Influence of change in body position on choroidal blood flow in normal subjects

To compare subfoveal choroidal blood flow (ChBF) in sitting and supine positions in normal volunteers.

ChBF was measured with laser Doppler flowmetry in 22 healthy volunteers of mean (SD) age 24 (5) years. Six independent measurements of ChBF were obtained in one randomly selected eye of each subject while seated. The subjects then assumed a supine position for 30 minutes and a new series of six measurements was obtained. The mean values of the two series were calculated. Systemic brachial artery blood pressure and intraocular pressure were measured in the sitting and supine positions. Ocular perfusion pressure (OPP) was calculated based on formulae derived from ophthalmodynamometric studies. The influence of changing OPP during change in body posture on the change in ChBF was assessed by linear regression analysis.

ChBF decreased by 6.6% (p = 0.0017) in the supine position. The estimated ophthalmic blood pressure in the supine position was adjusted to obtain a result of no change in OPP for no change in ChBF, yielding a mean decrease in the estimate of OPP of 6.7% (p = 0.0002). The necessary adjustment for the estimate of OPP in the supine position suggested a marked buffering of the change in perfusion pressure by the carotid system. The relative decrease in OPP correlated significantly with the relative decrease in ChBF (R(2) = 0.20; p = 0.036) with a slope for the regression line of 1.04.

The comparable degree of change in ChBF and OPP and the linear relationship between the two parameters suggest a passive response of the choroidal circulation to a change in posture. In contrast, the carotid system seems to control the gradient in perfusion pressure closely between the heart and its branches.