Effect of Rapid Pacing and T-Wave Scanning on the Relation Between the Defibrillation and Upper-Limit-of-Vulnerability Dose-Response Curves

Abstract

Background The critical-point and upper-limit-of-vulnerability (ULV) hypotheses predict that the ULV dose-response curve should be steeper and to the right of the defibrillation (DF) curve. Yet, some recent experimental data contradict this prediction. Two studies are presented that test two explanations for the contradiction: (1) Testing at a single 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). Methods and Results A nonthoracotomy lead system with a biphasic waveform was used throughout. In eight dogs, the dose-response curve widths (a measure of steepness) were compared between DF data and ULV data gathered at the peak (ULV PK ), middownslope (ULV DWN ), midupslope (ULV UP ), and all times (scanning or ULV SCN ) in the T wave. In another eight 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 pacing interval was chosen to more closely mimic the hemodynamics and activation rate of early VF. ULV data (ULV STD ) at normal heart rates were gathered for all animals. In the first study, scanning significantly 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 significantly 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; DF 80 , 427±60.9 V; P <.001). The subscript 80 signifies that the subject was left in normal sinus rhythm 80% of the time after that stimulus strength was delivered. In the second study, the ULV RAP curve was shifted dramatically to the right, the average ULV 50 RAP being greater than the average DF 90 . Furthermore, 92% of the ULV RAP VF inductions occurred between 10 ms before and 50 ms after the peak of the T wave, suggesting that scanning of the entire T wave may not be necessary. Conclusions With a single rapidly paced ULV sequence with limited T-wave scanning, it may be possible to estimate highly effective defibrillation doses with few VF episodes and high-voltage stimuli.