Late potentials in mitral valve prolapse.

To determine the incidence and significance of late potentials in patients with mitral valve prolapse, we performed surface signal-averaged electrocardiography and 24-hour ambulatory electrocardiographic (ECG) monitoring in 41 patients with moderate to severe mitral valve prolapse on two-dimensional echocardiograms. Late potentials were defined as the presence of either a root mean square voltage of the last 40 msec of the QRS (RMS-40) of less than 20 mu v or a low-amplitude signal duration (LAS-40) of greater than 39 msec. Despite the absence of clinically significant ventricular tachycardia by history and on ambulatory ECG monitoring, 12 patients had late potentials on their signal-averaged electrocardiograms. Clinical characteristics could not differentiate patients with from patients without late potentials, and all patients were doing well at a mean follow-up of 34 months except for one noncardiac death. We conclude that late potentials on the surface signal-averaged electrocardiogram are a common and benign finding in patients with mitral valve prolapse and their clinical significance should be determined only in the presence of other findings.