How Low Should We Go?

Current guidelines from the American Heart Association recommend use of therapeutic hypothermia (TH) after witnessed cardiac arrest (CA) to mitigate posthypoxic injuries. This is based on results of 3 randomized, controlled trials (RCTs) enrolling 385 patients, 43 before–after studies enrolling 10 442 patients, and supporting evidence from the field of neonatal asphyxia where 7 RCTs enrolling 1329 patients also demonstrated neuroprotective effects of hypothermia. However, this has been called into question by a recently published RCT enrolling 939 patients, which found no benefit of cooling to 33°C compared with maintaining 36°C. In this article we review the literature, with extra attention for strengths and weaknesses of the recently published RCT. In view of potential weaknesses in the new study (including a possibility of selection bias, long delays before initiation of cooling, a time to target temperature of 10 hours, and a rapid rewarming rate), we conclude that there are sufficient grounds to continue using hypothermia in most patients with witnessed ventricular fibrillation (VF)/ventricular tachycardia (VT) arrest, pending results of further studies which should examine multiple temperature levels (32–36°C) and multiple treatment durations (24–72 hours). The bases for these conclusions are discussed in detail below.The use of TH to mitigate various types of injury, in particular posthypoxic injury to the brain, has been studied since the late 1930s. Interest was initially kindled by reports of survival after prolonged exposure to cold, or submersion in ice-cold water, indicating a possible protective effect of low temperature on hypoxic injuries.1Use of hypothermia after CA was first described in the late 1950s,2,3 but conclusive proof that hypothermia could improve outcome in these patients remained elusive.4,5 At the time it was thought that protective effects of TH were purely a result of hypothermia-induced lowering of metabolism; therefore, it …