Fig 1.14 The ventricular activation process

The depolorisation of the ventricules is different in category A mammals (man ans small animals) compared with ungulates (including horses) (upper and lower diagrams respectively). In horses, the majority of the myocardium is depolorised simultaneously from multiple points and the voltages from the resulting mini-wavefronts cancel each other out when measured from the body surface. The surface ECG therefore provides very limited information about the size of the ventricular myocardium in this species.