MICROVASCULAR RESEARCH 51, 378–392 (1996) ARTICLE NO. 0034 Autoregulation in the Human Retinal Circulation: Assessment Using Isometric Exercise, Laser Doppler Velocimetry, and Computer-Assisted Image Analysis MARTIN J. DUMSKYJ, 1 JESPER E. ERIKSEN,CAROLINE J. DORE ´ ,* AND EVA M. KOHNER Department of Medicine and *Department of Medical Statistics, Royal Postgraduate Medical School, Hammersmith Hospital, DuCane Road, London W12 ONN, England Received August 2, 1995 Autoregulation of blood flow in response to changes in perfusion pressure is known to occur in a number of tissues including the human retina. Defective autoregulation may play a part in the pathophysiology of several retinal diseases. Laser Doppler velocimetry has been used to study retinal blood flow. Technically superior measurements are obtained from veins by this method but arterial measurements might provide additional information. The response of the normal human retina to an acute elevation of systemic blood pressure induced by isometric exercise was investigated in nine normal volunteers using laser Doppler velocimetry and com- puter-assisted image analysis. Measurements were taken from retinal veins and arteries. Autoregu- lation was demonstrated by an 8.4% rise in flow in response to a 34% rise in perfusion pressure (P Å 0.0007) using data derived from veins and a 4.8% rise in flow in response to a 33% rise in perfusion pressure (P Å 0.01) using data derived from arteries. Arteries constricted by 3.4% (P Å 0.002) and veins dilated by 1.6% (P Å 0.02). Red cell velocity rose in veins by 5.0% (P Å 0.008) and in arteries by 12.2% (P Å 0.02). The variability in velocity change derived from veins (SD 3.4%) was lower than that from arteries (SD 12.1%). A similar pattern of flow change was found in both sets of data. This makes venous measurements more useful for obtaining statistically reliable results from these techniques.  1996 Academic Press, Inc. INTRODUCTION The retina provides an opportunity for the in vivo noninvasive investigation of a part of the human microcirculation. Study of the retinal circulation is important to further our understanding of normal physiology (Riva et al., 1985; Feke et al., 1989) and also of a number of pathological conditions such as diabetic retinopathy (Patel et al., 1992; Grunwald et al., 1992), glaucoma (Grunwald et al., 1984), and retinal vascular occlusive disease (Wolf et al., 1994). Previous work has shown that blood flow in the human retina is autoregulated (Alm and Bill, 1987; Bill, 1984; Ffytche et al., 1974; Riva et al., 1986; Robinson et al., 1986). This autoregulation occurs both in response to changes in perfusion pressure (Grunwald et al., 1982; Robinson et al., 1986; Rassam et al., 1995) and in response to metabolic stimuli (Riva et al., 1983, 1986). Intrinsic autoregulation is thought to 1 To whom correspondence should be addressed at Fax: /44 181 383 2182. 378 0026-2862/96 $18.00 Copyright  1996 by Academic Press, Inc. All rights of reproduction in any form reserved.