Detection of Rapid Changes in Ventricular Refractoriness in Human Studies.

Conventional determination of the ventricular effective refractory period (VERP) is unsuitable for detection of rapid fluctuations in the effective refractory period (ERP). A programmed stimulation system was developed that adapts continuous atrioventricular sequential pacing, incremental extrastimulus interval (S₁S₂) scanning, and automatic detection of extrastimulus capture which is followed by shortening of S₁S₂ to execute repeated scanning. The accuracy of ERP determination was tested using variable incremental (2 and 4 ms) and decremental (4-16 ms) steps of the S₁S₂ interval. Based on a mean of 82 determinations in eight patients, the average VERP values stayed at 249.8-251.0 ms except during the highest capture frequency. Standard deviation of ERP values ranged from 1.1 to 2.5 ms on average at the tested incremental and decremental steps. One determination was accomplished within 7.8-15.6 seconds on average. The ability to track changes in ERP was tested by changing the drive cycle length. Time constants for the adaptation rate of VERP and ventricular monophasic action potential duration at a 90% level of repolarization were determined from each test, and were similar, 51 ± 8 seconds (mean ± SEM) for ERP and 51 ± 6 seconds for the action potential duration. Thus, the developed method provides accurate ERP measurements during rapid variation in ventricular refractoriness. It allows studying the recovery of excitability and the action potential duration simultaneously, and would be valuable particularly in pathological conditions and pharmacologic interventions where these electrophysiological variables become dissociated. [ABSTRACT FROM AUTHOR]