ST segment alternans

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Synonyms and keywords: ST alternans

Related chapter: electrical alternans

Electrical alternans is an electrocardiographic phenomenon which describes teh beat to beat alternation in the height (ie.e amplitude), duration (the length of the interval) or the direction (up or down) of any of the EKG complexes or intervals. While electrical alernans is usually thought of as alternation in the height or direction of the QRS complex, there can also be P wave, PR interval, QRS complex, R-R interval, ST segment, T wave, or U wave forms of alternans (see classification below).

• 1909: First described by Hearing
• 1910: Elaborated upon by Sir Thomas Lewis
• 1948: First description of electrical alternans on the surfce EKG by Kalter and Schwartz ^[1]
• 1978: The term ‘pseudoelectrical alternans’ was first described by Klein, Segni and Kaplinsky^[2]

ST segment alternans is defined as beat to beat variation in the magnitude or amplitude of ST segment elevation. It is due to abnormalities of calcium handling in the sarcoplastic reticulum as a result of myocardial ischemia.

It is most often due to myocardial ischemia and includes causes such as:

• Vasospastic angina pectoris
• Acute myocardial infarction
• Angina pectoris
• Myocardial ischemia during an exercise tolerance test
• Percutaneous transluminal coronary angioplasty (PTCA)
• Subarachnoid hemorrhage

The presence of ST segment alternans in the setting of myocarial ischemia is associated with ventricular tachycardia and ventricular fibrillation.

The electrical form of alternans should be differentiated from the mechanical form alternans in which case there is alternation of the strength of the pulse as is observed in pulsus alternans. Electrical and mechanical alternans may coexist.