An Exploratory Study on Vectorcardiographic Identification of the Site of Origin of Focally Induced Premature Depolarizations in Horses, Part I: The Atria

Simple Summary Arrhythmias are common in horses, but in order to offer adequate treatment, the anatomical location from where the arrhythmia originates must first be known. The objective of this study was to differentiate between the various anatomical origins of atrial premature depolarizations and to further differentiate these from normal sinus rhythm based upon the vectorcardiography characteristics of seven horses without cardiovascular disease. With vectorcardiography, the magnitude and direction of the cardiac electrical forces were derived from an electrocardiographic recording and were plotted in three dimensions. The three directions were represented by the right-left axis, head-tail axis, and dorsoventral axis. Under general anaesthesia, atrial premature depolarizations were induced by pacing inside the heart, guided by a 3D mapping system that allowed us to see the 3D position of the pacing catheter inside the heart in real-time. An adapted statistical test optimized for spherical data was used to demonstrate that the maximal directions of the first and second half of the P wave showed significant differences between the paced locations and between the paced complexes and sinus rhythm. The current study provides a practical approach to identifying the approximate site of origin of an atrial arrhythmia using a custom 12-lead ECG. In human cardiology, the anatomical origin of atrial premature depolarizations (APDs) is derived from P wave characteristics on a 12-lead electrocardiogram (ECG) and from vectorcardiography (VCG). The objective of this study is to differentiate between anatomical locations of APDs and to differentiate APDs from sinus rhythm (SR) based upon VCG characteristics in seven horses without cardiovascular disease. A 12-lead ECG was recorded under general anaesthesia while endomyocardial atrial pacing was performed (800-1000 ms cycle length) at the left atrial free wall and septum, right atrial free wall, intervenous tubercle, as well as at the junction with the cranial and caudal vena cava. Catheter positioning was guided by 3D electro-anatomical mapping and transthoracic ultrasound. The VCG was calculated from the 12-lead ECG using custom-made algorithms and was used to determine the mean electrical axis of the first and second half of the P wave. An ANOVA for spherical data was used to test if the maximal directions between each paced location and the maximal directions between every paced location and SR were significantly (p < 0.05) different. Atrial pacing data were not available from the LA septum in three horses, the intervenous tubercle in two horses, and from the LA free wall in one horse. The directions of the maximal electrical axes showed significant differences between all paced locations and between the paced locations and SR. The current results suggest that VCG is useful for identifying the anatomical origin of an atrial ectopy.