title: Acute hypercarbia increases the lower limit of cerebral blood flow autoregulation in a porcine model.
abstract: OBJECTIVE
  In the present study, our objective was to determine if hypercarbia would alter cerebral blood flow (CBF) autoregulation and reduce the ability of cerebrovascular reactivity monitoring to identify the lower limit of cerebrovascular autoregulation (LLA).
  
  
  METHODS
  Anaesthetised juvenile pigs were assigned between two groups: normocarbia (control group, n = 10) or hypercarbia [high carbon dioxide (CO2) group, n = 8]. Normocarbia subjects were maintained with an arterial CO2 of 40 Torr, while the hypercarbia subjects had an increase of inspired CO2 to achieve an arterial pCO2 of >80 Torr. Gradual hypotension was induced by continuous haemorrhage from a catheter in the femoral vein, and the LLA was determined by monitoring cortical laser Doppler flux (LDF). Vascular reactivity monitoring was performed using the pressure reactivity index (PRx) and haemoglobin volume index (HVx).
  
  
  RESULTS
  There were no sustained differences in ICP between groups. Autoregulation was present in both groups, despite elevation in pCO2.The control group had an average LLA of 45 mmHg (95% CI: 43-47 mmHg) and the high CO2 group had a LLA of 75 mmHg (95% CI: 73-77 mmHg). The detected LLA for each subject correlated with the level of pCO2 (spearman R = 0.8243, P < 0.0001). Both the PRx and HVx accurately detected the LLA despite the presence of hypercarbia.
  
  
  CONCLUSIONS
  Hypercarbia without acidosis increases the observed LLA independent of alterations in ICP. Elevations in CO2 can impair cerebrovascular autoregulation, but if there is a sufficient increase in blood pressure above the CO2 altered LLA, then autoregulation persists.
authors: a Department of Medicine , Baylor College of Medicine , Houston , TX , USA.|b Department of Anesthesiology, Center for Space Medicine , Baylor College of Medicine , Houston , TX , USA.
authors: Derek M
authors: DM
authors: Nusbaum
authors: c Department of Anesthesiology , Baylor College of Medicine, Texas Children's Hospital , Houston , TX , USA.|d Department of Pediatrics , Baylor College of Medicine, Texas Children's Hospital , Houston , TX , USA.
authors: Kenneth M
authors: KM
authors: Brady
authors: c Department of Anesthesiology , Baylor College of Medicine, Texas Children's Hospital , Houston , TX , USA.|d Department of Pediatrics , Baylor College of Medicine, Texas Children's Hospital , Houston , TX , USA.
authors: Kathleen K
authors: KK
authors: Kibler
authors: c Department of Anesthesiology , Baylor College of Medicine, Texas Children's Hospital , Houston , TX , USA.|d Department of Pediatrics , Baylor College of Medicine, Texas Children's Hospital , Houston , TX , USA.
authors: Ronald
authors: R
authors: Blaine Easley
mesh_terms: D000818:Animals; D001783:Blood Flow Velocity / Q000187:drug effects; D001784:Blood Gas Analysis; D001794:Blood Pressure / Q000187:drug effects; D002245:Carbon Dioxide / Q000494:pharmacology*; D000071937:Cerebral Blood Volume / Q000187:drug effects; D002560:Cerebrovascular Circulation / Q000187:drug effects* / Q000502:physiology; D006706:Homeostasis / Q000187:drug effects* / Q000502:physiology; D007427:Intracranial Pressure / Q000187:drug effects; D017078:Laser-Doppler Flowmetry; D023421:Models, Animal; D008991:Monitoring, Physiologic; D012372:ROC Curve; D013552:Swine
chemical_list: D002245:Carbon Dioxide
keywords: Cerebrovascular reactivity; Hypercarbia; Intracranial pressure; Lower limit; Near-infrared spectroscopy; Static rate of autoregulation