title: Spontaneous hyperventilation and brain tissue hypoxia in patients with severe brain injury.
abstract: 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.
authors: Department of Neurology, Division of Neurocritical Care, Columbia University Medical Center, 177 Fort Washington Avenue, New York, NY 10032, USA.
authors: Emmanuel
authors: E
authors: Carrera
authors: J Michael
authors: JM
authors: Schmidt
authors: Luis
authors: L
authors: Fernandez
authors: Pedro
authors: P
authors: Kurtz
authors: Maxwell
authors: M
authors: Merkow
authors: Morgan
authors: M
authors: Stuart
authors: Kiwon
authors: K
authors: Lee
authors: Jan
authors: J
authors: Claassen
authors: E
authors: E
authors: Sander Connolly
authors: Stephan A
authors: SA
authors: Mayer
authors: Neeraj
authors: N
authors: Badjatia
mesh_terms: 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
chemical_list: D002245:Carbon Dioxide