Arrhythmogenic Delayed Afterdepolarizations Are Promoted by Severe Hypothermia But Not Therapeutic Hypothermia

Background: Severe hypothermia (SH) is known to be arrhythmogenic, but the effect of therapeutic hypothermia (TH) on arrhythmias is unclear. It is hypothesized that susceptibility to Ca-mediated arrhythmia triggers would be increased only by SH. Methods and Results: Spontaneous Ca release (SCR) and resultant delayed afterdepolarizations (DADs) were evaluated by optical mapping in canine wedge preparations during normothermia (N, 36 degrees C), TH (32 degrees C) or SH (28 degrees C; n=8 each). The slope (amplitude/rise time) of multicellular SCR (mSCR) events, a determinant of triggered activity, was suppressed in TH (24.4 +/- 3.4%/s vs. N: 41.5 +/- 6.0%/s), but significantly higher in SH (96.3 +/- 8.1%/s) producing higher amplitude DADs in SH (35.7 +/- 1.6%) and smaller in TH (5.3 +/- 1.0% vs. N: 10.0 +/- 1.1%, all P<0.05). Triggered activity was only observed in SH. In isolated myocytes, sarcoplasmic reticulum (SR) Ca release kinetics slowed in a temperature-dependent manner, prolonging Ca transient rise time [33 +/- 3 (N) vs. 50 +/- 6 (TH) vs. 88 +/- 12 ms (SH), P<0.05], which can explain the decreased mSCR slope and DAD amplitude in TH. Although the SR Ca content was similar in TH and SH, Ca spark frequency was markedly increased only in SH, suggesting that increased ryanodine receptor open probability could explain the increased triggered activity during SH. Conclusions: Temperature dependence of Ca release can explain susceptibility to Ca-mediated arrhythmia triggers in SH. This may therefore explain the increased risk of lethal arrhythmia in SH, but not during TH.