10-27: Techniques for Evaluating Rhythm Disturbances

The ideal way of establishing a causal relationship between a symptom and a rhythm disturbance is to demonstrate the presence of the rhythm during the symptom. Unfortunately, this is not always easy because symptoms are often infrequent or sporadic.

Patients with aborted sudden death and recent or recurrent syncope are often evaluated and monitored in the hospital. Those with less ominous symptoms may be monitored as outpatients. When episodes are infrequent, use of an event recorder (either implantable or external) is preferable to 24-hour continuous monitoring. Exercise testing may be helpful when the symptoms are associated with exertion or stress. If symptomatic bradyarrhythmias or supraventricular tachyarrhythmias are detected, therapy can usually be initiated without additional diagnostic studies. Further electrophysiologic studies may be useful in evaluating ventricular arrhythmias and some supraventricular arrhythmias.

Caution is required before attributing a patient’s symptom to rhythm or conduction abnormalities observed during monitoring without concomitant symptoms. In many cases, the symptoms are due to a different arrhythmia or to noncardiac causes. For instance, dizziness or syncope in older patients may be unrelated to concomitantly observed bradycardia, sinus node abnormalities, or ventricular ectopy.

HEART RATE VARIABILITY

Although it has long been appreciated that there are periodic fluctuations in heart rate even under basal conditions, considerable interest has been focused on measurements of heart rate variability. These measurements can be made under controlled conditions in the ECG laboratory or from recordings obtained during ambulatory monitoring. Greater fluctuations in heart rate correspond to greater parasympathetic activity, and studies have indicated that greater heart rate variability is associated with a better prognosis and fewer life-threatening arrhythmias in a variety of cardiac conditions. Analyses have used frequency transformation of the R-wave to R-wave cycle length variability to provide indices of the relative balance between parasympathetic and sympathetic activity. In general, a greater contribution of the parasympathetic system is considered to confer a better prognosis. In studies of postinfarction patients and patients with symptomatic arrhythmias, these indices have some prognostic value. Studies of heart rate variability in patients with heart failure have shown that decreases in heart rate variability are associated with worse outcomes.

ELECTROPHYSIOLOGIC TESTING

Electrophysiologic testing using intracardiac ECG recordings and programmed atrial or ventricular (or both) stimulation is useful in the diagnosis and management of many arrhythmias or conduction disturbances. The primary indications for electrophysiologic testing are (1) evaluation of recurrent syncope of presumed cardiac origin, when the ambulatory ECG has not provided the diagnosis; (2) differentiation ...