The T-wave in the V10 precordial electrocardiographic lead is negative in healthy Chihuahua dogs

Marieke Dijkstra DVM, Viktor Szatmári DVM, PhD

Department of Clinical Sciences of Companion Animals, Faculty of Veterinary Medicine, Utrecht University, Yalelaan 108, 3508 TD Utrecht, The Netherlands

Corresponding author: M. Dijkstra

Email address of the corresponding author: m.dijkstra2@uu.nl

Telephone: +31 30 253 9228

Abstract

Objectives

The T-wave polarity in the V10 precordial electrocardiographic lead in Chihuahuas is described as positive in veterinary literature. The aim of this study was to investigate the polarity of the T-wave in V10 precordial ECG lead in clinically healthy Chihuahuas. Our null hypothesis was that healthy Chihuahuas have negative T-waves in V10.

Animals, materials and methods

In the prospective study 67 healthy, breeder-owned Chihuahuas were used. A physical examination, 10-lead electrocardiogram and an echocardiogram was performed on each dog.

Results

No cardio-respiratory abnormalities during physical examination were revealed in any of the otherwise healthy dogs.

Three out of sixty-seven ECGs were of insufficient quality because of baseline artifacts due to movement of the animal. Two other ECGs showed a nearly iso-electrical T-wave in the V10 lead. The remaining 62 ECGs showed negative T-waves in the V10 lead. Right ventricular hypertrophy was excluded with echocardiography in all dogs.

Conclusion

In contrast to a previous report ¹, we found that healthy Chihuahuas have negative T-wave in the V10 precordial ECG lead.

Keywords: Electrocardiography; Cardiology-small animal

Introduction

A surface ECG is a recording of changing potentials generated by electrical activity of the heart from the surface of the body. The ECG is primarily used as a diagnostic tool for characterizing cardiac arrhythmias and intra-cardiac conduction disturbances. However, changes on the ECG tracing may also indicate electrolyte disturbances and cardiac chamber enlargement. The ability of the ECG to detect cardiac chamber enlargement is limited because conduction from the heart to the body surface is inhomogeneous, moreover the surface ECG records the cardiac potentials as mille-volts. Nowadays, cardiac chamber enlargement in dogs is primarily and more accurately documented by echocardiography.

 The limb leads (I, II, III, aVR, aVL & aVF) are the most commonly used leads in veterinary medicine. The pre-cordial (i.e. chest) leads were introduced later in electrocardiographic history to measure electrical events limited to specific regions of the heart and in a plane that is perpendicular to that of the limb leads. Although pre-cordial leads are of limited value in dogs and cats, in some cases do provide additional information, e.g. P-waves can be identified more easily in the left pre-cordial leads than in the limb leads.

On an ECG the T-wave is directly related to the repolarisation of the ventricular myocardial cells. Because the genesis of the T-wave is very complex, the determinants of the T-wave polarity are still poorly understood ^2,3,4. The characteristics of the T-wave are, among others, related to the differences in the shape of the action potential curves in various regions of the ventricles. The relationship between the shape of the action potential curve and the surface ECG waveform is more complex for the repolarisation than that for depolarization, as the repolarisation involves the three-dimensional shape of the heart and its surrounding conductive medium ². Therefore, approximations made for the description of depolarization, e.g. a localized source, uniform propagation, etc. are invariably unsuitable for describing repolarisation. Furthermore, the T-wave is very sensitive to changes in the environment of the heart, various drugs, nerve stimulation, changes in temperature or ion concentration, etc.². Unlike in humans ⁴, the T-wave concordance (i.e. polarity of the T-wave compared to that of the QRS complex) has no diagnostic significance in dogs, with the only documented exceptions in certain pre-cordial leads as in V10 ⁶.

In 1965 Detweiler and Patterson ¹ described that the T-waves of normal dogs in a certain precordial ECG lead, the V10 lead, were negative. The Chihuahua breed was mentioned as an exception, as the T-waves of healthy Chihuahuas were positive. Positive T-waves in all the other breeds were reported to be indicative of right ventricular hypertrophy ¹. Modern, leading veterinary textbooks keep on citing this reference ^7,8.

The motivation to perform the following study was that the ECG-recordings of the 4 Chihuahuas that have undergone an ECG-examination at the authors’ clinic between 2004 and 2006 all had negative T-waves in lead V10. The underlying heart disease was patent ductus arteriosus in 2 dogs and myxomatous mitral valve degeneration in the other two. None of the 4 dogs had echocardiographic evidence of right ventricular hypertrophy.

The aim of this study was to investigate the polarity of the T-wave in V10 precordial ECG lead in clinically healthy Chihuahuas. Our null hypothesis was that healthy Chihuahuas have negative T-waves in V10.

Animals, materials and Methods

 Animals

This prospective study was performed on 67, breeder-owned pure breed pedigreed Chihuahua dogs. Forty-one dogs were female and 26 were male. Fifty-nine dogs had Dutch pedigrees, 7 dogs were imported from the United States of America and 1 dog was imported from Russia. Median age was 3.6 years (range 0.3 years to 13.4 years). All dogs were healthy according to the owners. Screening for cardiac abnormalities was performed by physical examination and any dog that had a cardiac murmur, detected at cardiac auscultation, was excluded from entering the study. In all dogs physical examination, an ECG and an echocardiogram were performed. The owners entered the study voluntarily. In every case owner consent was obtained. The study was performed in compliance with institutional guidelines for research on animals.

ECG

The dogs were held in right lateral recumbency with manual restrain. The 10-lead ECG included the 3 standard bipolar limb leads (I, II & III), the 3 augmented unipolar limb leads (aVR, aVL & aVF) and 4 unipolar precordial leads (CV5RL or rV2, CV6LL or V2, CV6LU or V4 and V10). All ECG electrodes were attached to the animal’s skin by crocodile clips. Commercially available ECG gel was used to obtain good conduction between the skin and the crocodile clips. The electrodes for the precordial leads were placed, according to the description of Detweiler and Patterson ¹, in the following four positions: CV5RL, the 5th right intercostal space near the edge of the sternum; CV6LL, the 6th left intercostal space near the edge of the sternum; CV6LU, the 6th left intercostal space at the costochondral junction; V10, over the dorsal spinous process of the 7th thoracic vertebra ¹. The ECGs were performed without sedation and were recorded using 25 and 100 mm/s paper speeds and a 10 mm/mV sensitivity on ECG paper.

The T-wave was defined as negative, if the deflection was below the baseline, and positive, if the deflection was above the baseline. In cases of biphasic T-waves, the sum of the negative and positive deflections was taken.

Echocardiography

B-mode, M-mode, color and spectrum Doppler echocardiographic examinations were performed from the right parasternal window. The reason of performing an echocardiogram was to exclude a clinically silent right ventricular eccentric or concentric hypertrophy. The echocardiograms were performed in right lateral recumbency without sedation by a board certified veterinary cardiologist (VS). Because no reference values for the right ventricular dimensions (in the Chihuahua) are available in the literature ^9,10 we chose to follow the criteria of Johnson and others ¹¹ to define a normal diastolic right ventricular lumen and wall thickness: “Right ventricular chamber size was reported as normal if it was less than or equal to one half of the size of the left ventricle in the right parasternal long-axis view of the left ventricular outflow tract” and “right ventricular wall thickness less than one half of left ventricular free wall thickness was recorded as normal” ¹¹. The left ventricular free wall thickness was measured in diastole and compared to published reference values ⁹.

 Statistics

The confidence interval was calculated with the following formula:

n = number of dogs (n = 62), p = the fraction of  dogs with a positive T-wave (p = 0), z = value when confidence of 95% is taken into account (z = 1.96).

Results

No clinically silent right ventricular concentric or eccentric hypertrophy was detected with echocardiography in any of the 67 dogs. The only abnormality that was found with echocardiography in a few of the otherwise healthy dogs were thickened mitral valve leaflets  and with color Doppler echocardiography trivial mitral valve regurgitation.

Three out of the 67 ECGs were of insufficient quality for interpretation of the T-waves in V10 lead because of baseline artifacts due to movement of the animals. Two other ECGs showed a nearly iso-electrical T-wave in the V10 lead. The remaining 62 ECGs showed negative T-waves in the V10 lead.

Discussion

In this study, none of the clinically healthy Chihuahuas showed positive T-waves in the precordial V10 ECG lead. This is in contradiction to earlier reports. The chance that we missed healthy Chihuahuas with positive T-waves in V10 is smaller than 5.8 % (confidence interval according to standard procedure) when a population of 62 dogs (n=62) are taken into account.

In 1971 Hill developed a valid method for the ECG diagnosis of right ventricular hypertrophy in dogs ⁶. Thirty-six abnormal ECG criteria were considered, of which 12 were finally selected. With the presence of a minimum of 3 out of 12 criteria, a diagnostic accuracy of 92.9% was obtained to detect abnormal dogs without false positives. In normal dogs, only 11.4% had one of these criteria, whereas none had two or more criteria. The polarity of the T-wave was found to have diagnostic significance only in leads CV5RL and V10. In normal dogs, 87.1% had a negative T-wave in V10 and 98.5% had a negative T-wave in CV5RL. The exact number of dogs with right ventricular hypertrophy and a positive T-wave in V10 was not mentioned in this study, but the combination of a negative T-wave in CV5RL and a positive T-wave in V10 occurred in 14.3% of dogs with right ventricular hypertrophy.

We speculate that the Chihuahuas in the study of Detweiler & Patterson ¹ probably had either a clinically silent right ventricular hypertrophy or were normal dogs with a positive T-wave in V10. It is not known whether these Chihuahuas had other ECG criteria of right ventricular hypertrophy as described by Hill ⁶. Also, it is unknown how many Chihuahuas were involved in Detweiler & Patterson’s study ¹. Because 40 years ago echocardiography was not available, it is possible that a clinically silent right ventricular hypertrophy in the Chihuahuas of the study of Detweiler & Patterson was missed.

The age of the dogs also influences the T-wave polarity ^12,13,14. During the first post-natal weeks, the electrocardiographic pattern resembles that seen in pathologic right ventricular hypertrophy because the right ventricular mass exceeds that of the left ventricle. A drop in right ventricular pressure to adult levels occurs in 5 to 6 weeks after birth and as a consequence a relative increase of the left ventricular mass takes place. In contrast to human infants, in which the T-wave in the right precordial leads are usually positive the first 24 hours after birth ¹², the T-waves in V10 produced in normal puppies are often negative ¹⁴. In one study, 16 healthy pups were compared to 20 other pups who were normal littermates from dog families with congenital heart disease ¹⁴. In the first group, T-waves in V10 were all negative like in adult dogs. In 82% of the second group, T-waves in V10 at birth were positive and at 12 weeks of age this pattern retained in 10.5% of this pups. No reason for this difference was found. In our study no dog was younger than 12 weeks of age.

Despite the fact that ECG has become an invaluable, routinely used diagnostic tool in human as well as in veterinary medicine, ironically, the genesis and the determinants of T-wave polarity are still incompletely understood ^3,4,5. Moreover, as the T-wave is sensitive to miscellaneous changes, various causes of inverted T-wave deflection may easily remain undetected.

In conclusion, in contrast to the previous report, we found that healthy Chihuahuas have negative T-waves in the V10 precordial ECG lead.

Acknowledgements

 A part of this study was presented: Dijkstra M, Szatmari V. “T-wave peculiarity” on Chihuahua-ECGs is a myth, In Proceedings, “Voorjaarsdagen” European Veterinary Conference 2008;243.

The authors wish to thank Dr. Arnold A. Stokhof for pointing out the discrepancy between the literature and his experience regarding T-wave polarity, Dr. Erik Teske for his advice regarding statistical issues and the owners of the Chihuahuas involved in this study.

References

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