EFFECT OF RAPID PACING AND T-WAVE SCANNING ON THE RELATION BETWEEN THE DEFIBRILLATION AND UPPER-LIMIT-OF-VULNERABILITY DOSE-RESPONSE CURVES

Background The critical-point and upper-limit-of-vulnerability (ULV) h ypotheses predict that the ULV dose-response curve should be steeper a nd to the right of the defibrillation (DF) curve. Yet, some recent exp erimental data contradict this prediction. Two studies are presented t hat test two explanations for the contradiction: (1) Testing at a sing le point in the T wave underestimates the ULV dose-response curve and (2) ULV testing at normal heart rates does not mimic the mechanical or electrical state of the heart in ventricular fibrillation (VF). Metho ds and Results A nonthoracotomy lead system with a biphasic waveform w as used throughout. In eight dogs, the dose-response curve widths (a m easure of steepness) were compared between DF data and ULV data gather ed at the peak (ULV(PK)), middownslope (ULV(DWN)), midupslope (ULV(UP) ), and all times (scanning or ULV(SCN)) in the T wave. In another eigh t dogs, ULV data (ULV(RAP)) were gathered by scanning the T wave after 15 rapidly paced beats (166- to 198-ms pacing interval). The rapid pa cing interval was chosen to more closely mimic the hemodynamics and ac tivation rate of early VF. ULV data (ULV(STD)) at normal heart rates w ere gathered for all animals. In the first study, scanning significant ly reduced the ULV curve width (ULV(SCN), 63.5+/-29.7 V; ULV(PK), 81.9 +/-45.2 V; ULV(DWN), 116+/-36.5 V; DF, 105+/-22.0 V; P<.03) and signif icantly shifted the ULV curve to the right (ULV(80) (SCN), 410+/-62.6 V; ULV(80) (PK), 266+/-35.3 V; ULV(80) (DWN), 355+/-80.4 V; DF80, 427/-60.9 V; P<.001). The subscript 80 signifies that the subject was lef t in normal sinus rhythm 80% of the time after that stimulus strength was delivered. In the second study, the ULV(RAP) curve was shifted dra matically to the right, the average ULV(50) (RAP) being greater than t he average DF90. Furthermore, 92% of the ULV(RAP) VF inductions occurr ed between 10 ms before and 50 ms after the peak of the T wave, sugges ting that scanning of the entire T wave may not be necessary. Conclusi ons With a single rapidly paced ULV sequence with limited T-wave scann ing, it may be possible to estimate highly effective defibrillation do ses with few VF episodes and high-voltage stimuli.