7.8.9 Ventricular tachycardia
Ventricular tachycardia is defined as four or more VPCs in succession. It may be paroxysmal or sustained (Figures 7.11 and 7.12). Ventricular tachycardia nearly always indicates the presence of serious underlying disease. Non-conducted P waves may be seen or may be hidden by the abnormal QRS complexes. Fusion beats (when the depolarisation starts from both the AV node and an ectopic focus simultaneously) or capture beats (when a sinus beat is conducted) may be seen. If no normal QRS complexes are seen it may be difficult to be sure that the QRS complexes are abnormally wide, because there is considerable overlap between the normal range of QRS duration and the QRS duration of ectopic ventricular beats in horses. Auscultation will reveal a rapid rhythm (> 60 bpm) which is regular in susventricular tachycardia and periodically irregular if paroxysms occur. A weak, variable pulse is palpated. Signs of underlying cardiac or systemic disease may be seen. CHF may be present. Treatment is aimed at reversing underlying disease. Antidysrhythmic drugs may be required in some cases (see above). The prognosis is likely to be unfavourable unless the underlying disease can be resolved. When ventricular tachycardia develops as a result of a primary cardiac condition, it usually indicates severe disease. In this case, the prognosis is likely to be poor. Identification of the primary cause may indicate that euthanasia is the only humane course of action.