title: Therapeutic hypothermia after cardiac arrest - cerebral perfusion and metabolism during upper and lower threshold normocapnia.
abstract: BACKGROUND
  During cardiac arrest and after successful resuscitation a continuum of ischaemia-reperfusion injury develops. Mild hypothermia exerts protective effects in the postresuscitation phase but also alters CO₂ production and solubility, which may lead to deleterious effects if overlooked when adjusting the ventilation of the resuscitated patient. Using a multimodality approach, the effects of different carbaemic states on cerebral perfusion and metabolism were evaluated during therapeutic hypothermia.
  
  
  METHODS
  Eight comatose survivors of prehospital cardiac arrest were cooled to 33°C for 24 h and underwent a 60 min phase of interventional lower threshold normocapnia according to temperature non-corrected pCO₂ (4.2 kPa) and higher threshold normocapnia according to corrected pCO₂ (6.0 kPa) in a random order. Prior to, during and after each phase, cerebral perfusion and metabolites via a microdialysis catheter were measured.
  
  
  RESULTS
  During upper-threshold pCO₂, an increase in middle cerebral artery mean flow velocity (MFV) and jugular bulb oxygen saturation (jSvO₂) were observed with a concomitant decrease in cerebral lactate concentration. Lower threshold normocapnia was associated with a decrease in MFV in most patients. In all patients jSvO₂ decreased but no change in cerebral lactate was observed. In seven patients jSvO₂ decreased below 55%. These changes were not reflected to intracranial pressure or cerebral oximetry.
  
  
  CONCLUSIONS
  During induced hypothermia, lower threshold normocapnia was associated with decreased cerebral perfusion/oxygenation but not reflected to interstitial metabolites. Upper threshold pCO₂ increased cerebral perfusion and reduced cerebral lactate. Vigilance over the ventilatory and CO₂ analysis regimen is mandatory during mild hypothermia.
authors.affiliation: Critical Care Medicine Research Group, Department of Critical Care Medicine, Tampere University Hospital, Tampere, Finland.
authors.forename: Lauri
authors.initials: L
authors.lastname: Pynnönen
authors.forename: Patrik
authors.initials: P
authors.lastname: Falkenbach
authors.forename: Antti
authors.initials: A
authors.lastname: Kämäräinen
authors.forename: Kimmo
authors.initials: K
authors.lastname: Lönnrot
authors.forename: Arvi
authors.initials: A
authors.lastname: Yli-Hankala
authors.forename: Jyrki
authors.initials: J
authors.lastname: Tenhunen
mesh_terms: D000368:Aged; D001784:Blood Gas Analysis; D002245:Carbon Dioxide / Q000097:blood*; D016887:Cardiopulmonary Resuscitation / Q000379:methods / Q000401:mortality; D002560:Cerebrovascular Circulation; D003128:Coma / Q000097:blood / Q000503:physiopathology; D018592:Cross-Over Studies; D005260:Female; D005387:Finland; D015600:Glasgow Coma Scale; D017052:Hospital Mortality / Q000639:trends; D006785:Hospitals, University; D006801:Humans; D007036:Hypothermia, Induced / Q000379:methods* / Q000401:mortality; D007362:Intensive Care Units; D008297:Male; D008875:Middle Aged; D008991:Monitoring, Physiologic / Q000379:methods; D015428:Myocardial Reperfusion Injury / Q000097:blood* / Q000401:mortality / Q000503:physiopathology; D058687:Out-of-Hospital Cardiac Arrest / Q000175:diagnosis / Q000401:mortality / Q000628:therapy*; D010100:Oxygen / Q000097:blood*; D011446:Prospective Studies; D011659:Pulmonary Gas Exchange; D012720:Severity of Illness Index; D016019:Survival Analysis; D016896:Treatment Outcome
chemical_list: D002245:Carbon Dioxide; D010100:Oxygen