The polarity and shape of the T wave in lead V-10 in Chihuahuas

The T-wave in the canine electrocardiogram can be positive, negative, or biphasic in most leads, while his amplitude should not be greater than 25% of the R wave. However, since the 1960 ’ s persists controversy regarding the polarity of the T wave in Chihuahuas when compared to other canine breeds. This study aimed to clarify the polarity and shape of the T-wave in the lead V-10 in a group of healthy Chihuahuas. Retrospective eletrocardiographic study with a sample of 20 private client-owned Chihuahuas. The conclusion was that the polarity of the T-wave is usually negative and the shape is asymmetric, type “ slow-fast/down-up” , in all dogs studied.

On the last 30 years veterinary electrocardiography gained continuous progress becoming an attractive, relatively inexpensive and useful technique to get information in cases of suspicion heart disease, electrolyte disturbances, drug toxicity, pre-surgical evaluation, diagnostic of cardiac arrhythmias, and in experimental research.
Although the history of canine electrocardiography dates back to the 19th century, only recently that the T-wave in the dog has become better characterized (Romito et al, 2022).In healthy dogs and cats, the T-wave shows variable shape and polarity in most leads, and may be positive, negative, biphasic or flat, and his two parts are asymmetric (Oliveira, 2018).
In the thoracic lead RV2 the T-wave usually is positive while in V10 is negative in normal dogs (Hill, 1968;Santilli et al., 2018).However, specifically in relation to Chihuahua's breed, persists controversy about the polarity of the T-wave in this lead, when compared to all other canine breeds.According the classical report of Detweiller and Patterson (1965), the canine T-wave have few requirements: it may be positive, negative or biphasic in most leads, his amplitude cannot be greater than 1/4 of amplitude of the corresponding R-wave, should be positive in the lead RV2 and negative V10, except in Chihuahuas in witch should be positive.This possible exception or "peculiarity" of Chihuahuas have been replicated in many subsequent publications (Ettinger & Sutter, 1970;Bolton, 1975;Tilley et al., 1985;Miller & Tilley,1988;Ware, 1994;Smith Jr. & Hadlock, 1995;1;Smith Jr. & Fox, 1999;Nelson 2000;Montoya & Ramirez, 2007;Cotté & Ettinger, 2008;Filippi 2011;Macintire et al., 2012;Mazzini & Prada, 2020;Pascon 2021;Schroeder, 2021).Contradicting this discrepancy, Dijkstra and Szatmári (2009) reported that in Chihuahuas the T-wave in lead V10 is usually negative, rather than positive; therefore, it would not constitute an exception among the different canine breeds.This research study aimed to clarify the polarity and shape of the T-wave in the lead V-10 in a group of healthy Chihuahuas.It followed the ethical principles of the Resolução Nº 55, de 5 de outubro de 2022 (Ministério da Ciência, Tecnologia e Inovações/Conselho Nacional de Controle de Experimentação Animal).

Material and Methods
Retrospective study with a convenient sample of 20 private client-owned Chihuahuas (10 female), in a random order.

Results
All electrocardiographic values were within the normal reference ranges for the canine general population (Hill, 1968;Ettinger & Sutter, 1970;Tilley et al., 1992;Tilley et al., 2008;Willis R., 2018;Santilli et al., 2020, Romito el al., 2022).Six representatives ECG tracings of studied dogs are seen on the Figure 2.  As seen in the Figure 3, the mean vectors of QRS (yellow arrow) and T (red arrow) are both pointing to the left inferior quadrant.The mean QRS-vector is orientated at +68 0 , while the mean T-vector is at +58 0 .Aarrowheads indicate the usual orientation of the QRS loop.
Specifically in relation to T waves in lead V10, all tracings showed Qr pattern, with polarity systematically negative, while the shape was asymmetrical, type "slow/fast" and "down/up".There was a remarkable uniformity in the configuration of the QRS-T deflections of the V10 lead, in all ECG tracings (Figures 1 and 2).

Discussion
Because all Chihuahuas presented negative T-wave, i.e. his polarity was concordant to the polarity of QRS in the lead V10, it also contradicts several modern publications.In fact, our findings are in line with a previous report of Hill (1968) who, in a study with 25 different breeds, including Chihuahuas, first stated that T wave in lead V10 is primarily negative while in lead RV2 is positive.Our data are also concordant with Brownlie (2000), and with Digtsra & Zatmari (2009) who reported that the polarity of the T wave in V10 in healthy Chihuahuas is not positive.Because T-positivity in V10 could be indicative of right ventricular hypertrophy (Hill, 1971), was speculated that the Chihuahuas studied by Detweiller and Patterson (1965) could have silent right ventricular hypertrophy and was not detected, since echocardiography was not available at that time (Digtsra & Zatmari 2009).
Ventricular depolarization and repolarization are three-dimensional phenomena by nature and can be represented, therefore, by projection of QRS-T forces onto the three orthogonal planes X, Y and Z, in order to generate the vectorcardiogram (VCG) (Riera et al 2007).
The vectorcardiogram is a form of electrocardiography that represents the direction and values of vectorial forces generated during the cardiac electrical activation, moment-to-moment, and is analyzed in the planes transverse, frontal and sagittal (Riera et al 2007).
In the VCG of normal dogs the direction of the mean QRS vector forces are usually orientated leftward, ventral and caudal, giving QRS positivity in D1, D2, and D3, and in V10 negativity (Hamlin & Smith, 1960;Trautvetter et al., 1981).In fact, the direction of the QRS-T vectors in the transverse plane are scattered between 45 and 90 degrees (Hellerstein & Hamlin, 1960;Detweiller 2010).In this case, the rationale is that the polarity of the QRS and of T should be concordant with each other in lead V-10, as shown in the figure 3, in which the modal QRS and T vectors are projecting to the same quadrant.
In adult normal dogs and cats, as well as in humans, the loop of ventricular activation is orientated in counterclockwise rotation, considering the three orthogonal planes (Detweiller 2010;Ettinger and Sutter, 1970;Hellerstein H. K. and Hamlin, 1960;Pastore C.A. et al., 2019).Regarding morphology of the T-wave in lead V10, all ECGs presented asymmetrical pattern, with two components, type "down/up and slow/fast ", which is in line with study of Romito et al, 2022.

Conclusion
The results obtained in this study confirm that, in adult healthy Chihuahuas, the polarity of T wave in the lead V10 is usually negative, concordant with QRS polarity, his morphology is asymmetric, type "down/up and slow/fast".

Figure 3 -
Figure 3 -Tomographic view of the canine thorax, with vectorcardiogram constructed from the scalar leads of the Figure 1.