Effects of mild hypothermia on hemodynamics in cardiac arrest survivors and isolated failing human myocardium.

Post-cardiac arrest myocardial dysfunction is a common phenomenon after return of spontaneous circulation (ROSC) and contributes to hemodynamic instability and low survival rates after cardiac arrest. Mild hypothermia for 24 h after ROSC has been shown to significantly improve neurologic recovery and survival rates. In the present study we investigate the influence of therapeutic hypothermia on hemodynamic parameters in resuscitated patients and on contractility in failing human myocardium. We analyzed hemodynamic data from 200 cardiac arrest survivors during the hypothermia period. The initial LVEF was 32.6 +/- 1.2% indicating a significantly impaired LV function. During hypothermia induction, the infusion rate of epinephrine could be significantly reduced from 9.1 +/- 1.3 microg/min [arrival intensive care unit (ICU) 35.4 degrees C] to 4.6 +/- 1.0 microg/min (34 degrees C) and 2.8 +/- 0.5 microg/min (33 degrees C). The dobutamine and norepinephrine application rates were not changed significantly. The mean arterial blood pressure remained stable. The mean heart rate significantly decreased from 91.8 +/- 1.7 bpm (arrival ICU) to 77.3 +/- 1.5 bpm (34 degrees C) and 70.3 +/- 1.4 bpm (33 degrees C). In vitro we investigated the effect of hypothermia on isolated ventricular muscle strips from explanted failing human hearts. With decreasing temperature, the contractility increased to a maximum of 168 +/- 23% at 27 degrees C (n = 16, P < 0.05). Positive inotropic response to hypothermia was accompanied by moderately increased rapid cooling contractures as a measure of sarcoplasmic reticulum (SR) Ca(2+) content, but can be elicited even when the SR Ca(2+) release is blocked in the presence of ryanodine. Contraction and relaxation kinetics are prolonged with hypothermia, indicating increased Ca(2+) sensitivity as the main mechanism responsible for inotropy. In conclusion, mild hypothermia stabilizes hemodynamics in cardiac arrest survivors which might contribute to improved survival rates in these patients. Mechanistically, we demonstrate that hypothermia improves contractility in failing human myocardium most likely by increasing Ca(2+)-sensitivity.