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; DF80, 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 DF90. 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.