Real time estimation of brain water content in comatose patients.
OBJECTIVE
Although brain swelling is an important cause of neurological deterioration, real time measurement of brain edema does not currently exist. Because thermal conductivity is proportional to percentage water content, we used the thermal conductivity constant to estimate brain water content (BWC).


METHODS
Between June 2008 and November 2010, 36 comatose brain-injured patients underwent cerebral blood flow monitoring using a thermal diffusion probe in our neurocritical care unit. BWC was estimated hourly utilizing the measured thermal conductivity and the known temperature-adjusted thermal conductivity of water. In vitro experiments were performed to validate this formula using agar, glycerol, and water mixtures with different water content.


RESULTS
Thermal conductivity was highly correlated (R(2) = 0.99) and estimated water content was well correlated with actual water content (mean difference, 0.58%) in the in vitro preparations. The majority of the 36 patients (median age, 57 years; 44% female) had subarachnoid hemorrhage (n = 14) or cardiac arrest (n = 9). Initial BWC at the time of monitoring ranged from 67.3 to 85.5%. Brain regions appearing edematous on computed tomography showed higher estimated BWC than normal-appearing brain regions (79.1 vs 70.2%; p < 0.01). Bolus osmotherapy (20% mannitol or 23.4% hypertonic saline) decreased BWC from 77.2 ± 0.7% (mean ± standard error) at baseline to 76.1 ± 0.5% at 1 hour, 76.5 ± 0.3% at 2 hours, and 76.7 ± 0.2% at 3 hours (all p ≤ 0.03).


CONCLUSIONS
Real time monitoring of BWC is feasible using thermal conductivity. Further studies are needed to confirm the clinical utility of this technique.
Department of Neurology, Seoul National University Hospital, Seoul, South Korea.
Sang-Bae
SB
Ko
H Alex
HA
Choi
Gunjan
G
Parikh
J Michael
JM
Schmidt
Kiwon
K
Lee
Neeraj
N
Badjatia
Jan
J
Claassen
E Sander
ES
Connolly
Stephan A
SA
Mayer
D000368:Aged; D001921:Brain / Q000000981:diagnostic imaging / Q000378:metabolism*; D001929:Brain Edema / Q000175:diagnosis / Q000209:etiology*; D002560:Cerebrovascular Circulation / Q000502:physiology*; D003128:Coma / Q000150:complications*; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D012189:Retrospective Studies; D013223:Statistics as Topic; D013814:Thermal Conductivity; D014057:Tomography, X-Ray Computed; D014867:Water / Q000378:metabolism*
D014867:Water