Electrical alternans and the onset of rate-induced pulsus alternans during acute regional ischaemia in the anaesthetised pig heart.

OBJECTIVES: Electrical alternans and mechanical alternans are both associated with cardiac ischaemia and in the case of electrical alternans there is a strong link with serious ventricular arrhythmia. We elected to investigate the relationship between electrical and mechanical alternans in control and acutely ischaemic myocardium in the intact porcine heart to determine the nature of their interaction and in particular to determine if abnormal mechanical events play a role in arhythmogenesis as has been suggested in non-ischaemic preparations.

METHODS: We used rapid atrial pacing to induce regional mechanical alternans and pulsus alternans before and then at 5-min intervals after the onset of acute ischaemia induced by a 30-min ligation of a diagonal branch of the left anterior descending artery. Regional mechanical activity is measured with epicardial tripodal strain gauges and regional electrical activity is measured using suction-based monophasic action potential electrodes. To test whether alternate stretching of ischaemic segments during pulsus alternans contributed to electrical alternans we simulated pulsus alternans by clamping the proximal aorta on alternate beats.

RESULTS: In control areas there was a constant discordant relationship between peak systolic pressure during alternans and action potential duration. In contrast, the ischaemic areas showed electromechanical alternans that was most frequently concordant. Clamping the proximal aorta on alternate beats produced an electrical alternans in control areas but not in the ischaemic area.

CONCLUSIONS: Pulsus alternans during acute ischaemia is associated with electrical alternans that can be out of phase in control and ischaemic areas. This could increase electrical dispersion which may be pro-arrhythmic.