Ventricular Premature Complex and Ventricular Tachycardia
• Ventricular premature complex (VPC): A spontaneous premature ventricular depolarization that originates from the ventricles, resulting in a QRS′ complex and T′ wave on the surface electrocardiogram (ECG) that have a different morphology (and duration) (Figure 1).
FIGURE 1 A VPC occurs earlier than expected, has no preceding P wave, and shows an abnormal morphology and duration. The ectopic beat is followed by a compensatory pause to “fall back” into the underlying sinus rhythm. The duration of the interval normal–abnormal–normal equals twice the normal RR interval.
• Depending on the site of origin, the QRS′ complex may be completely abnormal or may be difficult to distinguish from the normal complexes (when it originates high in the ventricle near the His-Purkinje system).
• VT of less than about 140 beats/min, especially if polymorphic, is usually caused by increased automaticity. Monomorphic VT of more than about 140 beats/min is usually caused by a rapidly firing focus, triggered activity, or reentry.
• Aberrant conduction in horses with AF: In AF, sympathetic stimulation or exercise may result in extremely high ventricular rates (up to >400 beats/min). Occasionally, QRS broadening and an R-on-T phenomenon may be found. Although AF and VPC/VT may occur concurrently, these extremely high rates are probably supraventricular in origin (sudden conduction of atrial impulses through the atrioventricular (AV) node because of the change in autonomic tone) whereby an aberrant conduction to the ventricles results in broader QRS complexes and R-on-T.
• Ventricular escape rhythm: Because of the absence of a normal ventricular depolarization, an ectopic beat is generated in the ventricles, resulting in a QRS′ complex that occurs later than normal and has an abnormal morphology and duration.