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Spontaneous hyperventilation and brain tissue hypoxia in patients with severe brain injury
  1. Emmanuel Carrera1,
  2. J Michael Schmidt1,
  3. Luis Fernandez1,
  4. Pedro Kurtz1,
  5. Maxwell Merkow2,
  6. Morgan Stuart2,
  7. Kiwon Lee1,2,
  8. Jan Claassen1,2,
  9. E Sander Connolly2,
  10. Stephan A Mayer1,2,
  11. Neeraj Badjatia1,2
  1. 1Department of Neurology, Division of Neurocritical Care, Columbia University Medical Center, New York, USA
  2. 2Department of Neurosurgery, Columbia University Medical Center, New York, USA
  1. Correspondence to Dr Neeraj Badjatia, Columbia University Medical Center, 177 Fort Washington Avenue, MHB 8-300, New York, NY 10032, USA; nbadjatia{at}


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 CO2 (EtCO2) was associated with an increased in brain tissue hypoxia (BTH).

Method We studied 21 consecutive patients (mean age 50±16 years; 15 women) undergoing continuous monitoring for brain tissue oxygenation (PbtO2), intracranial pressure (ICP), cerebral perfusion pressure (CPP) and EtCO2; mean values were recorded hourly BTH was defined as brain tissue oxygen tension (PbtO2) <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 EtCO2 (35–44 mm Hg) to 33.9% in patients with EtCO2<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 EtCO2 (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).

Conclusion The risk of brain tissue hypoxia in critically brain-injured patients increases when EtCO2 values are reduced. Unintentional spontaneous hyperventilation may be a common and under-recognised cause of brain tissue hypoxia after severe brain injury.

  • Hyperventilation
  • end-tidal CO2
  • hypercapnia
  • hypocapnia
  • brain tissue oxygen pressure
  • brain injury
  • coma
  • mechanical ventilation
  • head injury
  • intensive care
  • subarachnoid haemorrhage

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  • Funding This work was supported by research grants from the Swiss National Science Foundation (EC) (PBLAB-119620), Bern, Switzerland, the SICPA Foundation, Lausanne, Switzerland (EC) and the Charles A. Dana Foundation (SAM). NB received funding from a K 12 Career Development Award (RR024157; PI: H Ginsberg) from the National Center for Research Resources (NCRR), a component of the National Institutes of Health (NIH).

  • Competing interests None.

  • Provenance and peer review Not commissioned; externally peer reviewed.