PREMATURE VENTRICULAR CONTRACTIONS AND NON-SUSTAINED VT
Premature ventricular contractions (see Fig. 247-1A) can be due to automaticity or reentry (see Chap. A9). They are often sensitive to sympathetic stimulation and can be a sign of increased sympathetic tone, myocardial ischemia, hypoxia, electrolyte abnormalities, particularly hypokalemia, or underlying heart disease. During myocardial ischemia or in association with other heart disease, PVCs can be a harbinger of sustained VT or VF.
The ECG characteristics of the arrhythmia are often suggestive of whether structural heart disease is present. PVCs with smooth uninterrupted contours and sharp QRS deflections suggest an ectopic focus in relatively normal myocardium whereas broad notching and slurred QRS deflections suggest a diseased myocardial substrate. The QRS morphology also suggests the likely site of origin within the ventricle (see Fig. 247-4). PVCs that have a dominant S-wave in V1, referred to as left bundle branch block–like configuration originate from the right ventricle or interventricular septum. Those with a dominant R-wave in V1 originate from the left ventricle. A superior frontal plane axis (negative in II, III, AVF) indicates initial depolarization of the inferior wall (diaphraghmatic aspect of the heart), while an inferior frontal plane axis (positive in II, III, AVF) indicates an origin in the cranial aspect of the heart. The location of arrhythmia origin often suggests the nature of underlying heart disease. Most ventricular arrhythmias that are not associated with structural heart disease have a left bundle branch block–like configuration. PVCs with RBBB configuration are more likely to be associated with structural heart disease. Multiple morphologies of PVCs (multifocal PVCs) are also more likely to indicate structural heart disease (see Fig. 247-1B). In patients with heart disease, a greater frequency and complexity (couplets and non-sustained VT) of these arrhythmias are associated with more severe disease.
PVCs AND NON-SUSTAINED VT DURING ACUTE ILLNESS
These arrhythmias are often encountered in patients who are being evaluated in the emergency room, or who have been hospitalized and are on a cardiac monitor. When encountered during acute illness or as a new finding, evaluation should focus on detection and correction of potential aggravating factors and causes, specifically myocardial ischemia, ventricular dysfunction and electrolyte abnormalities, most commonly hypokalemia. Underlying heart disease should be defined.
PVCs AND NON-SUSTAINED VT IN PATIENTS WITHOUT HEART DISEASE
The most frequent site of origin for idiopathic ventricular arrhythmias is the right ventricular outflow tract, giving rise to PVCs or VT that have a left bundle branch block–like configuration, with an inferiorly directed frontal plane axis as discussed below (see Fig. 247-2). However, QRS morphology alone is not reliable as an indicator of disease or subsequent risk. Non-sustained VT is usually monomorphic with rates <200 beats/min and typically lasting <8 beats (see Fig. 247-2). Non-sustained VT that is very rapid, polymorphic, or with a first beat that ...