Induced hypothermia -- that is, deliberate cooling of the body -- can prevent or mitigate permanent injuries if implemented in the first few hours after a clinical event takes place. Yet in various countries, among them the UK and USA, such strategies are not widely applied. The issues are discussed in a Review in this week's edition of The Lancet, authored by Dr Kees Polderman, University Medical Centre Utrecht, Netherlands.
Increasing evidence suggests that induction of mild hypothermia by lowering the body temperature from 37ºC to between 32-35 ºC has a positive effect after ischaemic injury takes place. This effect has been shown most clearly for brain injury but could also apply to other organs such as the heart and the kidneys. It has been used as a treatment for heart attack and stroke. Dr Polderman says: "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."
There are many physiological reasons why lower body temperatures can help protect against injury -- all are discussed at length in the Review. For example, in brain injury lower temperatures reduce the permeability of the blood brain barrier, which can be disrupted by trauma or blood vessel blockages. Small blood clots (thrombi) can form post injury, and lower temperatures reduce the rate of formation of these clots. The immune response is also depressed by hypothermia -- this helps by preventing potentially harmful proinflammatory reactions of the immune system in the brain or other injured organs following an injury. The flip side is that development of fever can be harmful for brain-injured patients -- hence fever prevention should be an important aspect of care for these patients.
Induced hypothermia can be divided into three phases. The first, induction, is cooling the body to the target temperature. One highly effective and safe-method to jump start cooling is through infusion of cold fluids (4ºC). Then comes the maintenance phase, and finally the rewarming phase. The maintenance phase must be far longer (several days) if hypothermia is used for traumatic brain injury. Rewarming should be slow and controlled, eg, 0.2-0.5 ºC per hour in cardiac arrest patients, and even slower in patients with traumatic brain injury. Animal studies have that rapid rewarming can adversely affect outcome and that slow rewarming preserves the benefits of cooling.
Dr Polderman concludes: "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."
Dr Kees Polderman, University Medical Centre Utrecht, Netherlands T) +31 655 157 833 E) k.polderman@tip.nl
http://multimedia.thelancet.com/pdf/press/inducedhypothermia.pdf
Journal
The Lancet