J. ELECTROCARDIOLOGY, 8 (2) 167-172, 1975
The Normal Electrocardiogram of the Domestic Pony BY DARYL D. BUSS, D.V.M., M.S., CLARENCE A. RAWLINGS, D.V.M., PH.D.,* AND GERALD E. BISGARD, D.V.M., PH.D.
in t h e study. 25 Thus, i n t e r p r e t a t i o n of t he ECG of t he domestic pony has of necessity been based on normal d a t a reported for various breeds of full-sized horses. 8'5'7'14-16'2~ The p r e s e n t study was designed to identify n o r m a l electrocardiographic p a r a m e t e r s of the domestic pony, and, in so doing, to confirm or deny t he tacit assumption made in t he past t h a t t h e E CG of the pony is equivalent to t h a t of t h e horse.
SUMMARY Twelve-lead electrocardiograms (ECGs) were recorded f r o m 52 resting m a t u r e female domestic grade ponies. E v a l u a t i o n of t he 50 E C G S u l t i m a t e l y selected f or analysis revealed n u m e r o u s differences f r om accepted n o r m a l values o f horses. Among these differences are s h o r t e r d u r a t i o n s o f the P and QRS complexes as well as P-R and Q-T intervals and a lower amplitude of the P wave. P o n y ECGs displayed a lower incidence of wandering p a c e m a k e r and complete absence of second degree a t r i o v e n t r i e u l a r block, in cont ra st to th e relatively r o u t i n e o c c u r r e n c e o f these p h e n o m e n a in the horse. The existence Of significant differences between the E C G o f the pony and horse must be recognized and considered in f u t u r e clinical and r e s e a r c h a p plicatio n s o f e l e c t r o c a r d i o g r a p h y in t h e domestic pony.
MATERIALS A N D METHODS Fifty-two mature female domestic grade ponies ranging in weight from 145-256 kg (mean 199-+4 SEM) were utilized for the study. All were found to be in good health and free of detectable cardiovascular abnormalities on physical examination which included cardiac auscultation. A twelve-lead ECG was subsequently recorded from each pony on a Burdick EK4 single channel electrocardiographic recorder (Burdick Corp., Milton, Wis.) at a paper speed of 25 mm/sec and at calibration settings of 1 cm/mV and 2 cm/mV for each lead. The recordings were obtained with the pony standing on a rubber mat with minimal physical restraint. No tranquilization or other form of chemical restraint was employed during the study. In view of the established influence of forelimb placement on the configuration of the ECG of the pony,25 special care was taken to assure that the forelimbs were parallel with one another throughout the recording of the ECG. The leads recorded included the standard bipolar limb leads I, II, and III, and the augmented unipolar limb leads aVR, aVL, and aVF. Electrodes were attached approximately 6 cm below the olecranon and stifle joint by means of alligator clips and moistened with alcohol. Additionally, unipolar chest leads consisting of CV~LL, CVsLU, Vlo, CVsRU, and CV6RL were recorded, using Wilson's central terminal as the indifferent electrode. The placement of these electrodes, basically similar t o those described by Van Zijl, 28 was as follows: CVsLL (CV6 left lower) - - on the left side of the thorax over the sixth intercostal space at the level of a horizontal line drawn through the highest point of the olecranon; CVsLU (CV6 left upper) -on the left side at the sixth intercostal space at the
The suitability of t he domestic grade pony as a subject for a va r i et y of types of cardiop u l m o n a r y r e s e a r c h ha s b e e n r e p e a t e d l y emphasized. 1'2,8'1s'22'23 Little i n f o r m a t i o n is c u r r e n t l y available concerning the n o r m a l E C G of t h e pony, however, as a p r e v i o u s study on equine electrocardiography which included ponies failed to separate the dat a from .ponies and thoroughbreds also utilized
From the Department of Veterinary Science and Cardiovascular Research Laboratory, Department of Medicine, University of Wisconsin, Madison, Wisconsin 53706. *Dept. of Medicine & Surgery, College of Veterinary Medicine, University of Georgia, Athens, Georgia. Reprint requests to: Daryl D. Buss, D.V.M., Department of Veterinary Science, University of Wisconsin, 1655 Linden Drive, Madison, Wisconsin 53706. 167
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level of a horizontal line intersecting the scapulohumeral joint; CV6RL (CVs right lower) and CV6RU (CV6 right upper) were placed similarly only on the right side of the thorax; Vlo - - over the vertebral column vertically above CV6LL. 17 The final lead recorded was a bipolar base-apex lead, basically similar to that described by Ndrr, 21 with one electrode designated as the negative electrode placed over the left lower thorax at the cardiac apex while the positive electrode was attached over the upper V3 of the right scapula. Uniformity of electrode placement was aided by having the same individual attach electrodes throughout the study. Upon initial review of the ECGs, two of the ponies were noted to have recurrent episodes of sinoatrial block. Because insufficient information is presently available to judge the significance of this arrhythmia in the pony, these ECGs were e l i m i n a t e d from f u r t h e r c o n s i d e r a t i o n . The remaining 50 ECGs were found to be of suitable quality for more detailed measurement. Complexes selected for measurement were chosen when possible from areas where the preceding and following T-P interval baselines were at the same level, with all measurements representing averages over at least 3-5 complexes. Amplitudes were measured to the nearest 0.05 mV and durations to the nearest 0.01 second. In those cases where P or T waves were biphasic in nature, the amplitudes were summed while taking note of their respective signs, as has been previously advocated for the horse. 3'1s The greater amplitude was recorded in those cases where the P or T wave was bifid in nature. The QRS mean manifest electrical axes were calculated for each pony from the scalar ECG. Leads I and aVF were employed to calculate t h e mean electrical axis in the frontal plane, in which the pony is viewed from the ventral aspect, while leads I and Vlo were employed for axis determination in the transverse plane, viewing the pony in a cranial-caudal direction. The mean electrical axis for the sagittal plane, in which the pony is viewed from the left side, was determined utilizing leads V~o and aVF. These combinations of leads were selected on the basis of their approximately perpendicular relationships with one another, thus providing a semiorthogonal lead system for vector analysis. ~
RESULTS T h e d o m i n a n t c a r d i a c r h y t h m was basically r e g u l a r , a l t h o u g h slight v a r i a t i o n s in heart rate with respiration were common. T h r e e of t h e p o n i e s d e m o n s t r a t e d a m o r e m a r k e d sinus a r r h y t h m i a , a l t h o u g h no ins t a n c e of second d e g r e e or c o m p l e t e atriov e n t r i c u l a r h e a r t block w e r e n o t e d in a n y of the ECGs examined. T h e P w a v e was g e n e r a l l y u p r i g h t in leads I, II, III, aVF, CV6LL, CV6LU, CVsRL a n d base-apex, while g e n e r a l l y b e i n g i n v e r t e d in leads aVR a n d V~o (Table 1). A l t h o u g h v a r i a b i l i t y b e t w e e n i n d i v i d u a l a n i m a l s m a y be
n o t e d in s e v e r a l leads, t h i s v a r i a b i l i t y was particularly marked in leads aVL and CVGRU. T h e P - w a v e d u r a t i o n was v a r i a b l e a m o n g t h e v a r i o u s leads selected, as d e m o n s t r a t e d in T a b l e 2. T h e b a s e - a p e x lead provided t h e l a r g e s t a m p l i t u d e a n d longest d u r a tion P wave, w h i c h was u p r i g h t in all cases (Tables 1-3). V a r i a b i l i t y b e t w e e n p o n i e s also e x i s t e d w i t h r e g a r d to QRS c o n f i g u r a t i o n in some l e a d s (Table 2, 3). T h e s m a l l e s t degree of variability b e t w e e n individuals was n o t e d in lead Vlo a n d t h e b a s e - a p e x lead, w i t h t h e QRS complex a s s u m i n g a QR a n d RS configuration, respectively. T h e d u r a t i o n of t h e QRS complex r a n g e d from a m e a n of 0.066 s e c in leads aVR a n d CVsRL to a m e a n of 0.086 sec in the base-apex lead. T h e m e a n QRS e l e c t r i c a l axis in t h e f r o n t a l p l a n e was e x t r e m e l y v a r i a b l e , r a n g ing from - 1 7 5 ~ t o + 165 ~ w i t h a m e a n of - 3 6 ~ ---14(SEM). T h e m e a n electrical axes for t h e t r a n s v e r s e and sagittal p l a n e s d e m o n s t r a t e d a m u c h lesser d e g r e e of v a r i a b i l i t y , a v e r a g i n g -89~ and -97~ respectively. If t h e v a l u e s for m e a n electrical axis in t h e frontal, t r a n s v e r s e , a n d sagittal p l a n e s are used to o b t a i n 99% confidence i n t e r v a l s for t h e p o p u l a t i o n m e a n / ~ y t h e calculated i n t e r v a l s a r e f o u n d to be: F r o n t a l : - 2 ~ to - 7 4 ~ T r a n s v e r s e : - 8 1 ~ to - 9 7 ~ S a g i t t a l : - 8 6 ~ to - 108 ~ T h e d u r a t i o n of the T w a v e could not be a c c u r a t e l y d e t e r m i n e d , b e c a u s e the S-T segm e n t in t h e pony E C G t e n d s to follow a slanting course u p w a r d or d o w n w a r d d e p e n d i n g on the direction of t h e T w a v e , similar to t h a t described in t h e horse. 3'16'2s T h e a m p l i t u d e s a n d c o n f i g u r a t i o n s of t h e T w a v e s in t h e various leads are d e m o n s t r a t e d in Tables 1 and 3. The d u r a t i o n of the P-R a n d Q-T i n t e r v a l s of t h e p o n y E C G are listed in T a b l e 2. T h e P-R a n d Q-T i n t e r v a l s w e r e s u b s e q u e n t l y comp a r e d w i t h t h e h e a r t r a t e d u r i n g each lead. When these comparisons were considered over all leads, a significant (P
The Normal Electrocardiogram of the Domestic Pony BY DARYL D. BUSS, D.V.M., M.S., CLARENCE A. RAWLINGS, D.V.M., PH.D.,* AND GERALD E. BISGARD, D.V.M., PH.D.
in t h e study. 25 Thus, i n t e r p r e t a t i o n of t he ECG of t he domestic pony has of necessity been based on normal d a t a reported for various breeds of full-sized horses. 8'5'7'14-16'2~ The p r e s e n t study was designed to identify n o r m a l electrocardiographic p a r a m e t e r s of the domestic pony, and, in so doing, to confirm or deny t he tacit assumption made in t he past t h a t t h e E CG of the pony is equivalent to t h a t of t h e horse.
SUMMARY Twelve-lead electrocardiograms (ECGs) were recorded f r o m 52 resting m a t u r e female domestic grade ponies. E v a l u a t i o n of t he 50 E C G S u l t i m a t e l y selected f or analysis revealed n u m e r o u s differences f r om accepted n o r m a l values o f horses. Among these differences are s h o r t e r d u r a t i o n s o f the P and QRS complexes as well as P-R and Q-T intervals and a lower amplitude of the P wave. P o n y ECGs displayed a lower incidence of wandering p a c e m a k e r and complete absence of second degree a t r i o v e n t r i e u l a r block, in cont ra st to th e relatively r o u t i n e o c c u r r e n c e o f these p h e n o m e n a in the horse. The existence Of significant differences between the E C G o f the pony and horse must be recognized and considered in f u t u r e clinical and r e s e a r c h a p plicatio n s o f e l e c t r o c a r d i o g r a p h y in t h e domestic pony.
MATERIALS A N D METHODS Fifty-two mature female domestic grade ponies ranging in weight from 145-256 kg (mean 199-+4 SEM) were utilized for the study. All were found to be in good health and free of detectable cardiovascular abnormalities on physical examination which included cardiac auscultation. A twelve-lead ECG was subsequently recorded from each pony on a Burdick EK4 single channel electrocardiographic recorder (Burdick Corp., Milton, Wis.) at a paper speed of 25 mm/sec and at calibration settings of 1 cm/mV and 2 cm/mV for each lead. The recordings were obtained with the pony standing on a rubber mat with minimal physical restraint. No tranquilization or other form of chemical restraint was employed during the study. In view of the established influence of forelimb placement on the configuration of the ECG of the pony,25 special care was taken to assure that the forelimbs were parallel with one another throughout the recording of the ECG. The leads recorded included the standard bipolar limb leads I, II, and III, and the augmented unipolar limb leads aVR, aVL, and aVF. Electrodes were attached approximately 6 cm below the olecranon and stifle joint by means of alligator clips and moistened with alcohol. Additionally, unipolar chest leads consisting of CV~LL, CVsLU, Vlo, CVsRU, and CV6RL were recorded, using Wilson's central terminal as the indifferent electrode. The placement of these electrodes, basically similar t o those described by Van Zijl, 28 was as follows: CVsLL (CV6 left lower) - - on the left side of the thorax over the sixth intercostal space at the level of a horizontal line drawn through the highest point of the olecranon; CVsLU (CV6 left upper) -on the left side at the sixth intercostal space at the
The suitability of t he domestic grade pony as a subject for a va r i et y of types of cardiop u l m o n a r y r e s e a r c h ha s b e e n r e p e a t e d l y emphasized. 1'2,8'1s'22'23 Little i n f o r m a t i o n is c u r r e n t l y available concerning the n o r m a l E C G of t h e pony, however, as a p r e v i o u s study on equine electrocardiography which included ponies failed to separate the dat a from .ponies and thoroughbreds also utilized
From the Department of Veterinary Science and Cardiovascular Research Laboratory, Department of Medicine, University of Wisconsin, Madison, Wisconsin 53706. *Dept. of Medicine & Surgery, College of Veterinary Medicine, University of Georgia, Athens, Georgia. Reprint requests to: Daryl D. Buss, D.V.M., Department of Veterinary Science, University of Wisconsin, 1655 Linden Drive, Madison, Wisconsin 53706. 167
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level of a horizontal line intersecting the scapulohumeral joint; CV6RL (CVs right lower) and CV6RU (CV6 right upper) were placed similarly only on the right side of the thorax; Vlo - - over the vertebral column vertically above CV6LL. 17 The final lead recorded was a bipolar base-apex lead, basically similar to that described by Ndrr, 21 with one electrode designated as the negative electrode placed over the left lower thorax at the cardiac apex while the positive electrode was attached over the upper V3 of the right scapula. Uniformity of electrode placement was aided by having the same individual attach electrodes throughout the study. Upon initial review of the ECGs, two of the ponies were noted to have recurrent episodes of sinoatrial block. Because insufficient information is presently available to judge the significance of this arrhythmia in the pony, these ECGs were e l i m i n a t e d from f u r t h e r c o n s i d e r a t i o n . The remaining 50 ECGs were found to be of suitable quality for more detailed measurement. Complexes selected for measurement were chosen when possible from areas where the preceding and following T-P interval baselines were at the same level, with all measurements representing averages over at least 3-5 complexes. Amplitudes were measured to the nearest 0.05 mV and durations to the nearest 0.01 second. In those cases where P or T waves were biphasic in nature, the amplitudes were summed while taking note of their respective signs, as has been previously advocated for the horse. 3'1s The greater amplitude was recorded in those cases where the P or T wave was bifid in nature. The QRS mean manifest electrical axes were calculated for each pony from the scalar ECG. Leads I and aVF were employed to calculate t h e mean electrical axis in the frontal plane, in which the pony is viewed from the ventral aspect, while leads I and Vlo were employed for axis determination in the transverse plane, viewing the pony in a cranial-caudal direction. The mean electrical axis for the sagittal plane, in which the pony is viewed from the left side, was determined utilizing leads V~o and aVF. These combinations of leads were selected on the basis of their approximately perpendicular relationships with one another, thus providing a semiorthogonal lead system for vector analysis. ~
RESULTS T h e d o m i n a n t c a r d i a c r h y t h m was basically r e g u l a r , a l t h o u g h slight v a r i a t i o n s in heart rate with respiration were common. T h r e e of t h e p o n i e s d e m o n s t r a t e d a m o r e m a r k e d sinus a r r h y t h m i a , a l t h o u g h no ins t a n c e of second d e g r e e or c o m p l e t e atriov e n t r i c u l a r h e a r t block w e r e n o t e d in a n y of the ECGs examined. T h e P w a v e was g e n e r a l l y u p r i g h t in leads I, II, III, aVF, CV6LL, CV6LU, CVsRL a n d base-apex, while g e n e r a l l y b e i n g i n v e r t e d in leads aVR a n d V~o (Table 1). A l t h o u g h v a r i a b i l i t y b e t w e e n i n d i v i d u a l a n i m a l s m a y be
n o t e d in s e v e r a l leads, t h i s v a r i a b i l i t y was particularly marked in leads aVL and CVGRU. T h e P - w a v e d u r a t i o n was v a r i a b l e a m o n g t h e v a r i o u s leads selected, as d e m o n s t r a t e d in T a b l e 2. T h e b a s e - a p e x lead provided t h e l a r g e s t a m p l i t u d e a n d longest d u r a tion P wave, w h i c h was u p r i g h t in all cases (Tables 1-3). V a r i a b i l i t y b e t w e e n p o n i e s also e x i s t e d w i t h r e g a r d to QRS c o n f i g u r a t i o n in some l e a d s (Table 2, 3). T h e s m a l l e s t degree of variability b e t w e e n individuals was n o t e d in lead Vlo a n d t h e b a s e - a p e x lead, w i t h t h e QRS complex a s s u m i n g a QR a n d RS configuration, respectively. T h e d u r a t i o n of t h e QRS complex r a n g e d from a m e a n of 0.066 s e c in leads aVR a n d CVsRL to a m e a n of 0.086 sec in the base-apex lead. T h e m e a n QRS e l e c t r i c a l axis in t h e f r o n t a l p l a n e was e x t r e m e l y v a r i a b l e , r a n g ing from - 1 7 5 ~ t o + 165 ~ w i t h a m e a n of - 3 6 ~ ---14(SEM). T h e m e a n electrical axes for t h e t r a n s v e r s e and sagittal p l a n e s d e m o n s t r a t e d a m u c h lesser d e g r e e of v a r i a b i l i t y , a v e r a g i n g -89~ and -97~ respectively. If t h e v a l u e s for m e a n electrical axis in t h e frontal, t r a n s v e r s e , a n d sagittal p l a n e s are used to o b t a i n 99% confidence i n t e r v a l s for t h e p o p u l a t i o n m e a n / ~ y t h e calculated i n t e r v a l s a r e f o u n d to be: F r o n t a l : - 2 ~ to - 7 4 ~ T r a n s v e r s e : - 8 1 ~ to - 9 7 ~ S a g i t t a l : - 8 6 ~ to - 108 ~ T h e d u r a t i o n of the T w a v e could not be a c c u r a t e l y d e t e r m i n e d , b e c a u s e the S-T segm e n t in t h e pony E C G t e n d s to follow a slanting course u p w a r d or d o w n w a r d d e p e n d i n g on the direction of t h e T w a v e , similar to t h a t described in t h e horse. 3'16'2s T h e a m p l i t u d e s a n d c o n f i g u r a t i o n s of t h e T w a v e s in t h e various leads are d e m o n s t r a t e d in Tables 1 and 3. The d u r a t i o n of the P-R a n d Q-T i n t e r v a l s of t h e p o n y E C G are listed in T a b l e 2. T h e P-R a n d Q-T i n t e r v a l s w e r e s u b s e q u e n t l y comp a r e d w i t h t h e h e a r t r a t e d u r i n g each lead. When these comparisons were considered over all leads, a significant (P
