A review published in this week’s The Lancet claims that induced hypothermia is underused in the UK and in the USA. This practice of deliberately cooling the body is capable of preventing or limiting permanent injuries if it is employed within the first couple hours of a clinical event.
Dr Kees Polderman (University Medical Center Utrecht, Netherlands) first cites evidence that has demonstrated improved outcomes after ischemic injury (reduction in blood supply) when body temperature is reduced from 37ºC to between 32- 35ºC – a level of mild hypothermia. Though positive effects have been shown most clearly for brain injuries, it is probable that body temperature reduction can positively benefit injuries to the heart and kidneys, among other organs. The practice has already been used to treat heart attack and stroke.
“Hypothermia is a highly promising treatment in neurocritical care; thus, physicians caring for patients with neurological injuries, both in and outside the intensive care unit, are likely to be confronted with questions about temperature management more frequently,” says Polderman.
There are three phases in the process of lowering body temperature. Induction first cools the body to a specified temperature – usually through the highly effective and safe method of cold fluid (4ºC) infusion. The second phase involves maintenance, which can be for several days if the hypothermia is induced to treat traumatic brain injuries. The third phase, rewarming, must be slow and controlled. The rates are usually about 0.2 to 0.5ºC per hour in cardiac arrest patients and even slower in patients with traumatic brain injury. Studies on animals have demonstrated that rapid rewarming leads to adverse outcomes whereas slow rewarming maintains the benefits of the temperature reduction.
The review discusses several physiological reasons for lower body temperature’s injury protection capabilities. Since lower temperatures reduce the permeability of the blood brain barrier (the membrane that protects the brain from chemicals in the blood), brain injury patients can limit damage from trauma or blood vessel blockage. Induced hypothermia can also limit the rate of formation of small blood clots, or thrombi, which can occur after brain injuries. Additionally, the immune response is depressed by lower body temperatures, preventing inflammatory reactions that could harm the brain or other organs after injury. Fever prevention is also seen as a practical use, as fever development can further harm patients with brain injuries.
“Use of mild hypothermia seems to be a major breakthrough in the treatment of neurological injuries… Studies that establish optimum depth and duration of cooling are also needed. Increasing evidence suggests that fever is harmful to the injured brain, and it seems reasonable to maintain normothermia in most patients with neurological injuries who have decreased consciousness – especially in those previously treated with hypothermia – for at least 72 hours after injury. Hypothermia remains widely underused in many countries, especially in the USA and, to a lesser extent, the UK and Germany; therefore, applying the existing evidence and working on implementation strategies should be a priority,” concludes Polderman.
Induced hypothermia and fever control for prevention and treatment of neurological injuries
K H Polderman
The Lancet (2008). 371: p 1955.
Written by: Peter M Crosta