Hypothermia
The effects of hypothermia on cerebral physiology and its potential
cerebral protective mechanisms have been presented in Chapter
21
(see Table 21-8 and text). There have been numerous laboratory demonstrations
of the efficacy of mild hypothermia (32°C to 34°C) in reducing the neurologic
injury occurring after standardized cerebral and spinal cord ischemic insults. These
investigations have resulted, in spite of the absence of any demonstration of efficacy
in humans, in the relatively widespread use of induced hypothermia in the management
of cerebral vascular procedures, in particular, aneurysms[152]
[153]
and occasionally AVMs. An international multicenter
trial of mild hypothermia in aneurysm surgery has been undertaken. Enrollment of
the intended 1000 patients was completed in April 2003 (M.M. Todd, personal communication),
but the results were unavailable at the time of this writing.
Despite the fact that mild hypothermia is perceived to convey
certain hazards, including coagulation dysfunction and an increased postoperative
wound infection rate, neither of these problems has been evident in the context of
aneurysm surgery, although once again no systematic study has been conducted. Anecdotally,
hypertension on emergence has been noted to occur in patients who are not adequately
rewarmed, and a modest overshoot in temperature has been observed to occur in patients
who were cooled intraoperatively.[154]
The issue
of where body temperature should be recorded to best reflect brain temperature during
craniotomy has been addressed.[155]
It appears
that esophageal, tympanic membrane, pulmonary artery, and jugular bulb temperatures
are all very similar and provide a reasonable reflection of deep brain temperature
whereas bladder temperature does not. Superficial layers of cortex may be substantially
cooler than deep brain and central temperatures.
Because ischemia is recognized to make a postinsult contribution
to neuronal injury after head injury,[28]
[156]
hypothermia was also studied in the laboratory in the context of traumatic brain
injury.[157]
Its efficacy resulted in at least
four single-institution trials of 24 to 48 hours of mild hypothermia after head injury.
These trials demonstrated either significant improvement in outcome or favorable
trends.[158]
[159]
[160]
[161]
The
investigations revealed some physiologic dysfunction associated with more prolonged
mild hypothermia, all of which was reversible with restoration of normal temperature.
Physiologic dysfunction included decreased creatinine clearance, elevation of pancreatic
enzymes, and a suggestion of an increased infection rate. A decreased incidence
of seizures was an apparent adjunctive benefit in one study. As a result, a multicenter,
prospective trial of hypothermia was performed but did not show a benefit of hypothermia
(33°C) induced within 8 hours of injury and maintained for 48 hours.[162]
Post hoc subgroup analysis indicated that patients younger than 45 years who arrived
at the tertiary care facility with a temperature lower than 35°C and were randomized
to the cooling limb of the trial did, in fact, have an improved outcome. A follow-up
trial in which the protocol seeks to achieve cooling more rapidly is under way.
On the basis of lack of demonstrated efficacy in humans, routine
use of hypothermia in neurosurgery cannot be advocated in a standard text. The decision
to use it, generally in the context of aneurysm surgery, will be a local one. If
used, attention should be paid to the possibility of cardiac dysrhythmias and coagulation
dysfunction if temperatures become too low, as well as the necessity of adequate
rewarming before emergence to avoid shivering and hypertension.
Yet another application of mild hypothermia is in evolution.
Very recently, two multicenter trials have demonstrated improved neurologic outcome
in survivors of witnessed cardiac arrest who were cooled to 32°C to 34°C
within 4 hours and maintained at that temperature for 12 to 24 hours.[163]
[164]
Consideration of bringing this therapy to
bear on victims of intraoperative cardiac arrest or other neurologically threatening
events[165]
seems inevitable, but to our knowledge,
it has not yet been attempted.
