Therapeutic Hypothermia: What's Hot about Cold.

Reducing body temperature to 33 °C in patients who have been resuscitated from cardiac arrest but who remain comatose can ameliorate anoxic encephalopathy and improve recovery. Experimental animal studies have suggested that cooling to 33 °C also aids the resuscitative process itself, facilitating the resumption of spontaneous circulation (ROSC). The mechanism of cooling benefit is probably the reduction of metabolic demand of most organs, and reduced production of toxic metabolites and reactive oxygen species. External cooling by application of ice or pads through which cold water circulates is effective but requires up to 8 hours to achieve the target temperature of 33 °C. Our goal was to develop a faster method of cooling that could be initiated during cardiopulmonary resuscitation. In anesthetized swine, we induced ventricular fibrillation by passing alternating current down an electrode catheter in the right ventricle. We then ventilated the animals' lungs with liquid perfluorocarbons (PFCs), a technique known as total liquid ventilation (TLV). Perfluorocarbons are oxygen-carrying modules; we pre-oxygenated the PFCs by bubbling 100% O(2) through the solution for 2 minutes before use, and pre-cooled the PFCs to -15 °C. The cold oxygenated PFCs reduced pulmonary artery temperature (a surrogate for myocardial temperature) to 33 °C in about 6 minutes. Using this technique we achieved ROSC in 8 of 11 (82%) animals given TLV versus 3 of 11 (27%) control animals receiving conventional CPR without PFCs (P<0.05). We also compared the cold TLV technique with the administration of intravenous iced saline to achieve hypothermia. Both the cold TLV and cold saline techniques produced rapid hypothermia, but we could achieve ROSC in only 2 of 8 (25%) animals given cold saline versus 7 of 8 (88%) given cold TLV. This result is likely due to the rise in right atrial pressure and corresponding reduction in coronary perfusion pressure caused by volume loading with IV saline, in addition to the higher pO(2) associated with pre-oxygenated PFCs. Cold TLV is a promising technique for achieving rapid intra-arrest and post-resuscitation hypothermia in patients experiencing cardiac arrest.