARDIOGRAPHY
0
Specimen S
l
Specimen L
883
OF SPHENODON
I
I
I
I
10
15
20
25
BODY
TEMPERATURE
(“C)
FIG. 1. Influence of body temperature on heart rate in two unrestrained tuataras. Over most of the temperature range heart rates were recorded while body temperature was slowly changing. The proportionally greater dispersion of rates in the vicinity of 25°C is due in part to (a) samples recorded while specimens were in thermal equilibrium and undisturbed for some time, and (b) samples reflecting excitement attending attachment of electrodes. 60 -
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@e Specimen Specimen
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FIG. 2 A composite graph of five episodes illustrating the pattern of decline in heart rate after animals were restrained for attachment of electrodes. consisted
only of
P, R and T waves.
No regular deflection that could be identified ECG’s,
as an SV wave was apparent in any of the records. As in other vertebrate the ventricular depolarization wave was the strongest and most definitive, is therefore
described
first.
and it
884
HARRY S. MCDONALDANDJAMESE. HEATH
With but one exception no distinct Q or S waves appeared in any of the records. In some samples the descending portion of the R wave overshot the baseline slightly. However, in these cases the return to the baseline was very gradual over a period many times longer than the R duration. Maximum R amplitude was 0.34 and 0.20 mV for Specimens L and S, respectively, and these values were obtained with Lead III *. Although sets of “standard” leads were not
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FIG. 3. Selected ECG records from Sphenodon. A, B, C. Records from Specimen L at three temperatures. Waves are designated in C because the T wave is shallow in that sample. D. Record from Specimen S at 25”C, illustrating lengthening of the P-R and shortening of the R-T. Note that the Pz-T, does not differ greatly from that in A. E, F, G, H, I. Samples recorded from Specimen L at 25°C illustrating lengthening of the R-T interval during disturbance despite increased heart rate. In E, specimen was resting quietly with a heart rate of 18.5 beats/min. F, G and H were recorded after progressively greater disturbance. Sample I was recorded after the specimen had been allowed to rest for several minutes. Note that the P-R shortened with tachycardia as would be expected. All samples were recorded with Lead III* arrangement (see text). The time and amplitude scale is applicable to all samples.
taken, transverse leads (from electrodes located on the right and left sides of the neck, or in right and left axillary regions) indicated a small right to left component in the mean axis of ventricular depolarization. The duration of the R wave was approximately 0.10 set at 23-25°C and 0.28 set at 7°C. The only record that exhibited anything more than a simple R wave in the ventricular depolarization complex was taken from the small specimen at room temperature. Electrodes were located on the left side of the neck and in the right axillary region. The initial record was biphasic with an S wave about half the amplitude of the R wave. The specimen was then induced to bend its body to the right, whereupon the R and S deflections became equal and the overall amplitude of the complex increased slightly to 0.08 mV.
El.ECTROCARDIOGRAPHY
OF SPHENODON
885
The P wave was low, monophasic and upright. We did not attempt to measure its duration because of the gently sloping leading and trailing edges and the low amplitude, but P wave duration was always clearly less than that of the accompanying R wave. The waveform did not always seem to be symmetrical, and in some records at low temperature the P seemed to be bimodal. As in other reptiles, the T wave was variable in amplitude and configuration. Most commonly, it was inverted and moderate in amplitude (30-60 per cent of R amplitude), but in both specimens it was occasionally upright, and in Specimen S there were a few instances when an upright T wave exceeded the R wave in amplitude by as much as 25 per cent. In several samples transitional biphasic T waves appeared ; these were always moderate in amplitude with the initial deflection opposing the R wave polarity. We did not attempt to measure T wave duration because frequently either the leading or trailing edge (or both) merged with the baseline very gradually. Electrocardiographic
intervals
Graphs illustrating the effect of temperature on atrioventricular conduction time (P-R interval), duration of ventricular depolarization (R-T interval) and total length of the active electrical cycle of the heart (P,T, interval) are presented in Fig. 4. Estimated mean absolute and relative values at selected temperatures are given in Table 1. Measurements of P-R and R-T intervals were made from peak to peak of the respective waves because of the aforementioned difficulty in identifying the beginning and end of some waveforms. The P,Tt interval, on the other hand, was measured from the beginning of the P wave to the end of the T wave, and in cases of doubt the longest conceivable interval was recorded to avoid underestimating the active duty cycle of the heart. A similar approach has previously been used in analyzing other reptilian ECG’s (see, for example, Dawson, 1960; Dawson & Templeton, 1963; Moberly, 1968). The P-R values recorded ranged from 0.32 to 1.98 sec. As can be seen in Fig. 4, at a given temperature level the P-R was proportionally more variable than either R-T or P,T,. A straight line fitted by eye to mean values for one-degree intervals yielded a Q10 of 2.14 for the reciprocal of P-R over the entire temperature range. Since the semilogarithmic plots did not seem to indicate true linear relationships between the logarithms of the intervals and temperature, a line was also fitted to the most linear portion of the graph, between 7 and lS”C, and a Q10 of 2.40 was obtained for this narrower range. The relative P-R interval, P-R : R-R, ranged from 0.15 to O-38, and tended to be greater for Specimen S than Specimen L. It never exceeded 0.30 for Specimen L under any condition, nor for either specimen at heart rates of 20 beats/min or less (regardless of temperature). The overall range of R-T was 0.50-3.64 sec. Though less variable (on a proportional basis) than P-R at any given temperature level, it was more strongly influenced by temperature than either P-R or P,T,. Estimates of Q1a’s for the reciprocal of R-T were 2.46 for the entire temperature range, and 2.80 for the region between 7 and 18°C. The relative R-T value, R-T : R-R’, ranged from
886
HARRY S. MCDONALD AND JAMES E. HEATH
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FIG. 4. Influence of body temperature on electrocardiographic intervals in two tuataras. Each scatter-graph includes values from both specimens. The P-R and R-T intervals were measured from peak to peak of the indicated waves, and are intended as estimates of atrio-ventricular conduction time and duration of ventricular depolarization, respectively. The Pi-T, interval was measured from the beginning of the P wave to the end of the T wave, and represents the active portion of the electrical cycle of the heart.
0.21 to 0.58, and both of these extreme values were from samples at room temperature. A plot of R-T vs. R-R’ at room temperature (Fig. 5) reveals that this relationship is quite variable. The duration of the active phase of the cardiac electrical cycle, the Pi--T, interval, varied from 1.0 to 5.8 sec. There was a trend towards greater variability in relative P,-T, at higher temperatures; at 7-8°C it ranged only from 0.64 to 0.80, while at 25-26°C minimum and maximum values were 0.42 and 0.95. Estimated Q1,,‘s for the reciprocal of Pi-T1 were 2.16 and 2.37 for temperature ranges of 7-26°C and 7-18°C respectively. At all temperatures Specimen S tended to have higher relative P,--T, values than Specimen L. This is due to the fact that temperature influenced interval durations in the two specimens about equally,
ELE~R~~I~~P~
OF
887
SPHENODON
while the smaller specimen exhibited higher heart rates throughout the temperature range used. A few specific observations relating to ECG intervals bear comment. The first concerns an elongated P-R interval that appeared on two separate occasions in Specimen S (see Fig. 3). In both cases the specimen was at room temperature and the P-R exceeded 0.70 set while the accompanying R-T values were relatively low (less than 0.60 set). One sample in which this phenomenon was noted was preceded by a lengthy record at 0.1 cm/set, and although precise measurements could not be made from the low-speed record, it was clear that the relative duration of the P-R and R-T intervals had been maintained for over 90 min prior to .
/I’
. I
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I
1
,/’
2. R-R’
1NTERVAL
, 3
1
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4
(seconds)
FIG. 5. Duration of R-T plotted vs. the beat-to-beat interval in two tuataras at room temperature (23-25”(J), showing the great variability of this relationship. The dashed lines represent the boundaries of the area within which R-T accounted for 20 to 60 per cent of the beat-to-beat interval.
recording the 25 cm/set sample. It is also interesting, though perhaps not significant, that the changes in P-R and R-T intervals so balanced one another that the Pa-T, was near the mean value for that interval at room temperature. The long P-R intervals were quite regular, and there was no indication of abnormality in the ECG or distress in the specimen. The other observation involved both specimens. In several episodes disturbance led to increased R-T intervals while the heart rate was also increasing. In two cases the elongation of R-T could be traced through several consecutive samples as the specimen was intentionally disturbed. In all cases the accompanying P-R intervals decreased with rising heart rate, as would be anticipated. This phenomenon is illustrated in Fig. 3.
888
HARRY S. MCDONALD AND JAMES E. HEATH
DISCUSSION
In a laboratory study utilizing a thermal gradient chamber, Stebbins (1958) found that Sphenodon would voluntarily tolerate body temperatures from 3.5 to 27*9”C. The mean body temperature, based on over 200 measurements from each of two specimens, was 18*3”C, and fewer than 3 per cent of all measurements were outside of the 7 to 26°C range used in our own study. Thus, at least as far as temperature is concerned, our data probably reflect the electrocardiology of Sphenodon under normal resting conditions. To qualify the temperature sensitivity of heart rate in our specimens, the relationship between log heart rate and temperature was evaluated. Though some of our samples were recorded while specimens were known to be spontaneously moving or disturbed, we did not feel that adequate data were on hand to justify separation of “resting” and “active” rates. The least-squares regression equation based on all samples is log H. R. = 0.760 + 0.027 Tb. The regression coefficient corresponds to a Qr,, of 1.86. Between 7 and 26”C, our mean heart rate values fall below those reported by Wilson & Lee (1970) for “resting” conditions, although the extrapolated regression lines would intersect at 2.6”C. The difference is striking at higher temperatures because the Qn, for their heart rate curve is 2.34, and at 25°C even the ranges of rates recorded in the two studies barely overlap. In view of the marked effect of restraint on heart rate in our specimens and the fact that the tuataras studied by Wilson & Lee were restrained for all recording, we are inclined to attribute the difference to that factor. A parallel situation in a lizard was reported by Licht (1965) who noted that heart rates in untethered Dipsosaurus dorsalis were lower than “resting” rates for restrained specimens of the same species (Dawson & Bartholomew, 1958). Although our heart rate results for Sphenodon differ strikingly from those of Wilson & Lee, they resemble the results obtained on several species of large lizards: Tiliqua scincoides (Bartholomew et al., 1965), T. rugosa (Licht, 1965), Amphibolurus barbatus (Bartholomew & Tucker, 1963), Varanus spp. (Bartholomew & Tucker, 1964) and Iguana iguana (Moberly, 1968). Within the common temperature range our data are closest to the resting rates for I. iguana and T. scincoides. Our regression coefficient corresponds to a lower Qr,,, however, and the regression line intersects that for I. iguana between 10 and 15°C and that for T. scincoides between 20 and 25°C. In those reports wherein data for both resting and active conditions were presented (Bartholomew & Tucker, 1963, 1964; Bartholomew et al., 1965; Moberly, 1968) the jQrs’s for resting heart rates were above 2.0, while those for active rates were below 2.0. The unusual combination of low absolute values and low &,, that we obtained may be a sampling artifact in that most of the rates between 15 and 23°C were recorded during change in body temperature, while many of the values between 23 and 25”C, on the other hand, were from specimens in thermal equilibrium. If changing body temperature acted as a minor stress, this combination of circumstances may have led to underestimation of the
QlW
ELECTROCARDIOGRAPHY
OF SPHENODON
889
On the basis of their work in Australian varanid lizards, Bartholomew & Tucker (1964) developed an equation for predicting resting heart rate at 30°C in lizards of various sizes. Using the mean weight of our specimens, a predicted rate of 35.5 beats/mm was calculated. Although we did not warm our specimens beyond 26”C, extrapolation of the regression line to 30°C yields a heart rate of 37 beats/min. Since Sphenodon apparently will not voluntarily allow its body temperature to reach 30°C even when adequate heat sources are available (Stebbins, 1958), it is unlikely that a true “resting” rate could be recorded at that temperature, the close agreement of the two predictions notwithstanding. Although our specimens did not appear to be easily disturbed by the presence of observers, or even by threatening actions (other than restraint), the gradual decline in heart rate after handling indicates that at least 1 hr must be allowed after attachment of electrodes before “resting” rates can be expected. A similar pattern of heart rate change after disturbance has been reported in crocodilians (Huggins et aZ., 1969). The absolute values of the ECG intervals in Sphenodon do not differ greatly from those of resting I. iguana at corresponding temperatures (Moberly, 1968), and the P-R intervals are almost identical. The Q1s’s for the reciprocals of P-R and P,T, are also similar in these two species. However, the effect of temperature on the duration of ventricular depolarization is much more pronounced in Sphenodon (QIO = 2.46 for l/R-T) than in I. t&ma (QIO = 1.5 for l/S-T). For two smaller lizards, Eumeces obsoletus (Dawson, 1960) and Crotuphytus collaris (Dawson & Templeton, 1963) the QiO’s for l/S-T were 2.1 and 2.2 respectively, and it is possible that the marked difference between Sphenodon and Iguana in this respect results from less striking deviations in opposite directions from a more typical intermediate value. Because the QlO’s for the reciprocals of ECG intervals were all greater than the Q10 for heart rate in Sphenodon, the mean active fraction of the heart’s electrical cycle decreased with rising temperature (see Table 1). Without additional information, the combined effects of changes in filling time and duration of ventricular systole (to the extent that it is influenced by R-T duration) on heart performance cannot be estimated. It should also be recalled that while mean relative P,-Tf is inversely related to temperature, the variability of this quantity increases with temperature and higher individual P,-T,/R-R’ values occur at 25°C than at lower temperatures (see Fig. 6). The great variability of relative R-T values is surprising and seems related in part at least to the unexpected lengthening of R-T with rising heart rate seen in some instances following disturbances. It is not unusual for the relative R-T to increase after disturbance, but an increase in absolute value of R-T under such circumstances has not been observed in a number of species of snakes (H. S. McDonald, unpublished observations), nor has it, to our knowledge, been reported in other reptiles. An increase in sympathetic activity would be expected to accompany excitement, and endogenous or exogenous catecholamines sometimes lengthen the ventricular action potential (Hoffman & Cranefield, 1960; Schaefer & Haas,
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IO
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AND RELATIVEECG
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TABLE I-ABSOLUTE
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(set)
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040
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INTERVALSOF Sphenodon AT SELECTED TEMPERATURES
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P,-T,/R-R’
iz
2
3
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ELECTFtOCARDIOGRAPHY
891
OF SPHENODON
1962), but if this is the basis of the response in Sphenodon the sensitivity of its cardiac tissue to catecholamines must differ from that of other reptiles. The lengthening of R-T was noted only when increases in heart rate were moderate; it did not accompany the greatly elevated rates that occurred after handling. In Specimen L it was associated with a shift in form of the T wave from inverted to upright. ,.o-
0 Specimen
S
Specimen
L
l
.
.
b $~~“~~~~o~
. .
. 0.
l
.
*.’
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I 10 BODY
15 TEMPERATURE
I 20
I 25
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FIG. 6. Influence of body temperature on the active fraction of the cardiac electrical cycle in two tuataras. The solid line connecting the x ‘s represents the trend of mean relative P,-T, values derived from Table 1.
The general orientation of the ventricular depolarization vector, predominately from anterior to posterior with a small right to left component, agrees with that of squamate reptiles (Mullen, 1967; Valentinuzzi et aE., 1969). We did not attempt to evaluate the P or T wave vectors. Although an SV wave was not identifiable in our records, its occurrence in squamate ECG’s suggests that more extensive electrocardiographic study of Sphenodon will reveal it in that species also. Attempts to record ECG’s from our specimens using standard limb leads were quickly abandoned, because they generally yielded weak signals which were sometimes completely masked by noise from skeletal muscles. This result is not surprising since the Sphenodon heart is located within the region of the pectoral girdle (Simons, 1965) and, as previously mentioned, the mean electrical axis is almost coincident with the longitudinal axis of the animal, having only a small lateral component. With the accesssion wave moving almost at right angles to electrodes located on the forelimbs, a very small Lead I signal would be expected. Lead II or Lead III, having one electrode more nearly in line with the electrical axis would produce a stronger signal, but the forelimb electrode would still contribute little to the amplitude. An actual Lead II record yielded a 0.1 mV R wave. By locating one electrode anterior to the heart, and the other posterior to but close to the heart (Lead III*), a consistently stronger signal was obtained. Given the present lack of knowledge of reptilian electrocardiology, and the variability in 30
892
HARRY S. MCDONALDANDJAMESE. HEATH
heart position and thoracic structure among reptiles, we feel that electrode placements yielding strong signals are preferable to standard leads, as long as the former are identified well enough to permit replication. REFERENCES BARTHOLOMEW G. A. & TUCKER V. A. (1963) Control of changes in body temperature, metabolism, and circulation by the agamid lizard, Amphibolurus barbatus. Physiol. 2051. 36, 199-218. BARTHOLOMEW G. A. & TUCKERV. A. (1964) Size, body temperature, thermal conductance, oxygen consumption, and heart rate in Australian varanid lizards. Physiol. Zodl. 37, 341-354. BARTHOLOMEW G. A., TUCKER V. A. & LEE A. K. (1965) Oxygen consumption, thermal conductance, and heart rate in the Australian skink Tiliqua scincoides. Copeia 169-173. LAWSON W. R. (1960) Physiological responses to temperature in the lizard Eumeces obsoletus. Physiol. ZoBl. 33, 87-103. DAWSONW. R. & BARTHOLOMEW G. A. (1958) Metabolic and cardiac responses to temperature in the lizard Dipsosaurus dorsalis. Physiol. Zoiil.31, 100-l 11. DAWSONW. R. & TEMPLETONJ. R. (1963) Physiological responses to temperature in the lizard Crotaphytus collaris. Physiol. Zoiil. 36, 219-236. HOFFMANB. F. & CRANEFIELDP. F. (1960) Electrophysiology of the Heart. McGraw-Hill, New York. HUGGINSS. E., HOFF H. E. & PENA R. V. (1969) Heart and respiratory rates in crocodilian reptiles under conditions of minimal stimulation. Physiol. Zoiil. 42, 320-333. LICHT P. (1965) Effects of temperature on heart rates of lizards during rest and activity. Physiol. Zoiil. 38, 129-137. MOBERLYW. R. (1968) The metabolic responses of the common iguana, Iguana iguana, to activity under restraint. Camp. Biochem. Physiol. 27, l-20. MULLEN R. K. (1967) Comparative electrocardiography of the Squamata. Physiol. Zoiil. 40, 114-126. SCHAEFERH. & HAAS H. G. (1962) Electrocardiography. In Handbook of Physiology, Section 2, Circulation (Edited by HAMILTONW. F. & Dow P.), Vol. I, pp. 323-415. American Physiological Sot., Washington, D.C. SIMONSJ. R. (1965) The heart of the tuatara Sphenodon punctutus. J. 2001. 146, 451-466. STEBBINSR. C. (1958) An experimental study of the “third eye” of the tuatara. Copeiu 183190. VALENTINUZZI M. E., HOFF H. E. & GEDDESL. A. (1969) Electrocardiogram of the snake: effect of the location of the electrodes and cardiac vectors. r. Electrocardiol. 2, 245-252. WALLIS A. T., MEEK A. P. & Nc J, (1968) Clinical and experimental applications of a central recording system. N. 2. Med. J. 67, 356-361. WHITE F. N. (1959) Circulation in the reptilian heart (Squamata). Anut. Rec. 135, 129-134. WILSON K. J. & LEE A. K. (1970) Changes in oxygen consumption and heart rate with activity and body temperature in the tuatara, Sphenodon punctatum. Camp. Biochem. Physiol. 33, 311-322. Key Word Index-Electrocardiology; Iguana iguana; reptile heart; Tuatara; Sphenodon punctatus.
heart rate; temperature-heart;