Sinus bradycardia is the term for a heart rate of less than 60 beats/min and may be seen in the normal adult population. Sinus bradycardia during exercise, fever or congestive heart failure is abnormal. Persistent rates of less than 45 beats/min are also considered abnormal, and in the absence of drugs such as digitalis, beta-blockers and calcium channel blockers, reflect abnormality in the sinus node.
Sinus bradycardia can be present in otherwise normal individuals and is common in well-trained athletes and in most people during deep sleep. It is part of the normal reaction to vagal stimulation. Sinus bradycardia may also be related to metabolic abnormalities, including hypothermia and myxedema. Nonvagally mediated sinus bradycardia also may occur as a manifestation of organic heart disease, including ischemic heart disease, particularly when the SA node is damaged, as with certain types of acute MI and in association with severe chest pain of acute MI. Sinus bradycardia may also be a complication of myocardial disease in which the SA node is damaged by scarring or infiltrative processes associated with aging as part of a degenerative conduction system process.
Asymptomatic sinus bradycardia requires no treatment. If sinus bradycardia is so extreme, however, that symptoms result, including syncope, congestive heart failure (CHF), angina pectoris and hypertension, and/or if it leads to the development of ventricular ectopic beats (slow sinus rates predispose to re-entry mechanisms), then it should be treated, and in some instances temporary and sometimes permanent ventricular pacing is necessary.