Abnormalities of cardiac rhythm and conduction can be lethal (sudden cardiac death), symptomatic (syncope, near syncope, dizziness, or palpitations), or asymptomatic.

They are dangerous to the extent that they reduce cardiac output, so that perfusion of the brain or myocardium is impaired, or tend to deteriorate into more serious arrhythmias with the same consequences. Stable supraventricular tachycardia is generally well tolerated in patients without underlying heart disease but may lead to myocardial ischemia or congestive heart failure in patients with coronary disease, valvular abnormalities, and systolic or diastolic myocardial dysfunction.

Ventricular tachycardia, if prolonged (lasting more than 10-30 seconds), often results in hemodynamic compromise and is more likely to deteriorate into ventricular fibrillation.

Whether slow heart rates produce symptoms at rest or on exertion depends upon whether cerebral perfusion can be maintained, which is generally a function of whether the patient is upright or supine and whether left ventricular function is adequate to maintain stroke volume. If the heart rate abruptly slows, as with the onset of complete heart block or sinus arrest, syncope or convulsions may result.

Arrhythmias are detected either because they present with symptoms or because they are detected during the course of monitoring. Arrhythmias causing sudden death, syncope, or near syncope require further evaluation and treatment unless they are related to conditions that are unlikely to recur (eg, electrolyte abnormalities or acute myocardial infarction). In contrast, there is controversy over when and how to evaluate and treat rhythm disturbances that are not symptomatic but are possible markers for more serious abnormalities (eg, nonsustained ventricular tachycardia). This uncertainty reflects two issues: (1) the difficulty of reliably stratifying patients into high-risk and low-risk groups, and (2) the lack of treatments which are both effective and safe. Thus, screening patients for these so-called “premonitory” abnormalities is often not productive.

A number of procedures are employed to evaluate patients with symptoms who are felt to be at risk for life-threatening arrhythmias, including in-hospital and ambulatory electrocardiographic monitoring, event recorders (instruments that can be worn for prolonged periods in order to record or transmit rhythm tracings when infrequent episodes occur), exercise testing, intracardiac electrophysiologic studies (to assess sinus node function, atrioventricular conduction, and inducibility of arrhythmias), signal-averaged ECGs, and tests of autonomic nervous system function (especially tilt-table testing). These are discussed below and in the subsequent sections on individual rhythm disturbance and symptomatic presentation. In general, these techniques are more successful in diagnosing symptomatic arrhythmias than in predicting the outcome of asymptomatic ones.