Two‐Dimensional Vector Analysis of Beat‐to‐Beat Dynamics of Ventricular Repolarization

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.