title: Ophthalmic artery blood flow velocity increases during hypocapnia.
abstract: OBJECTIVE
  The effects of anesthetic management on blood flow to the optic nerve have not been well-studied. The ophthalmic artery provides the majority of the blood supply to the optic nerve via several smaller branches. Retinal blood flow has been shown to react to carbon dioxide (CO(2)) similar to intracranial vessels, but insufficient data exist for the ophthalmic artery. The purpose of this study is to examine the CO(2)-reactivity of the ophthalmic artery.
  
  
  METHODS
  Eight healthy awake subjects aged 28 to 50 yr were tested for CO(2)-reactivity in the ophthalmic artery using transcranial Doppler (TCD) insonation of blood flow velocity (V(op)), while simultaneously recording the V(op) of the middle cerebral artery (V(mca)) as an internal control. V(op) and V(mca) recordings were made under hypocapnic, normocapnic and hypercapnic conditions.
  
  
  RESULTS
  The CO(2)-reactivity slope of V(mca) was 3.27% per mmHg PaCO(2). From normocapnia to hypercapnia, V(op) did not change significantly (mean +/- SD, 18 +/- 4 cm*sec(-1) to 18 +/- 6 cm*sec(-1)), (end-tidal CO(2), etCO(2), = 43 +/- 5 mmHg to 53 +/- 4 mmHg, respectively). In contrast, V(op) increased significantly under hypocapnic conditions (etCO(2) = 26 +/- 4 mmHg) to 25 +/- 5 cm*sec(-1) (P < 0.05). The CO(2)-reactivity slope of V(op) from normocapnia to hypocapnia was 2.57% per mmHg.
  
  
  CONCLUSIONS
  This study demonstrates that V(op) increases with hypocapnia, but is unaffected by hypercapnia. The anastomoses of the ophthalmic artery with the external carotid artery, which displays a relatively fixed resistance, may account for these findings.
authors.affiliation: Department of Anesthesiology, Harborview Medical Center, University of Washington, Seattle, Washington, USA. lorlee@u.washington.edu
authors.forename: Lorri A
authors.initials: LA
authors.lastname: Lee
authors.forename: Monica S
authors.initials: MS
authors.lastname: Vavilala
authors.forename: Arthur M
authors.initials: AM
authors.lastname: Lam
authors.forename: Colleen
authors.initials: C
authors.lastname: Douville
authors.forename: Anne
authors.initials: A
authors.lastname: Moore
authors.forename: Elizabeth
authors.initials: E
authors.lastname: Visco
authors.forename: David W
authors.initials: DW
authors.lastname: Newell
mesh_terms: D000328:Adult; D001783:Blood Flow Velocity / Q000187:drug effects / Q000502:physiology; D002245:Carbon Dioxide / Q000494:pharmacology; D002560:Cerebrovascular Circulation / Q000502:physiology*; D005260:Female; D006801:Humans; D006935:Hypercapnia / Q000503:physiopathology; D016857:Hypocapnia / Q000503:physiopathology*; D008297:Male; D008875:Middle Aged; D020768:Middle Cerebral Artery / Q000000981:diagnostic imaging / Q000502:physiology; D009880:Ophthalmic Artery / Q000000981:diagnostic imaging / Q000187:drug effects / Q000502:physiology*; D012016:Reference Values; D017585:Ultrasonography, Doppler, Transcranial; D014851:Wakefulness / Q000502:physiology
chemical_list: D002245:Carbon Dioxide