AUTOREGULATION OF HUMAN RETINAL BLOOD-FLOW - AN INVESTIGATION WITH LASER DOPPLER VELOCIMETRY

Abstract

The effect of acute changes in mean retinal perfusion pressure, .hivin.P (2/3 of mean brachial artery blood pressure minus IOP), on retinal volumetric blood flow rate, Q, was investigated in normal volunteers. Changes in Q were determined from Q = k .cntdot. Vmax .cntdot. D2, where Vmax is the center line red blood cell velocity measured from temporal veins by laser Doppler velocimetry, D is the vessel diameter obtained by monochromatic fundus photography, and k is a constant of proportionality. A suction cup was used to induce step changes in IOP and, consequently, in .hivin.P. The magnitude of the steps ranged from 10-32 mmHg. During the first 30 sec after a step decrease in .hivin.P, Vmax and Q were significantly smaller than at rest by an amount proportional to the decrease in .hivin.P. Thereafter, Vmax and Q increased markedly towards their values at rest, although .hivin.P changed comparatively little during this period of time. Time constant of the corresponding decrease in vascular resistance, R(t) = .hivin.P(t)/Q(t), was approximately 45 sec. There was no significant change in D during elevated IOP. Removal of the cup induced an immediate step increase in .hivin.P, Vmax, D, Q, and R. Thereafter, Vmax, D, Q, and R returned to their values at rest (time constant of the change in R was about 30 sec), while .hivin.P remained nearly constant. The rapid change in vascular resistance following a step decrease and increase in .hivin.P can be attributed to an active process that attempts to maintain blood flow close to normal, in spite of changes in perfusion pressure (autoregulation). The results show, however, that autoregulation, although present at .hivin.Ps as low as 10 mmHg (IOP .simeq. 42 mmHg) is fully effective only if .hivin.P is not lowered by more than 50% (IOP not above some value between 27 and 30 mmHg).