As previously reported in veterinary literature (Hill and Tilley 1985), our data confirmed that VPE is characterized by wide QRS complexes and short P-δQRS interval. A short P-δQRS complex interval depends on several variables such as the interatrial conduction times, the distance of the atrial origin of the bypass tract from the sinus node, the refractory period of the atrioventricular node and of the bypass tract, the autonomic nervous system tone (Steurer et al. 1994).
In human medicine, VPE is intermittent in 50% of cases. Intermittent VPE is defined by loss of the δ wave with concomitant prolongation in the PQ interval documented on at least one beat (Kein et al. 1983). In our study intermittent pre-excitation occurred in a lower percentage of cases (23.1%). Human patients with intermittent VPE present a longer anterograde refractory period of the AP and a poorer conduction over the pathway than patients with constant pre-excitation. This supports the hypothesis that intermittent pre-excitation is a manifestation of an AP with “precarious” conduction (Kein et al. 1983). The anterograde effective refractory period is not influenced by the AP location in humans (de Chillou et al. 1992) although in our study, none of the right anterior APs were correlated with intermittent VPE.
A reported approach to diagnose VPE is to test the presence of an initial R wave in lead aVR owing to the activation of the interventricular septum. dIn VPE, the first part of ventricular activation is usually from the basal aspect towards the apex, away from aVR, obscuring the usual R wave visible in this lead (Eisenberger et al. 2010). In our sample, 26/26 dogs presented no initial positive delta wave in lead aVR, so probably this electrocardiographic criterion is suggestive of VPE in the dog.
Radiofrequency catheter ablation has become the treatment of choice for patients with symptomatic and asymptomatic ventricular pre-excitation and correlated supraventricular arrhythmias (Santilli et al. 2018; Wright et al. 2018; Morady 2004). During recent years criteria for the localization of the bypass tract from the conventional 12-lead ECG have become increasingly apparent (Arruda et al. 1998; Boesma et al. 2002; Crinion et al. 2020; Ratner et al. 2012; Pambrun et al. 2018; Jamal et al. 2019). A non-invasive method, such as 12-lead surface ECG, to guide accurate localization of the anatomic substrate of the supraventricular arrhythmia would represent a significant adjunct to the electrophysiologic mapping (Silka et al. 1993). This is particularly true for the right anterior APs that must be differentiated from right postero-septal and right posterior APs due to their proximity to the His bundle and the relative increased risk of atrioventricular node damage with radiofrequency catheter ablation, which might require special techniques (Sriratanasathavorn et al. 2016).
More accurate localization of the bypass tracts could be possible and is based upon the following principles: analysis of the main QRS deflection and analysis of the delta wave.
Our study showed that the polarity of the QRS complexes in standard limb and precordial leads and the mean electrical axis in the frontal plane can help in the localization of an accessory pathway based on the analysis of the 12-lead surface ECG. Pre-excitation along a right anterior AP presents a wide δQRS complex with a normal mean electrical axis mimicking a complete left bundle branch block (median electrical axis in the frontal plane + 68°), while in most cases VPE associated with right posterior AP was characterized by QRS complex axis in the frontal plane deviated to the left (median electrical axis in the frontal plane -43.5°). On the other hand, postero-septal pathways, based on their position, usually have a less pronounced left axis deviation both in human medicine and in this study (-24°) (Steuer et al. 1994; Xie et al. 1994; Okajiana and Sotobata 1980).
The analysis of the polarity of the QRS complex in precordial leads seems to be at least as important as the analysis of the QRS mean electrical axis in the frontal plane. In human medicine, the right or left bypass tracts are easily distinguished by the main polarity in lead V1 and in leads V2-V6. Right sided APs presented an rS morphology with R/S < 1 in V1 and an Rs morphology with R/S > 1 in V2 to V6 (Lemery et al. 1987; Haghjoo et al. 2008). In our study all dogs had right-sided accessory pathway, V1 had rS morphology with R/S < 1 and from V2 to V6 R/S > 1 in all the cases.
The polarity of the δ wave in the frontal plane can be an additional factor that can help to localize the accessory pathway, although in human medicine it has been reported that the classification considering δ wave polarity has significant limitations in predicting APs localization (Kamakura et al. 1986). In human medicine, negative δ waves in lead I and aVL are never associated with right posterior or postero-septal APs (Haghjoo et al. 2008). . This data can be suggestive also in veterinary medicine since in our sample δ waves in lead I and aVL were positive in all right posterior and right postero-septal APs. Furthermore, in human medicine right anterior APs are usually associated with frontal plane δ wave axis in the region of +30°/+60° resulting in a positive δ wave in inferior leads II, III, aVF (Fitzpatrick et al. 1994; Chiang et al. 1995). We observed a similar pattern in our study in which the δ wave was positive in leads II, III, aVF in 5/5 cases presenting anterior APs.
Besides the limited number of dogs included in this study and the small number of dogs included in each group, any electrocardiographic classification of the VPE syndrome has important limitations. They include variable degrees of fusion between the normal and AP-derived wavefronts traveling to the ventricles, as VPE is a dynamic phenomenon, superimposition of the terminal portion of the P wave on the initial δ wave, and the effect of endocardial versus epicardial location of the AP (Gallagher et al. 1978; Bathia et al. 2016). Moreover, the δ wave and δ-QRS complex may further be modified by anatomic shift of the heart inside the thorax resulting from extracardiac factors (Hill and Tilley 1985; Wright at al. 1995). Furthermore, the interpretation of the VPE patterns can be complicated by the geometry of the chest in different morphotypes and the conductivity of the body tissue in relation to the body condition score (De Ambroggi et al. 1976). The right-sided APs were categorized in three main major categories. Finally, a variability and repeatability assessment on electrocardiographic measurement was not done.
In human medicine, different algorithms were able to differentiate right postero-septal from right posterior and right lateral APs (Xie et al. 1994; Fitzpatrick et al. 1994; Chiang et al. 1995; d’Avila et al. 1995; Iturralde et al. 1996; Arruda et al. 1998), while in our study, probably because of the smaller dimensions of the tricuspid valve annulus and the proximity of the different posterior locations, this differentiation was not possible. Furthermore, in this study, the electrocardiographic feature of VPE were studied only considering a right-sided position, since in our population no dog presenting a left-sided AP was represented.
In conclusion, based on the results of the present study, the localization of the right-sided AP based on electrocardiographic criteria is possible. Considering the mean electrical axis of the QRS complex in the frontal plane, pre-excitation along an anterior AP presents a wide δQR complex with a normal mean electrical axis mimicking a complete left bundle branch block, while pre-excitation along a posterior (lateral, posterior and postero-septal) AP presents a leftward deviation of the mean electrical axis in the frontal plane mimicking a left anterior fascicular block. Considering, the polarity of the δ wave in the frontal plane, δ wave is positive in lead I and aVL in right posterior and right postero-septal APs and positive in the inferior leads in case of right anterior APs. Furthermore, all right APs are characterized by an rS morphology with R/S < 1 in lead V1 and R/S > 1 from V2 to V6.