BJA Advance Access published online on May 16, 2007
British Journal of Anaesthesia, doi:10.1093/bja/aem118
Cerebrovascular reactivity during hypothermia and rewarming
1 Department of Clinical Neurosciences, Academic Neurosurgical Unit
2 Department of Anaesthesiology, Addenbrooke's Hospital, Cambridge, UK
3 Institute of Anaesthesiology and Intensive Care Medicine, University of Brescia, Brescia, Italy
* Corresponding author: Department of Clinical Neurosciences, Academic Neurosurgical Unit, Box 167, Addenbrooke's Hospital, Cambridge, UK. E-mail: mc141{at}medschl.cam.ac.uk
Background: Experimental evidence from a murine model of traumatic brain injury (TBI) suggests that hypothermia followed by fast rewarming may damage cerebral microcirculation. The effects of hypothermia and subsequent rewarming on cerebral vasoreactivity in human TBI are unknown.
Methods: This is a retrospective analysis of data acquired during a prospective, observational neuromonitoring and imaging data collection project. Brain temperature, intracranial pressure (ICP), and cerebrovascular pressure reactivity index (PRx) were continuously monitored.
Results: Twenty-four TBI patients with refractory intracranial hypertension were cooled from 36.0 (0.9) to 34.2 (0.5)°C [mean (SD), P<0.0001] in 3.9 (3.7) h. Induction of hypothermia [average duration 40 (45) h] significantly reduced ICP from 23.1 (3.6) to 18.3 (4.8) mm Hg (P<0.05). Hypothermia did not impair cerebral vasoreactivity as average PRx changed non-significantly from 0.00 (0.21) to –0.01 (0.21). Slow rewarming up to 37.0°C [rate of rewarming, 0.2 (0.2)°C h–1] did not increase ICP [18.6 (6.2) mm Hg] or PRx [0.06 (0.18)]. However, in 17 (70.1%) out of 24 patients, rewarming exceeded the brain temperature threshold of 37°C. In these patients, the average brain temperature was allowed to increase to 37.8 (0.3)°C (P<0.0001), ICP remained stable at 18.3 (8.0) mm Hg (P=0.74), but average PRx increased to 0.32 (0.24) (P<0.0001), indicating significant derangement in cerebrovascular reactivity. After rewarming, PRx correlated independently with brain temperature (R=0.53; P<0.05) and brain tissue O2 (R=0.66; P<0.01).
Conclusions: After moderate hypothermia, rewarming exceeding the 37°C threshold is associated with a significant increase in average PRx, indicating temperature-dependent hyperaemic derangement of cerebrovascular reactivity.
Keywords: autoregulation; brain, injury; hypothermia; monitoring, critical care
Declaration of interest. ICM+ software (www.neurosurg.cam.ac.uk/icmplus) is licensed by University of Cambridge, UK and P.S. and M.C. have a financial interest in the licensing fee.
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