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The electrocardiogram of the normal dog
The electrocardiogram of the normal dog Sheldon O. Burman, M.D.,* Photis Panagopoulos, M.D., and Steven Kahn, B.A., New York, N.Y.
-In most experimental surgical procedures upon the heart of laboratory animals, the electrocardiogram is routinely recorded and studied as one of the parameters of cardiac function. However, in studying the presumably normal tracings of a group of 20 control animals recently, we noted such wide disparity of the various components that we were uncertain as to which, if any, of these tracings was indeed normal, and whether, in fact, sufficient standardization exists in the dog's electrocardiogram to validate a definition of normalcy. It seemed essential to us, even when the dog was to serve as his own control, to recognize the range of normal variations so as to avoid introducing an unknown variable into the experiment by including animals whose abnormal electrocardiograms might be a reflection of preexisting disease or deficiency states. A review of the literature failed to clarify the definition of the normal canine electrocardiogram. Most reports make scant mention of the conditions under which these "normal" recordings were made, such as the animal's general state of health, the type and depth of anesthesia, the state of blood oxygen saturation, the dog's position on the table, and the exact placement of electrodes From the Department of Surgery, the New York Medical College, New York, N. Y. Supported by Grant No. U-1493 from the Health Research Council of New York. Received for publication July 30, 1965. *Career Investigator, Health Research Council of New York.
—all factors known to affect the character of the electrocardiogram recordings. In those rare studies in which these important variables were recognized and controlled, the number of animals measured was too few to yield a reliable spectrum of normal values. Since our own experimental problems, and those of other workers, so frequently involve the dog and its electrocardiogram, we have undertaken to define and delineate the normal electrocardiogram in a group of dogs domiciled and studied under relatively stable, standardized conditions. Considering the variability of the data derived, it was important that the size of the group to be sampled be sufficiently ample to ensure statistical significance. In this study we used anesthetized animals since most laboratory procedures in which such information would be useful are done with the dog under surgical anesthesia. Procedure One hundred and forty-five adult mongrel dogs of both sexes, ranging in weight from 14 to 29 kilograms, were originally selected for this study. Upon arrival in our kennels, all animals were vaccinated for distemper, sprayed for vermin, weighed, and quarantined for 3 weeks. Those animals appearing clinically ill, listless, or markedly undernourished within this time were excluded from the series. Following completion of the studies to be described, all animals were autopsied and those with heart worms, lung 379
380
Journal of
Burmcm, Panagopoulos, Kahn
disease, or other significant abnormality were also eliminated, leaving a total of 107 dogs whose electrocardiograms form the basis of this study. Anesthesia consisting of intravenous sodium pentobarbital 30 mg. per kilogram of body weight, was administered over a 3 minute interval. A second group of 12 animals was anesthetized by means of intravenous sodium Pentothal, 2.5 per cent, given over a period of 3 minutes. In every case a plane of anesthesia was achieved in which the animal failed to respond when a towel clip was placed through the skin. The animals were allowed to rest for 10 minutes, breathing ambient air, while their condition stabilized before tracings were recorded. The dog was positioned on the right side and subcutaneous needle electrodes were implanted in each extremity, 1 inch below the knee and elbow. Lead V2 was placed at the midsternal line. Samples of arterial blood were withdrawn and analyzed for pH, pC02, pOL>, sodium, potassium, and calcium in the first 31 animals. The paper speed in the Sanborn electrocardiograph was 50 mm. per second and the calibration was 1 mv. equals 10 mm. Leads I, II, III, aV lt , aV F , aV L , and V-2 were recorded for all dogs and the tracings analyzed for the presence and height of deflections of the P, Q, R, S, and T waves, and the P-R, QRS, and Q-T intervals were determined. The maximum, minimum, and mean values were calculated, together with their standard deviation and per cent incidence. Standard deviation was Ed 2 computed by means of the formula % = , n where Ed- equals the sum of each deviation squared and n equals the total number of the sample. The per cent incidence indicates the relative occurrence of P, Q, R, S, and T waves in the leads studied. This applies particularly to those cases in which the incidence of a given component is less than 100 per cent. Results Data derived from the electrocardiograms of 107 dogs are summarized in Tables I,
Thoracic and Cardiovascular Surgery
I, and III. Tables I and III taken together are actually a composite statistical representation of the normal dog's electrocardiogram. Table II demonstrates that in Leads II, III, and aVi;. the T wave usually has a negative deflection, whereas in aV It , aV L , and, especially in V2, it is usually positive. In Lead II, the T wave was diphasic in 32 per cent of the dogs and in Leads I and II the P wave was diphasic in 13 per cent. Deviations of the S-T segment of less than 1 mm. were found in 66 (62 per cent) of the tracings and S-T deviations greater than 1 mm. in 37 (35 per cent) tracings of the limb leads. The arterial pH, p0 2 , pCCK, and the values of blood sodium, potassium, and calcium were within normal limits in every dog in which these studies were done. Discussion It is of interest that, out of 145 dogs originally selected for this study, 37 of these, or more than one quarter, were unsuitable and were eliminated because of clinical or postmortem evidence of serious disease or deficiency states. The wisdom of a preliminary observation period before undertaking any experimental procedure is thus emphasized. It should be noted that in no animal in this series was an endotracheal tube inserted before the electrocardiogram was recorded, nor was an animal given any gas to breathe except room air. In several dogs, following the conclusion of these studies, an endotracheal tube was inserted and an electrocardiogram recording was again made. Such marked variations in rate, rhythm, and the characteristics of the various components of the tracings were encountered that in most cases the tracings following intubation bore little resemblance to those recorded beforehand. The profound effects of endotracheal intubation and manipulation upon electrocardiograms in humans has been pointed out previously.1 In the great majority of cases, our own animals, and those of other investigators, are anesthetized for surgical procedures with intravenous pentobarbital so this agent was selected for this study. However, in
Volume 51
Electrocardiogram of normal dog
Number 3 March, 1966
381
Table I. Vectoral data /
II
III
aVK
aV,.
aVF
V,
Mean S. D. Maximum Minimum % Incidence
+0.44 0.10 +2.5 +0.1 100%
+3.0 0.10 +5.0 + 1.0 100%
+2.5 0.09 +4.5 +0.8 100%
-1.6 0.09 + 1.5 -3.00 100%
-1.01 0.15 + 1.00 -3.00 100%
+2.91 0.12 +5.00 +0.50 100%
+ 1.80 0.21 + 4.00 + 0.10 100%
Mean S. D. Maximum Minimum % Incidence
0.27 0.10 0 2.2 52%
1.2 0.15 0 6.0 62%
1.3 0.10 0 8.0 58%
12.6 0.46 0 20.0 74%
8.5 0.48 0 15.0 68%
1.7 0.09 0 6.0 72%
0.02 0.05 0 2.0 26%
Mean S. D. Maximum Minimum % Incidence
2.2 0.50 32.0 0.5 100%
16.5 0.44 30.0 5.0 100%
14.8 0.43 28.0 1.60 100%
1.10 0.30 3.2 0.0 100%
2.10 0.30 4.5 0.3 100%
16.2 0.34 33.0 1.0 100%
29.5 0.55 47.0 5.0 100%
Mean S. D. Maximum Minimum % Incidence
0.08 0.09 0.00 3.0 36%
1.4 0.22 0.0 7.0 92%
1.30 0.15 0.0 7.0 100%
10.2 0.33 0.0 20.5 79%
0.75 0.50 0.0 22.0 71%
1.95 0.20 0.0 9.0 66%
12.0 0.31 0.0 21.0 98%
Mean S. D. Maximum Minimum % Incidence
0.10 0.20 +2.5 -4.0 76%
-1.10 0.11 +4.0 -5.0 98%
-1.00 0.18 +4.0 -5.0 100%
+ 1.10 0.23 +3.5 -2.2 100%
+ 1.12 0.23 +0.2 -3.0 100%
-0.95 0.17 +4.5 -6.0 98%
+ 4.6 0.10 + 15.0 -6.0 100%
Table II. Percentage of positive T waves Lead 11—40% Lead III—44% Lead aVn—56% Lead a V L — 5 8 % Lead aV F —42% Lead V 2 —76%
Table III Lead* /
II
HI
0.096 ± 0.024 0.037 + 0.021 0.167 + 0.054
0.098 ± 0.024 0.045 + 0.024 0.176 + 0.054
0.099 ± 0.024 0.044 + 0.024 0.177 ±0.054
Interval* PR QRS Q-T
•All intervals are in seconds.
common with others, we have achieved an increased survival rate, particularly in our open-heart procedures in the laboratory, when anesthesia is induced with sodium Pentothal and then maintained throughout the procedure with a mixture of 80 per cent nitrous oxide—20 per cent oxygen. Therefore, for purposes of comparison, as noted above, a second group of 12 dogs was given intravenous sodium Pentothal instead of pentobarbital, and their electrocardiograms were recorded. So far as the heart's electrical activity is concerned, no significant difference was noted between the 2 anesthetic agents, providing the depth of anesthesia achieved in the 2 groups was comparable. Some of these dogs were used later for other experiments and it is of interest to note parenthetically that when an animal's chest was opened, without any other manip-
Journal of
382
Burman, Panagopoulos, Kahn
ulation, the T wave would often suddenly reverse direction. Also, the presence of diphasic T waves in Lead V2 of 32 per cent of dogs and diphasic P waves in Leads II and III of 13 per cent indicate that these findings are not necessarily abnormal, a point not previously noted in the literature. Conclusions 1. The electrocardiograms of 107 normal dogs domiciled and studied under stable, standardized conditions were analyzed for the minimum, maximum, and mean values of each component as well as the standard deviations and per cent incidence. 2. The electrocardiogram of the normal anesthetized dog shows far greater variation than that of man. Within this broader range there is, however, sufficient standardization to validate a concept of normalcy. 3. In all future experiments in which the canine electrocardiogram plays a significant role it would seem safer to exclude as normal at the outset any animal whose tracing contains components outside the minimummaximum limits established herein.
Thoracic and Cardiovascular Surgery
The authors are grateful to Dr. Donald J. Weber of the Cardiopulmonary Laboratory for his assistance and advice in the recording and interpretation of the data. REFERENCES 1 Burman, S. O., and Gibson, T. C : Bronchoscopy and the Cardio-Respiratory Reflexes, Ann. Surg. 157: 134, 1963. 2 Fabre, H., Fabre, R., and Linquette, Y.: La variability d'electrocardiogramme du chien, Arch. mal. coeur, 7: 673, 1955. 3 Gyarnati, E.: Electrocardiographs Examination of Dogs. (Thesis) Budapest, 1939. 4 Horwitz, S. A., Spanier, M. R., and Wiggers, H. C : The ECG in Normal Dogs, Proc. Soc. Exper. Biol. & Med. 84: 121, 1953. 5 Lannek, N. A.: Clinical and Experimental Study on the ECG in Dogs. Proceedings of Medical Clinic of the Royal Veterinary College of Stockholm, 1949. 6 Lombard, E. A., and Wilham, C : Electrocardiogram of the Anesthetized Dog, Am. J. Physiol. 181: 567, 1955. 7 Petersen, E. S., Ticketts, H. T., Brewer, N. R., Lints, H. A., Test, C. E., and Tupikova, N. A.: Electrocardiogram of the Beagle Dog, Proc. Soc. Exper. Biol. & Med. 77: 330, 1951. 8 Smith, F. M.: The Ligation of Coronary Arteries With Electrocardiographic Study, Arch. Int. Med. 22: 8, 1918.
-In most experimental surgical procedures upon the heart of laboratory animals, the electrocardiogram is routinely recorded and studied as one of the parameters of cardiac function. However, in studying the presumably normal tracings of a group of 20 control animals recently, we noted such wide disparity of the various components that we were uncertain as to which, if any, of these tracings was indeed normal, and whether, in fact, sufficient standardization exists in the dog's electrocardiogram to validate a definition of normalcy. It seemed essential to us, even when the dog was to serve as his own control, to recognize the range of normal variations so as to avoid introducing an unknown variable into the experiment by including animals whose abnormal electrocardiograms might be a reflection of preexisting disease or deficiency states. A review of the literature failed to clarify the definition of the normal canine electrocardiogram. Most reports make scant mention of the conditions under which these "normal" recordings were made, such as the animal's general state of health, the type and depth of anesthesia, the state of blood oxygen saturation, the dog's position on the table, and the exact placement of electrodes From the Department of Surgery, the New York Medical College, New York, N. Y. Supported by Grant No. U-1493 from the Health Research Council of New York. Received for publication July 30, 1965. *Career Investigator, Health Research Council of New York.
—all factors known to affect the character of the electrocardiogram recordings. In those rare studies in which these important variables were recognized and controlled, the number of animals measured was too few to yield a reliable spectrum of normal values. Since our own experimental problems, and those of other workers, so frequently involve the dog and its electrocardiogram, we have undertaken to define and delineate the normal electrocardiogram in a group of dogs domiciled and studied under relatively stable, standardized conditions. Considering the variability of the data derived, it was important that the size of the group to be sampled be sufficiently ample to ensure statistical significance. In this study we used anesthetized animals since most laboratory procedures in which such information would be useful are done with the dog under surgical anesthesia. Procedure One hundred and forty-five adult mongrel dogs of both sexes, ranging in weight from 14 to 29 kilograms, were originally selected for this study. Upon arrival in our kennels, all animals were vaccinated for distemper, sprayed for vermin, weighed, and quarantined for 3 weeks. Those animals appearing clinically ill, listless, or markedly undernourished within this time were excluded from the series. Following completion of the studies to be described, all animals were autopsied and those with heart worms, lung 379
380
Journal of
Burmcm, Panagopoulos, Kahn
disease, or other significant abnormality were also eliminated, leaving a total of 107 dogs whose electrocardiograms form the basis of this study. Anesthesia consisting of intravenous sodium pentobarbital 30 mg. per kilogram of body weight, was administered over a 3 minute interval. A second group of 12 animals was anesthetized by means of intravenous sodium Pentothal, 2.5 per cent, given over a period of 3 minutes. In every case a plane of anesthesia was achieved in which the animal failed to respond when a towel clip was placed through the skin. The animals were allowed to rest for 10 minutes, breathing ambient air, while their condition stabilized before tracings were recorded. The dog was positioned on the right side and subcutaneous needle electrodes were implanted in each extremity, 1 inch below the knee and elbow. Lead V2 was placed at the midsternal line. Samples of arterial blood were withdrawn and analyzed for pH, pC02, pOL>, sodium, potassium, and calcium in the first 31 animals. The paper speed in the Sanborn electrocardiograph was 50 mm. per second and the calibration was 1 mv. equals 10 mm. Leads I, II, III, aV lt , aV F , aV L , and V-2 were recorded for all dogs and the tracings analyzed for the presence and height of deflections of the P, Q, R, S, and T waves, and the P-R, QRS, and Q-T intervals were determined. The maximum, minimum, and mean values were calculated, together with their standard deviation and per cent incidence. Standard deviation was Ed 2 computed by means of the formula % = , n where Ed- equals the sum of each deviation squared and n equals the total number of the sample. The per cent incidence indicates the relative occurrence of P, Q, R, S, and T waves in the leads studied. This applies particularly to those cases in which the incidence of a given component is less than 100 per cent. Results Data derived from the electrocardiograms of 107 dogs are summarized in Tables I,
Thoracic and Cardiovascular Surgery
I, and III. Tables I and III taken together are actually a composite statistical representation of the normal dog's electrocardiogram. Table II demonstrates that in Leads II, III, and aVi;. the T wave usually has a negative deflection, whereas in aV It , aV L , and, especially in V2, it is usually positive. In Lead II, the T wave was diphasic in 32 per cent of the dogs and in Leads I and II the P wave was diphasic in 13 per cent. Deviations of the S-T segment of less than 1 mm. were found in 66 (62 per cent) of the tracings and S-T deviations greater than 1 mm. in 37 (35 per cent) tracings of the limb leads. The arterial pH, p0 2 , pCCK, and the values of blood sodium, potassium, and calcium were within normal limits in every dog in which these studies were done. Discussion It is of interest that, out of 145 dogs originally selected for this study, 37 of these, or more than one quarter, were unsuitable and were eliminated because of clinical or postmortem evidence of serious disease or deficiency states. The wisdom of a preliminary observation period before undertaking any experimental procedure is thus emphasized. It should be noted that in no animal in this series was an endotracheal tube inserted before the electrocardiogram was recorded, nor was an animal given any gas to breathe except room air. In several dogs, following the conclusion of these studies, an endotracheal tube was inserted and an electrocardiogram recording was again made. Such marked variations in rate, rhythm, and the characteristics of the various components of the tracings were encountered that in most cases the tracings following intubation bore little resemblance to those recorded beforehand. The profound effects of endotracheal intubation and manipulation upon electrocardiograms in humans has been pointed out previously.1 In the great majority of cases, our own animals, and those of other investigators, are anesthetized for surgical procedures with intravenous pentobarbital so this agent was selected for this study. However, in
Volume 51
Electrocardiogram of normal dog
Number 3 March, 1966
381
Table I. Vectoral data /
II
III
aVK
aV,.
aVF
V,
Mean S. D. Maximum Minimum % Incidence
+0.44 0.10 +2.5 +0.1 100%
+3.0 0.10 +5.0 + 1.0 100%
+2.5 0.09 +4.5 +0.8 100%
-1.6 0.09 + 1.5 -3.00 100%
-1.01 0.15 + 1.00 -3.00 100%
+2.91 0.12 +5.00 +0.50 100%
+ 1.80 0.21 + 4.00 + 0.10 100%
Mean S. D. Maximum Minimum % Incidence
0.27 0.10 0 2.2 52%
1.2 0.15 0 6.0 62%
1.3 0.10 0 8.0 58%
12.6 0.46 0 20.0 74%
8.5 0.48 0 15.0 68%
1.7 0.09 0 6.0 72%
0.02 0.05 0 2.0 26%
Mean S. D. Maximum Minimum % Incidence
2.2 0.50 32.0 0.5 100%
16.5 0.44 30.0 5.0 100%
14.8 0.43 28.0 1.60 100%
1.10 0.30 3.2 0.0 100%
2.10 0.30 4.5 0.3 100%
16.2 0.34 33.0 1.0 100%
29.5 0.55 47.0 5.0 100%
Mean S. D. Maximum Minimum % Incidence
0.08 0.09 0.00 3.0 36%
1.4 0.22 0.0 7.0 92%
1.30 0.15 0.0 7.0 100%
10.2 0.33 0.0 20.5 79%
0.75 0.50 0.0 22.0 71%
1.95 0.20 0.0 9.0 66%
12.0 0.31 0.0 21.0 98%
Mean S. D. Maximum Minimum % Incidence
0.10 0.20 +2.5 -4.0 76%
-1.10 0.11 +4.0 -5.0 98%
-1.00 0.18 +4.0 -5.0 100%
+ 1.10 0.23 +3.5 -2.2 100%
+ 1.12 0.23 +0.2 -3.0 100%
-0.95 0.17 +4.5 -6.0 98%
+ 4.6 0.10 + 15.0 -6.0 100%
Table II. Percentage of positive T waves Lead 11—40% Lead III—44% Lead aVn—56% Lead a V L — 5 8 % Lead aV F —42% Lead V 2 —76%
Table III Lead* /
II
HI
0.096 ± 0.024 0.037 + 0.021 0.167 + 0.054
0.098 ± 0.024 0.045 + 0.024 0.176 + 0.054
0.099 ± 0.024 0.044 + 0.024 0.177 ±0.054
Interval* PR QRS Q-T
•All intervals are in seconds.
common with others, we have achieved an increased survival rate, particularly in our open-heart procedures in the laboratory, when anesthesia is induced with sodium Pentothal and then maintained throughout the procedure with a mixture of 80 per cent nitrous oxide—20 per cent oxygen. Therefore, for purposes of comparison, as noted above, a second group of 12 dogs was given intravenous sodium Pentothal instead of pentobarbital, and their electrocardiograms were recorded. So far as the heart's electrical activity is concerned, no significant difference was noted between the 2 anesthetic agents, providing the depth of anesthesia achieved in the 2 groups was comparable. Some of these dogs were used later for other experiments and it is of interest to note parenthetically that when an animal's chest was opened, without any other manip-
Journal of
382
Burman, Panagopoulos, Kahn
ulation, the T wave would often suddenly reverse direction. Also, the presence of diphasic T waves in Lead V2 of 32 per cent of dogs and diphasic P waves in Leads II and III of 13 per cent indicate that these findings are not necessarily abnormal, a point not previously noted in the literature. Conclusions 1. The electrocardiograms of 107 normal dogs domiciled and studied under stable, standardized conditions were analyzed for the minimum, maximum, and mean values of each component as well as the standard deviations and per cent incidence. 2. The electrocardiogram of the normal anesthetized dog shows far greater variation than that of man. Within this broader range there is, however, sufficient standardization to validate a concept of normalcy. 3. In all future experiments in which the canine electrocardiogram plays a significant role it would seem safer to exclude as normal at the outset any animal whose tracing contains components outside the minimummaximum limits established herein.
Thoracic and Cardiovascular Surgery
The authors are grateful to Dr. Donald J. Weber of the Cardiopulmonary Laboratory for his assistance and advice in the recording and interpretation of the data. REFERENCES 1 Burman, S. O., and Gibson, T. C : Bronchoscopy and the Cardio-Respiratory Reflexes, Ann. Surg. 157: 134, 1963. 2 Fabre, H., Fabre, R., and Linquette, Y.: La variability d'electrocardiogramme du chien, Arch. mal. coeur, 7: 673, 1955. 3 Gyarnati, E.: Electrocardiographs Examination of Dogs. (Thesis) Budapest, 1939. 4 Horwitz, S. A., Spanier, M. R., and Wiggers, H. C : The ECG in Normal Dogs, Proc. Soc. Exper. Biol. & Med. 84: 121, 1953. 5 Lannek, N. A.: Clinical and Experimental Study on the ECG in Dogs. Proceedings of Medical Clinic of the Royal Veterinary College of Stockholm, 1949. 6 Lombard, E. A., and Wilham, C : Electrocardiogram of the Anesthetized Dog, Am. J. Physiol. 181: 567, 1955. 7 Petersen, E. S., Ticketts, H. T., Brewer, N. R., Lints, H. A., Test, C. E., and Tupikova, N. A.: Electrocardiogram of the Beagle Dog, Proc. Soc. Exper. Biol. & Med. 77: 330, 1951. 8 Smith, F. M.: The Ligation of Coronary Arteries With Electrocardiographic Study, Arch. Int. Med. 22: 8, 1918.