| Abstrakt: |
This study compared the ventricular effective refractory periods measured by scanning diastole with an extrastimulus in incremental and decremental steps of 5 msec. The subjects of the study were 80 patients undergoing a clinically indicated electrophysiological test. Eight beat basic drive trains at a cycle length of 600 msec and an intertrain pause of 4 seconds were used to measure the ventricular effective refractory period (VERP). In the incremetal method, the extrastimulus initially was positioned at a coupling interval shorter than the VERP and the coupling interval then was progressively increased until ventricular capture occurred. In the decremental method, the initial extra-stimulus coupling interval was longer than the VERP and the coupling interval was progressively shortened until ventricular capture was lost. In 50 subjects, the mean VERP determined by the incremental method, 252 ± 18 (± standard deviation), was significantly longer than the mean VERP determined in the same patients by the decremental method, 248 ± 18 msec (P < 0.0001). In ten subjects, a subthreshold stimulus (S') positioned 10 msec earlier than the VERP had an inhibitory effect that lengthened the VERP by an average of 7 msec; however, when S' was positioned after the seventh beat of an eight beat drive train, no inhibitory effect could be demonstrated. In 20 subjects, VERP's were determined by the incremental and decremental methods using intertrain pauses of 1, 4, 8, 12, and 20 seconds. The mean VERP measured by the incremental method was significantly less than the mean VERP measured by the decremental method when the intertrain pause was 1, 4, or 8 seconds, but not when the pause was 12 or 20 seconds. The results of this study demonstrate that incremental scanning of an extrastimulus with eight blat basic drive trains yields a longer VERP than when the extrastimulus is scanned in decremental fashion. The discrepancy between the two methods is not attributable to inhibition by noncapturing extrastimuli in the incremental method, but rather to a decrease in the VERP caused by an effect of extrastimuli that capture the ventricle in the decremental method. Therefore, when a conventional eight beat drive train and 4 second intertrain pause are used to measure ventricular refractoriness, incremental scanning of an extrastimulus yields a more accurate VERP than does decremental scanning. [ABSTRACT FROM AUTHOR] |
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