The impact of hypercapnia on retinal capillary blood flow assessed by scanning laser Doppler flowmetry.

AIM

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.