title: Combined effects of nitrous oxide and propofol on the dynamic cerebrovascular response to step changes in end-tidal PCO2 in humans.
abstract: BACKGROUND
  Nitrous oxide (N2O) and propofol exhibit directionally opposite effects on the cerebral circulation, vasodilation and vasoconstriction, respectively. The authors investigated an interaction between the two anesthetic agents on the dynamic cerebrovascular response to step changes in end-tidal pressure of carbon dioxide (PetCO2) in humans.
  
  
  METHODS
  Participants with no systemic diseases were allocated into two groups, each of which was anesthetized sequentially with two protocols. Patients in group 1 were anesthetized with 30% O2 + 70% N2O. A continuous intravenous infusion of propofol (7-10 mg x kg(-1) x h(-1)) was then added to the N2O. Patients in group 2 were anesthetized first with continuous infusion of propofol (10 mg x kg(-1) h(-1)), and then 30% O2 + 70% N2O was added to the propofol anesthesia. Using transcranial Doppler ultrasonography, blood flow velocity at the middle cerebral artery (FV(MCA)) was measured during a step increase (on-response) followed by a step decrease (off-response) in PetCO2, with PetCO2 ranging between approximately 28 and 50 mmHg. The dynamic FV(MCA)-PetCO2 relationship was analyzed using a mathematical model that was characterized with a pure time delay, and a time constant and a gain each for the on- or off-response.
  
  
  RESULTS
  The addition of propofol to the N2O anesthesia increased the on-response time constant (P < 0.01), whereas the addition of N2O to the propofol anesthesia increased the time constants for on- (P < 0.01) and off-responses (P < 0.05). However, the addition of either anesthetic did not affect the gains.
  
  
  CONCLUSIONS
  Propofol and N2O, when one is added to the other, produce similar dynamic FV(MCA) responses to sudden changes in PetCO2. Addition of each anesthetic slows the dynamic response and produces the response whose magnitude is proportional to the baseline FV(MCA).
authors.affiliation: Department of Anesthesiology, Chiba University Graduate School of Medicine, Japan.
authors.forename: Shin
authors.initials: S
authors.lastname: Inaba
authors.forename: Jiro
authors.initials: J
authors.lastname: Sato
authors.forename: Mitsuo
authors.initials: M
authors.lastname: Aono
authors.forename: Tsutomu
authors.initials: T
authors.lastname: Numata
authors.forename: Takashi
authors.initials: T
authors.lastname: Nishino
mesh_terms: D000293:Adolescent; D000328:Adult; D001783:Blood Flow Velocity / Q000187:drug effects; D002245:Carbon Dioxide / Q000032:analysis*; D002560:Cerebrovascular Circulation / Q000187:drug effects*; D004347:Drug Interactions; D005260:Female; D006801:Humans; D008297:Male; D009609:Nitrous Oxide / Q000008:administration & dosage / Q000494:pharmacology*; D015742:Propofol / Q000008:administration & dosage / Q000494:pharmacology*
chemical_list: D002245:Carbon Dioxide; D009609:Nitrous Oxide; D015742:Propofol