Moderate Hypothermia (33 degrees C) Decreases the Susceptibility to Pacing-Induced Ventricular Fibrillation Compared with Severe Hypothermia (30 degrees C) by Attenuating Spatially Discordant Alternans in Isolated Rabbit Hearts

Abstract

Background: Severe hypothermia (SH, 30 degrees C) increases the risk of pacing-induced ventricular fibrillation (PIVF) by enhancing spatially discordant alternans (SDA). Whether moderate hypothermia (MH, 33 degrees C), which is clinically used for therapeutic hypothermia, also facilitates SDA remains unclear. We hypothesized that MH attenuates SDA occurrence compared with that achieved by SH, and decreases the susceptibility of PIVF. Methods: Using an optical mapping system, action potential duration (APD)/conduction velocity restitutions and thresholds of APD alternans were determined by Si pacing in Langendorff-perfused isolated rabbit hearts. In the MH group (n = 7), S-1 pacing was performed at baseline (37 degrees C), after 5-min MH, and after 5-min rewarming (37 degrees C). In the SH group (n = 9), pacing was also performed at baseline (37 degrees C), after 5-min SH, and after 5-min rewarming (37 degrees C). The thresholds of APD alternans were defined as the longest Si pacing cycle length at which APD alternans were detected. Results: Although the thresholds of APD alternans were not different between the MH (273 +/- 46 ms) and the SH (300 +/- 35 ms) (p = 0.281) groups, SDA threshold was shorter (at a faster heart rate) during MH (228 +/- 33 ms) than that during SH (289 +/- 42 ms) (p = 0.028). At APD alternans threshold, SH hearts showed more SDA than that during MH (SH: 7 hearts, MH: 2 hearts, p = 0.049). SDA could be induced in all 9 SH hearts (100%), while only 4 MH hearts (57%) had SDA (p = 0.029). The PIVF inducibility during SH (44 +/- 53%) was higher than that during MH (0%) (p = 0.043). Conclusions: Compared with SH, the MH group showed greater attenuation of SDA and decreased the susceptibility of PIVF. Therefore, MH is safer as a procedural guideline for use in clinical therapeutic hypothermia than SH.