SYMPATHETIC INFLUENCES ON ELECTRICAL AND MECHANICAL ALTERNANS IN THE CANINE HEART

Objective: The aim was to investigate me influence of the sympathetic nervous system on the induction of mechanical and electrical alternans in the intact canine heart. Methods: Experiments were performed on 8 open-chest dogs anesthetized with sodium pentobarbital. A micromanomet er-tipped catheter was used to measure left ventricular pressure, dp/d t and the time constant of isovolumic relaxation. Rapid atrial pacing was used to induce alternans and the left stellate ganglion was stimul ated electrically to alter sympathetic tone. The longest pacing cycle length that showed a significant alternation in peak systolic pressure was defined as the alternans threshold. Electrical alternans was dete cted by comparing the ST-T area in the surface ECG (lead II) on altern ate beats. Results: The alternans threshold was 305(s.e.m. 10.4) ms un der control conditions and decreased to 271(12.1), 225(33.4), and 177( 6.2) ms, as the frequency of left stellate stimulation was increased t o 1, 2, and 5 Hz, respectively (P < 0.001). Tau and peak -dp/dt began to alternate at the same pacing cycle length as peak +dp/dt and peak s ystolic pressure. Electrical alternans was only observed during mechan ical alternans and the ST-T area of the strong beat was 243(143)% grea ter than the ST-T area of the weak beat (P < 0.001). Timolol (1 mg . k g(-1)) blocked the effect of left stellate stimulation (1 and 2 Hz) on mechanical and electrical alternans. Conclusions: Left sympathetic ac tivation causes a frequency-dependent reduction in the threshold cycle length for global mechanical and electrical alternans. Alternation in relaxation occurs at the same pacing cycle length as does alternation in contraction. Repolarization alternans in the surface ECG appears t o reflect underlying mechanical events.