Sympathetic modulation of the relation between ventricular repolarization and cycle length.

Sympathetic influences on ventricular repolarization are not yet fully elucidated, despite their relevance to arrhythmogenesis. The sympathetic control of repolarization, measured from an endocardial monophasic action potential duration (APD) and from the QT interval, was investigated in 24 anesthetized cats. The effects of right and left stellectomy and of subsequent bilateral stellectomy or beta-blockade on the relation between APD (or QT) and cycle length (CL) at steady state, and on the kinetics of adaptation of APD to a sudden change in cycle length were studied separately. Steady-state APD/CL (or QT/CL) relations were obtained by atrial pacing at different cycle lengths. The kinetics of APD adaptation were evaluated for a sudden decrease of approximately 100 msec in pacing cycle length. The steady-state APD/CL (QT/CL) relation was fitted by the hyperbolic function APD = CL/[(a. CL) + b]. From this, two parameters were computed: 1) 1/a, that is, APD (QT) extrapolated at infinite cycle length (APDmax ...