| Abstract: | Objective: Duration of ventricular repolarization is a result of complex interaction between autonomic modulation and heart rate (HR). Methods: To study the dynamics of ventricular repolarization of the human right ventricle, beat-to-beat variability of ventricular repolarization was measured by plotting the duration of each monophasic action potential at the 90% phase of repolarization (APD90) as a function of the previous APD90 both in sinus rhythm and during steady-state atrial pacing (cycle length of 600 rns) in 12 subjects without structural heart disease. Results: Quantitative analyses of APD90 and RR interval variability in sinus rhythm showed that the total standard deviation (SD) of APD90 was only 8% of the SD of all RR intervals. Both longterm, continuous variability (SD2) of APD90 (3.9 ± 1.5 ms) and instantaneous beat-to-beat variability (SD1) of APD90 (1.2 ± 0.3 ms) were smaller than the SD2 and SD1 of RR intervals (46 ± 17 ms and 15 ± 9 ms, respectively) (P < 0.001 for both), but the shapes of the APD90 and RR interval plots and the SD1/SD2 ratio did not differ. SD1 of APD90 correlated well with the SD1 of RR intervals (r = 0.64, P < 0.05) but no significant correlation was observed between the SD2 of APD90 and SD2 of RR intervals (r = 0.32, NS). During steady-state atrial pacing, only minimal instantaneous beat-to-beat variability in APD90 (0.9 ± 0.3 rns) was observed but the SD2 was larger than the SD2 of RR intervals (2.3 ± 1.0 ms vs. 1.6 ± 0.8 ms, P < 0.001). Conclusion: The results suggest that instantaneous beat-to-beat variability of ventricular repolarization is mainly due to beat-to-beat fluctuation of HR, but the long-term dynamics of repolarization are partly influenced by other factors than the HR variability. |
