Spontaneous hyperventilation and brain tissue hypoxia in patients with severe brain injury.
BACKGROUND
Hyperventilation has been shown to be associated with cerebral vasoconstriction and increased risk of infarction. Our aim was to determine whether spontaneous reduction in end-tidal CO(2) (EtCO(2)) was associated with an increased in brain tissue hypoxia (BTH).


METHODS
We studied 21 consecutive patients (mean age 50+/-16 years; 15 women) undergoing continuous monitoring for brain tissue oxygenation (PbtO(2)), intracranial pressure (ICP), cerebral perfusion pressure (CPP) and EtCO(2); mean values were recorded hourly BTH was defined as brain tissue oxygen tension (PbtO(2)) <15 mm Hg.


RESULTS
Diagnoses included subarachnoid haemorrhage (67%), intracranial haemorrhage (24%) and traumatic brain injury (10%). Overall, BTH occurred during 22.5% of the study period (490/2179 hourly data). The frequency of BTH increased progressively from 15.7% in patients with normal EtCO(2) (35-44 mm Hg) to 33.9% in patients with EtCO(2)<25 mm Hg (p<0.001). The mean tidal volume and minute ventilation were 7+/-2 ml/kg and 9+/-2 1/min, respectively. Hypocapnia was associated with higher measured-than-set respiratory rates and maximal minute ventilation values, suggestive of spontaneous hyperventilation. Using a generalised estimated equation (GEE) and after adjustment for GCS, ICP and core temperature, the variables independently associated with BTH events were EtCO(2) (OR: 0.94; 95% CI 0.90 to 0.97; p<0.001) and CPP (OR: 0.98; 95% CI 0.97 to 0.99; p=0.004).


CONCLUSIONS
The risk of brain tissue hypoxia in critically brain-injured patients increases when EtCO(2) values are reduced. Unintentional spontaneous hyperventilation may be a common and under-recognised cause of brain tissue hypoxia after severe brain injury.
Carrera
Emmanuel
E
Department of Neurology, Division of Neurocritical Care, Columbia University Medical Center, 177 Fort Washington Avenue, New York, NY 10032, USA.
Schmidt
J Michael
JM
Fernandez
Luis
L
Kurtz
Pedro
P
Merkow
Maxwell
M
Stuart
Morgan
M
Lee
Kiwon
K
Claassen
Jan
J
Sander Connolly
E
E
Mayer
Stephan A
SA
Badjatia
Neeraj
N
D000328:Adult; D000368:Aged; D001784:Blood Gas Analysis; D001930:Brain Injuries / Q000150:complications*; D002245:Carbon Dioxide / Q000097:blood; D003128:Coma / Q000097:blood / Q000209:etiology; D005260:Female; D015600:Glasgow Coma Scale; D006339:Heart Rate / Q000502:physiology; D006801:Humans; D006985:Hyperventilation / Q000209:etiology*; D002534:Hypoxia, Brain / Q000209:etiology*; D007427:Intracranial Pressure / Q000502:physiology; D008297:Male; D008875:Middle Aged; D011446:Prospective Studies; D012121:Respiration, Artificial; D015656:Respiratory Mechanics / Q000502:physiology; D013990:Tidal Volume / Q000502:physiology
D002245:Carbon Dioxide