Why does inducing therapeutic hypothermia (TH) increase survival rates for out-of-hospital cardiac arrests triggered by ventricular fibrillation? And why have healthcare agencies, particularly EMS, been slow to implement this treatment?
Annually, nearly 450,000 people in the U.S. are victims of sudden death from cardiac arrest (CA).1 Despite numerous treatment options, survivals to hospital discharge have remained relatively unchanged over the last 50 years.2 While technological advances are increasing the survivability of cardiac arrest, patients are still not living long enough to be discharged from the hospital.
Therapeutic hypothermia has been associated with improving the survival rate of cardiac arrest patients, as well as helping reduce their neurological impairment. However, it may need to be implemented as soon as possible for maximal benefit. The earliest stage in the chain of survival at which TH can be implemented is in the EMS system. But EMS agencies can't begin TH if it can't be continued by receiving hospitals.
Several factors impede the implementation of TH in hospitals. This article evaluates the physiology of TH and analyzes the factors preventing its implementation.
Annually, around 300,000 adults in the United States experience out-of-hospital cardiac arrests.3 These patients have a variety of treatment regimens available. With the pharmacological therapy, intra-aortic balloon pumps, state-of-the-art intensive care units and extracorporeal membrane oxygenation treatments we have today,2 cardiac arrest care has progressed tremendously. However, according to the University of Pennsylvania's Raina M. Merchant, MD, "TH is the only post-resuscitation therapy shown to improve both survival and reduce disability after cardiac arrest."3
Within the past decade, several research papers focusing on the use of TH after CA have been published.4 However, there are only four trials of mild hypothermia after CA that describe any kind of comparative study, and as treatment could not be blinded, only two of these studies were randomized controlled trials (RCTs).5 Here are brief descriptions of these four studies.
- Bernard, et al, 1997: A prospective study using a historic control group of 22 adult patients who remained unconscious following initial VF out-of-hospital cardiac arrest (OHCA). The hypothermic group was cooled to 33 degrees [all temperatures Centigrade] for 12 hours with surface cooling and icepacks. Patients were then actively rewarmed over 6 hours. Findings: Good neurological outcome (GOS 1 or 2) in 11/22 of hypothermic group. Limitations: Study was not an RCT or multicenter, but a prospective study with a small sample size of 22 historic control. This was a pilot study for a later RCT.
- Yanagawa, et al, 1998: A prospective study from one site in Japan involving 28 adult patients who sustained OHCA and ROSC. Thirteen were selected for TH of 33-34 degrees for 48 hours. Cooling was achieved using cooling blankets. Patients were passively rewarmed at 1 degree a day. Findings: More survivors (7/13 vs. 5/15) and improved neurological outcomes in TH group. However, 11/13 in the TH group vs. 6/15 in the control group developed pneumonia. Limitations: Not an RCT. Used historic controls rather than randomization. Small scale, single center. Predominantly male sample, and TH group was 6 years younger than historic control group.
- Bernard, et al, 2002: Multicenter RCT involving 4 hospitals in Australia with 77 patients remaining unconscious following OHCA. Of these, 43 received TH to 33 degrees for 12 hours. Cooling was achieved using icepacks. Findings: TH group had good neurological recovery (21/43 vs. 9/34 for normothermic group). TH increased survival (21/43 vs. 11/34). Limitations: Odd and even day randomization may be difficult to achieve outside a controlled hospital environment. Strict inclusion criteria, only involved shockable CAs. Excluded women below age 50.
- HACA Study Group, 2002: Multicenter RCT across Europe involving 275 adult patients who sustained OHCA and ROSC. Of these, 137 received TH to 32-34 degrees. for 24 hours with an external cooling device consisting of a mattress and cover that deliver cold air to the entire body. After 24 hours, they were passively rewarmed over 8 hours. Findings: Of TH group, 55% had good neurological outcomes vs. 39% in the normothermic group. Six-month mortality was 41% in the TH group, 55% in the normothermic group. Limitations: Study ended prematurely because of funding difficulties. Strict inclusion criteria caused a delay in sample recruitment.