Cardiac alternans: diverse mechanisms and clinical manifestations.

OBJECTIVES: The purpose of this review is to assemble the widely dispersed information about cardiac alternans and to categorize the types and mechanisms of alternans, their clinical manifestations and possible therapeutic implications.

BACKGROUND: The phenomena of mechanical and electrical alternans have been of continuing interest to both physiologists and clinicians. Recent studies have enhanced this interest because of the reported association of alternans with experimental myocardial ischemia and cardiac arrhythmias.

METHODS: The review formulates concepts based on extensive review of published studies and personal observations.

RESULTS: Cardiac alternans has been subdivided into the following four categories: 1) mechanical, 2) electrical, 3) in association with myocardial ischemia, and 4) in association with cardiac motion. Mechanical alternans can be explained by hemodynamic or inotropic alterations, or both. Mechanical alternans in the ventricular muscle is accompanied by alternans of action potential shape. In the Purkinje fibers, action potential duration alternates without change in shape and is determined by the duration of the preceding diastolic interval. However, in ventricular muscle fiber, alternans can occur in the presence of constant diastolic intervals. T wave alternans reflects changes in action potential duration and is frequently associated with a long QT interval. Electrocardiographic manifestations of conduction alternans occur at many different sites within the conducting system and myocardium. During myocardial ischemia, additional mechanisms of repolarization alternans have been proposed. Alternans occurring in the presence of a large pericardial effusion is attributed to swinging motion of the heart maintaining two-beat periodicity.

CONCLUSIONS: Since its origin as "pulsus alternans" described by Traube in 1872, the definition of alternans has evolved into a term encompassing multiple physiologic and pathologic phenomena that, although united by the term cardiac alternans, diverge widely with respect to etiology, mechanism and clinical significance.