title: The effect of clonidine on cerebral blood flow velocity, carbon dioxide cerebral vasoreactivity, and response to increased arterial pressure in human volunteers.
abstract: BACKGROUND
  Because patients may be taking clonidine chronically or may be receiving it as a premedication before surgery, the authors investigated its effect on cerebral hemodynamics.
  
  
  METHODS
  In nine volunteers, middle cerebral artery mean blood flow velocity (Vm) was measured using transcranial Doppler ultrasonography (TCD). CO2 vasoreactivity was measured before clonidine administration (preclonidine), 90 min after clonidine, 5 microg/kg orally, then following restoration of mean arterial pressure (MAP) to the preclonidine level. In addition, Vm was measured after a phenylephrine-induced 30-mmHg increase in MAP.
  
  
  RESULTS
  After clonidine administration, Vm decreased from 62 +/- 9 to 48 +/- 8 cm/s (P < 0.01), and MAP decreased from 86 +/- 10 to 63 +/- 5 mmHg (P < 0.01; mean +/- SD). Clonidine decreased the CO2 vasoreactivity slope from 2.2 +/- 0.4 to 1.2 +/- 0.5 cm x s(-1) x mmHg(-1) (P < 0.05); restoring MAP to the preclonidine level increased the slope to 1.60 +/- 0.5 cm x s(-1) x mmHg(-1), still less than the preclonidine slope (P < 0.05). CO2 vasoreactivity expressed as a percentage change in Vm, decreased after clonidine, 3.5 +/- 0.8 versus 2.4 +/- 0.8 %/mmHg (P < 0.05); this difference disappeared after restoration of MAP, 3.1 +/- 1.2 %/mmHg. With a 30-mmHg increase in MAP, Vm increased by 13% before and after clonidine (P < 0.05).
  
  
  CONCLUSIONS
  Clonidine, 5 microg/kg orally, decreases Vm and slightly attenuates cerebral CO2 vasoreactivity, therefore decreased cerebral blood flow and mildly attenuated CO2 vasoreactivity should be anticipated.
authors.affiliation: Department of Anesthesia, University of California, San Francisco 94143-0648, USA.
authors.forename: H W
authors.initials: HW
authors.lastname: Lee
authors.forename: J E
authors.initials: JE
authors.lastname: Caldwell
authors.forename: B
authors.initials: B
authors.lastname: Dodson
authors.forename: P
authors.initials: P
authors.lastname: Talke
authors.forename: J
authors.initials: J
authors.lastname: Howley
mesh_terms: D000316:Adrenergic alpha-Agonists / Q000494:pharmacology*; D000328:Adult; D001783:Blood Flow Velocity / Q000187:drug effects; D001794:Blood Pressure / Q000187:drug effects*; D002245:Carbon Dioxide / Q000032:analysis*; D002560:Cerebrovascular Circulation / Q000187:drug effects*; D003000:Clonidine / Q000494:pharmacology*; D006801:Humans; D011292:Premedication
chemical_list: D000316:Adrenergic alpha-Agonists; D002245:Carbon Dioxide; D003000:Clonidine