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.