title: The impact of hypercapnia on retinal capillary blood flow assessed by scanning laser Doppler flowmetry.
abstract: OBJECTIVE
  To determine the effect of hypercapnia on retinal capillary blood flow using scanning laser Doppler flowmetry (SLDF).
  
  
  METHODS
  One randomly selected eye of each of 10 normal healthy subjects (mean age 25 years, SD 2.3) was studied. Subjects breathed unrestricted air for 15 min before (baseline) and after raising fractional (percent) end-tidal concentration of CO2 (FETCO2) for 15 min by adding low flows of CO2 to air entering a sequential gas delivery circuit attached to a nasal mask. Five good quality baseline SLDF images were acquired both of the optic nerve head (ONH) and of the macula. Subsequently, a minimum of 7 sequential images were acquired during hypercapnia. Five further images were acquired of the ONH, or of the macula, after returning to unlimited air breathing. The respiratory parameters of subjects were continually monitored.
  
  
  RESULTS
  The group mean increase in end-tidal CO2 was 14.13% (SD 4.10) relative to baseline. The nasal macula (P = 0.028) and foveal (P = 0.042) areas showed a significant increase in retinal capillary blood flow in response to hypercapnia while no significant change was noted in the ONH or temporal macula areas. Change in blood flow significantly correlated with change of FETCO2 and/or end-tidal PO2 for 3 of the 4 locations.
  
  
  CONCLUSIONS
  Hypercapnia provoked a significant increase in retinal capillary blood flow in 2 of 4 retinal locations. Hypercapnia also induced a change in respiratory parameters that significantly correlated with change in retinal capillary blood flow in 3 of the 4 locations.
authors.affiliation: Multi-Disciplinary Laboratory for the Research of Sight-Threatening Diabetic Retinopathy, School of Optometry, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1.
authors.forename: Subha T
authors.initials: ST
authors.lastname: Venkataraman
authors.forename: Chris
authors.initials: C
authors.lastname: Hudson
authors.forename: Joseph A
authors.initials: JA
authors.lastname: Fisher
authors.forename: John G
authors.initials: JG
authors.lastname: Flanagan
mesh_terms: D000328:Adult; D002196:Capillaries / Q000502:physiology; D006801:Humans; D006935:Hypercapnia / Q000000981:diagnostic imaging / Q000503:physiopathology*; D017078:Laser-Doppler Flowmetry; D008266:Macula Lutea / Q000098:blood supply; D008297:Male; D009900:Optic Nerve / Q000098:blood supply; D012039:Regional Blood Flow; D012171:Retinal Vessels / Q000502:physiology*; D014463:Ultrasonography