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OBJECTIVE: To investigate retinal lesions in children with severe falciparum malaria. METHODS: Color photography and fluorescein angiography were performed in consecutive children admitted to a pediatric high-dependency unit in Kenya during 1 malaria season. The presence and category of retinal lesion was compared with disease severity, clinical outcome, anemia, lactic acidosis, and parasite count. RESULTS: Twenty-six patients with cerebral malaria and 14 patients who were prostrate were studied. Thirty-one of the patients had clinical features of ocular disease, including round, flame-shaped, and white-centered hemorrhages; peripheral and foveal retinal opacification; peripheral vascular occlusion; venous dilation; disc edema with hyperemia; and arterial pulsatility. Of 8 patients with retinal opacification, only 2 showed small, infrequent zones of capillary nonperfusion on fluorescein angiography; the leakage of dye at sites of opacification was not seen. Retinal opacification was significantly associated with a higher parasite count (P < .02). White-centered hemorrhages were significantly associated with a higher parasite count (P < .05), severe disease (p < .05), and severe anemia (P < .02). CONCLUSIONS: The blood-retina barrier and retinal vascular flow remain substantially normal despite widespread pathological features. Retinal features in children with severe malaria are consistent with cellular hypoxia, nutritional deficiency, or both rather than with vascular occlusion; they support the concept of metabolic steal by parasites.

Much of our basic knowledge of retinal blood flow regulation is based on data obtained from animal experiments through the use of invasive techniques. However, during the last decades, major developments in the field of optics and lasers have led to a variety of noninvasive techniques, which have been applied to the human eye for the investigation of retinal hemodynamics, and more specifically the regulation of retinal blood flow in response to a number of physiological and pharmacological stimuli. The Retinal Vessel Analyzer has markedly simplified the measurement of the diameter of retinal vessels, as well as the change in this diameter evoked by various physiological stimuli (dynamic measurements). Bidirectional laser Doppler velocimetry allows the measurement of absolute red blood cell centerline velocity, which, when combined with the diameter allows the calculation of retinal blood flow in the main retinal vessels. Laser Doppler flowmetry and laser speckle flowgraphy are techniques that measure the velocities of blood in discrete areas of the retinal tissue microcirculation. Adding a scanning capability, a spatial map of velocities across the retinal tissue is obtained. The blue-field simulation technique allows the quantification of the velocity, number and velocity pulsatility of leukocytes moving in the retinal capillaries of the macular region. With color Doppler imaging, the peak systolic and end-diastolic values of blood velocity in the ophthalmic and central retinal artery are measured, from which a resistivity index is obtained. These techniques may help better understand the role of altered retinal blood flow and its regulation in the pathogenesis of retinal diseases of vascular origin.

Cerebral malaria (CM) is a disease of high mortality worldwide. It can be associated with malarial retinopathy (MR) resulting from impaired perfusion within the retinal microvasculature. Areas of capillary non-perfusion (CNP) appear white (retinal whitening) on ophthalmoloscopy. In this study, electrophysiological investigations were performed to investigate the physiological consequences of these hypoxic and ischaemic changes.Children admitted with CM were assessed for inclusion in the study. Those with MR underwent further detailed fundus assessment to quantify retinal whitening and were then designated a severity score. Electrophysiological recordings were performed using a miniganzfeldt stimulator with calibration to the International Society for Clinical Electrophysiology of Visual (ISCEV) standards. ERG data were then analysed with respect to presence of MR and also graded disease severity.Thirty-one children were recruited with a diagnosis of CM, 20 had MR (group 1), and 11 had absent MR (group 2). Statistical analyses of these two groups showed a significant relationship between reduced single flash cone b wave amplitude (CBWA) and increased severity of retinal whitening/CNP (P0.05). Cone and maximal response b : a wave ratios remained1 in all subjects.Retinal whitening/CNP in MR is associated with significant changes in ERG cone b wave function. The relatively high b : a ratio is compatible with the high frequency of MR resolution without sequelae.

The retina and optic nerve are both optically accessible parts of the central nervous system. They represent, therefore, highly valuable tissues for studies of the intrinsic physiological mechanism postulated more than 100 years ago by Roy and Sherrington, by which neural activity is coupled to blood flow and metabolism. This article describes a series of animal and human studies that explored the changes in hemodynamics and oxygenation in the retina and optic nerve in response to increased neural activity, as well as the mechanisms underlying these changes. It starts with a brief review of techniques used to assess changes in neural activity, hemodynamics, metabolism and tissue concentration of various potential mediators and modulators of the coupling. We then review: (a) the characteristics of the flicker-induced hemodynamical response in different regions of the eye, starting with the optic nerve, the region predominantly studied; (b) the effect of varying the stimulus parameters, such as modulation depth, frequency, luminance, color ratio, area of stimulation, site of measurement and others, on this response; (c) data on activity-induced intrinsic reflectance and functional magnetic resonance imaging signals from the optic nerve and retina. The data undeniably demonstrate that visual stimulation is a powerful modulator of retinal and optic nerve blood flow. Exploring the relationship between vasoactivity and metabolic changes on one side and corresponding neural activity changes on the other confirms the existence of a neurovascular/neurometabolic coupling in the neural tissue of the eye fundus and reveals that the mechanism underlying this coupling is complex and multi-factorial. The importance of fully exploiting the potential of the activity-induced vascular changes in the assessment of the pathophysiology of ocular diseases motivated studies aimed at identifying potential mediators and modulators of the functional hyperemia, as well as conditions susceptible to alter this physiological response. Altered hemodynamical responses to flicker were indeed observed during a number of physiological and pharmacological interventions and in a number of clinical conditions, such as essential systemic hypertension, diabetes, ocular hypertension and early open-angle glaucoma. The article concludes with a discussion of key questions that remain to be elucidated to increase our understanding of the physiology of ocular functional hyperemia and establish the importance of assessing the neurovascular coupling in the diagnosis and management of optic nerve and retinal diseases.

An ocular fundus reflectometer to measure non-invasively the light reflectance changes in response to diffuse flicker stimulation at two wavelengths (569 and 600 nm) simultaneously is presented. Image registration to minimize the effect of involuntary eye motion is described. Two-dimensional maps of reflectance changes of the optic disc and peripapillary retinal tissues in healthy volunteers are presented. These functional maps suggest increases in blood volume during stimulation, with changes in the optic disc as large as 10.7% at 569 nm and 3.5% at 600 nm.

A fundus camera-based instrument to investigate increased retinal neuronal activity effect on optic nerve blood flow is described. It incorporates (a) near infrared fundus illumination and observation, (b) near infrared laser Doppler flowmeter (LDF), (c) pupil position monitoring and (d) delivery of visual stimuli. Two types of stimulation are currently used: diffuse heterochromatic and chromatic luminance flicker generated by light emitting diodes, and contrast reversal pattern displayed on a video monitor. Recordings of the changes in the LDF parameters (blood velocity, volume and flow) from the optic nerve in response to these stimuli illustrate the potential of the technique.

Near-infrared laser Doppler flowmetry was applied in 15 normal volunteers to record the time course and magnitude of changes in the velocity (Vel), volume (Vol) and flow (F) of blood and tissue reflectance (R) at the optic disk in response to 40 and 50 s of increased retinal neural activity. This activity was evoked by diffuse luminance flicker of the retinal posterior pole. After 20 s of flicker, the group averages of Vel, Vol, and F were significantly higher than at baseline (pre-flicker) by 12, 24 and 38%. Time constants of the increases in Vel, Vol, and F were 3.4, 12.7 and 9.1 s, respectively. The group average change in R of 1% was not significant. However, in one subject, 15 recordings from the same site of the optic disk showed a significant increase in R of 8%, with a time course similar to that of Vol. Our findings show that, in the human optic nerve, a white matter tissue, the temporal dynamics and magnitude of the response of blood flow to an increase in retinal neural activity are similar to those reported for brain gray matter. Furthermore, although the R-response could be due, in part, to changes in blood volume, other factors, such as activity-evoked tissue scattering changes, may also affect this response.

The purpose of the present study was to investigate the contribution of basal nitric oxide (NO) on retinal vascular tone in humans. In addition, we set out to elucidate the role of NO in flicker-induced retinal vasodilation in humans. Twelve healthy young subjects were studied in a three-way crossover design. Subjects received an intravenous infusion of either placebo or NG-monomethyl-l-arginine (l-NMMA; 3 or 6 mg/kg over 5 min), an inhibitor of NO synthase. Thereafter, diffuse luminance flicker was consecutively performed for 16, 32, and 64 s at a frequency of 8 Hz. The effect of l-NMMA on retinal arterial and venous diameter was assessed under resting conditions and during the hyperemic flicker response. Retinal vessel diameter was measured with a Zeiss retinal vessel analyzer. l-NMMA significantly reduced arterial diameter (3 mg/kg: –2%; 6 mg/kg: –4%, P < 0.001) and venous diameter (3 mg/kg: –5%; 6 mg/kg: –8%, P < 0.001). After placebo infusion, flicker induced a significant increase in retinal vessel diameter ( P < 0.001). At a flicker duration of 64 s, arterial diameter increased by 4% and venous diameter increased by 3%. l-NMMA did not abolish these hyperemic responses but blunted venous vasodilation ( P = 0.017) and arterial vasodilation ( P = 0.02) in response to flicker stimulation. Our data indicate that NO contributes to basal retinal vascular tone in humans. In addition, NO appears to play a role in flicker-induced vasodilation of the human retinal vasculature.

BACKGROUND: To investigate the effect of isovolumic hemodilution on the tissue oxygenation of the optic nerve head (ONH). MATERIAL AND METHODS: In 9 miniature pigs (6 - 12 kg), hemodilution was performed by replacing 100 - 140 ml of blood by an equivalent volume of 6 % hydroxyethyl starch in saline. The blood flow parameters in the ONH microcirculation, namely velocity (BVel), volume (BVol) and flow (BF), were measured by laser Doppler flowmetry. Microelectrodes placed at approximately 50 micrometer from the disk surface and the phosphorescence quenching technique were used to measure pO 2 in the vitreous (pO 2 (vitr)) and in the ONH capillary blood (pO 2 (blood)), respectively. Arterial blood pressure, hematocrit (Hct) and gas content were documented. RESULTS: In all animals, hemodilution reduced Hct by 30 +/- 6 %. All the flow parameters increased markedly. Simultaneously, pO 2 (blood) (baseline: 31 +/- 3 mm Hg) dropped to approximately 75 % of its initial value, while pO 2 (vitr) (baseline: 26 +/- 8 mm Hg) increased by 15 +/- 8 %. CONCLUSIONS: Hemodilution leads to an increase of pO 2 (vitr) reflecting a similar rise of the ONH tissue pO 2. This effect results from an enhanced blood perfusion of the ONH, which over-compensates the decrease of the blood O 2 content.

Purpose To investigate the impact of different gas mixtures (hyperoxia, hypoxia, and hypercapnia) on the optic nerve head (ONH) and choroidal (Ch) hemodynamics. Methods Twenty-three healthy subjects (28 ± 6 years) took part in the study. Variations in inspired oxygen and carbon dioxide fraction were produced by a gas mixing device. Arterial oxygen saturation (SpO2) was measured continuously using a transcutaneous sensor and end-tidal carbon dioxide partial pressure by capnography. The experiment comprised three successive periods: 3-minute baseline (room air breathing), 15-minute gas mixture inhalation (normocapnic hypoxia, hypercapnia, or hyperoxia), and 15-minute recovery (room air breathing). Laser Doppler flowmeter parameters-velocity (VEL), volume (VOL), and flow (BF) of red blood cells-were measured. Two-way ANOVAs were performed for statistical analysis. Results In response to hyperoxia, ONHBF significantly decreased by -18% ± 6% (P = 0.04) from baseline, due to significant changes in VEL (-12% ± 3% P = 0.0002). During hypoxia at 85% SpO2, ONH VEL increased by +12% ± 3% (P = 0.0009), whereas VOL and BF did not change significantly. ChBF significantly increased by +7% ± 2% (P = 0.004) in response to hypoxia, due to significant changes in VEL +5% ± 2% (P = 0.03). Both Ch and ONHBFs did not vary significantly in response to hypercapnia. Conclusions The magnitude of the blood flow response is the most significant during hyperoxia for ONH and hypoxia for ChBF. For ONHBF, a 37% difference between hyperoxia and hypoxia can be useful when vasoreactivity to O2 will be tested in patients.

Purpose. Arterial carbon dioxide tension and arterial oxygen tension are important determinants of retinal and cerebral blood flow. In the present study the hypothesis that changes in arterial blood gases also influence choroidal blood flow was tested. Methods. The effect of breathing different mixtures of oxygen (O 2) and carbon dioxide (CO 2) on choroidal blood flow in the foveal region was investigated in healthy subjects. The study was performed in a randomized, double-masked four way cross-over design in 16 subjects. Using a compact laser Doppler flowmeter, red blood cell velocity (ChBVel), volume (ChBVol), and flow (ChBF) in the choroidal vasculature were measured during the breathing of various mixtures of O 2 and CO 2 (hyperoxia-hypercapnia): 100% O 2, 97% O 2 + 3% CO 2, 95% O 2 + 5% CO 2 (carbogen) and 92% O 2 + 8% CO 2. Arterial oxygen tension (pO 2) and carbon dioxide tension (pCO 2) were measured from arterialized blood samples from the earlobe. Results. Breathing 100% O 2 had no significant e...

BACKGROUND Previous studies have demonstrated, in the cat, a vasodilatation of retinal vessels in response to neuronal activity induced by diffuse luminance flicker. The aim of this study was to determine whether a similar diameter variation is detectable in humans. MATERIALS AND METHODS Nine normal subjects were exposed to 1 min of sinusoidally varying diffuse luminance flicker (10 Hz, 30 degrees around optic nerve head). Monochromatic fundus pictures before and after the stimulation were taken. The diameter of retinal arteries and veins was measured on the digitised photographs with the NIH-Image software and an own algorythm. RESULTS The diameter immediately after flicker was significantly larger than the pre-stimulus diameter by 4.2 +/- 2.2% (p < 0.014) (mean +/- SD) for the retinal arteries and 2.7 +/- 1.7% (p < 0.001) for the retinal veins. Six seconds after cessation of the flicker, arterial diameter was not significantly different from that of pre-flicker value. CONCLUSIONS Diffuse luminance flicker induces an increase in retinal vessel diameter. This suggest that retinal blood flow is coupled with neuronal activity as previously evidenced by the blue field simulation technique in the macula.

Increased neuronal activity induced by diffuse flickering illumination of the ocular fundus generates increases in blood flow and decreases in tissue pO 2 in the optic nerve head (ONH). This suggests that in the ONH, as in the brain, a coupling exists between function, perfusion, and metabolism, in accordance with the hypothesis of Roy and Sherrington. Following a review of the characteristics of the activity-induced blood flow response in the cat ONH, the relationship between blood flow and metabolism is discussed. We then examine possible mediators of the neurovascular coupling, some of the factors which may interfere with it, and point out various problems which remain to be addressed in order to better understand this coupling.

The effect of decreased ocular mean perfusion pressure (PPm), defined as mean arterial blood pressure minus intraocular pressure (IOP), on optic nerve head blood flow (Fonh) and on the response of this flow (RFonh) to diffuse luminance flicker was investigated in 19 anesthetized cats using laser Doppler flowmetry. PPm was decreased by increasing the IOP. The flicker stimulus consisted of 20-msec flashes delivered at 10 Hz for 30-60 sec. It illuminated a 30 degrees diameter area of the fundus, centered at the optic disk. Decreasing PPm by 10-35% from its resting value resulted in a 23% increase in RFonh (supranormal RFonh). With further decreases in PPm, RFonh decreased, reaching zero at a PPm below 20 mmHg. Fonh remained constant until PPm was < 40 mmHg and then decreased thereafter. When PPm was brought back to resting value after having been decreased for approximately 45 min, Fonh first increased by approximately 380% and then returned to its value at rest within approximately 4.5 min. At low PPm, hyperoxia decreased Fonh by 23% and restored the attenuated RFonh back to the value at resting PPm and hypoxia did not increase Fonh, as it did at normal PPm. This study confirms that the optic nerve head circulation is autoregulated over a wide range of PPm and reveals, for the first time, a hyperemic response to a prolonged decrease in PPm. It suggests that hypoxia plays a role in abolishing RFonh at low PPm and that the supranormal RFonh at moderately decreased PPm is due to an increase in the flicker-induced ganglion cell activity.

The role of nitric oxide (NO) in the regulation of optic nerve head (ONH) blood flow was investigated from direct electrochemical measurements of NO and simultaneous measurements of ONH blood flow by laser Doppler flowmetry during increased neuronal activity with diffuse luminance flickering light stimuli. For normal physiological conditions, there was a consistent increase in ONH blood flow with each flicker stimulus (n = 10 cats). Simultaneous NO measurements were made in the vitreous humor immediately in front of the ONH (n = 6 cats). During flicker, NO increased by 88 +/- 23 nM (P < 0.05) above basal NO levels. Measurements were repeated after inhibiting NO synthase by intravenous infusion of either NG-nitro-L-arginine (n = 5 cats) or NG-nitro-L-arginine methyl ester (n = 5 cats). After inhibition, NO levels were significantly reduced and both ONH blood flow and NO responses to flicker were significantly attenuated. We conclude that NO plays a key vasodilatory role in the ONH, modulating blood flow during greater neuronal activity with flicker stimuli.

AIMS/BACKGROUND: The retinal circulation was investigated in a group of 19 patients with insulin dependent diabetes mellitus with less than 4 years of disease duration and no evidence of diabetic retinopathy. Results of these patients were compared with those of 16 age-matched normal controls. METHODS: Venous diameter (D) was measured from monochromatic fundus photographs. Maximum erythrocyte velocity (Vmax) was assessed by bidirectional laser Doppler velocimetry in the major retinal veins of one eye of each subject. Total volumetric blood flow rate (QT) was calculated by adding the flow rates of the major retinal veins. RESULTS: Average QT was 12% larger than normal in diabetic patients (one tailed, non-paired Student's t test, p < 0.05). A statistically significant correlation was observed between QT and disease duration (r = 0.35, p < 0.04). Patients with longer disease duration tended to have somewhat larger QT. The average retinal vascular regulatory responses to hyperoxia were not significantly different from normal in diabetic patients. In these patients, however, higher blood glucose levels were associated with decreased regulatory responses to hyperoxia. CONCLUSIONS: Patients with diabetes mellitus of relatively short duration have mildly increased QT, suggesting that increased blood flow may play an early role in the development of diabetic retinal microangiopathy.

We determined the effects of endothelin-1 on optic nerve head (ONH) blood flow in anesthetized cats. Endothelin-1 (50-2500 pmol) injected into the vitreous produced a dose-related and sustained decrease (22 +/- 6% by 500 pmol and 36 +/- 11% by 2500 pmol) in the ONH blood flow. Blood pressure (BP) and heart rate (HR) remained, however, unchanged. In contrast, intravenous (i.v.) injected endothelin-1 (0.004-0.4 nmol kg-1) produced a dose-related and short-lasting increase in the ONH blood flow; its maximum increase by 0.4 nmol kg-1 was 135 +/- 34%. Endothelin-1, at 0.4 nmol kg-1, i.v., produced a transient decrease followed by a more sustained increase in BP, but had no remarkable effect on HR. The increase in ONH blood flow by i.v. injection of endothelin-1 was abolished by N(G)-nitro-L-arginine methyl ester (L-NAME, 5 mg kg-1 min-1, i.v.). The suppression of blood flow by L-NAME was reversed by the addition of L-arginine (50 mg kg-1 min-1). Pressor responses to endothelin-1 (0.4 nmol kg-1, i.v.) were augmented in the presence of L-NAME, but reversed in combination with L-arginine. These findings suggest that i.v. injection of endothelin-1 causes NO release from endothelial cells which increases ONH blood flow, whereas intravitreal injection of endothelin-1 decreases ONH blood flow by its vasoconstrictive effect. Different populations of ET receptors on vascular smooth muscles or endothelial cells would reflect the opposing effects of endothelin-1.

The present study was conducted to assess the feasibility of laser Doppler velocimetry in young infants, as a prelude to ultimately undertaking such measurements in premature infants. A portable, unidirectional laser Doppler velocimeter was developed based on a Kowa RC-2 hand-held fundus camera. Six infants between 1 and 21 weeks of age were studied. Relative red blood cell velocity (δfmax) at the centre of retinal arteries was measured over approximately 10 heart cycles. A pulsatility parameter (P=1−δfmax.dia/δfmax.sys), a summary index of vascular status, was determined from the average diastolic and systolic values of δfmax. Velocity waveforms were obtained in four of the six infants. Arterial pulsatility for the group was 0.63±0.13. Precise non-invasive measurement of arterial red blood cell velocity waveforms in young infants was achieved. The high signal-to-noise ratio and temporal resolution of this data suggest that relative measurements of retinal blood flow may permit assessment of haemodynamic changes in premature infants.

AIMS--The effect of rebreathing into a bag (RB) on retinal macular blood velocity was evaluated in healthy volunteers. METHODS--Ten normal volunteers, whose ages ranged from 17 to 34 years, performed RB over 135 to 260 seconds (mean (SD) 193 (38) seconds) while retinal macular blood velocity was determined non-invasively using the blue field simulation technique. RESULTS--Leucocyte velocity significantly increased (p < 0.05) at 2 minutes and at the end of RB by 53% (42%) and 92% (65%), respectively (95% confidence interval of the mean (CIM)). All subjects observed an increase in the density of leucocytes. At the end of RB, mean systolic brachial arterial pressure and heart rate were significantly increased by 24% (11%) and 37% (15%) respectively (p < 0.01). At 2 minutes, end tidal oxygen concentration in the exhaled air was 47% (8%) (95% CIM) below and carbon dioxide was 41% (16%) above baseline (p < 0.001). The RB produces a large increase in macular leucocyte velocity, suggesting an increase in blood flow. CONCLUSION--Although RB has some systemic risk due to hypoxia and hypercapnia, RB for a short period of 1 or 2 minutes might be of help in the treatment of retinal arterial obstructive diseases in young patients without cardiovascular disorders if other treatments do not show any beneficial effects.

PURPOSE: To assess subfoveal choroidal blood flow (ChBF) in patients with early manifest glaucoma (EMG) and to compare blood flow with functional measures of central retinal integrity, standard automated perimetry (SAP) and pattern electroretinogram (PERG). METHODS: Subfoveal ChBF was determined by confocal, real-time laser Doppler flowmetry in 24 patients with EMG [>-6 dB mean deviation (MD), age range: 29-77 years, visual acuity: 20/25-20/20] and 23 age-matched control subjects. All patients had a therapeutically (topical beta-blockers with or without prostaglandin analogues) controlled intraocular pressure (IOP < 20 mmHg). Subfoveal choroidal blood volume (ChBVol), velocity (ChBVel) and ChBF were determined as the average of three 60 second recordings. In all patients and controls, the PERG and SAP (Humphrey 30-2), following standardized protocols, were also recorded. RESULTS: In patients with EMG, reductions in average ChBVel and ChBF were roughly equal, respectively, by 30% and 33.4% (p < 0.01), when compared to control subjects, so that there was no significant difference in ChBVol between the two groups. Pattern electroretinogram amplitudes were reduced by 46% (p < 0.01) in patients compared to controls. No correlation was found between any of the ChBF parameters and PERG amplitude, or Humphrey 30-2 MD and pattern standard deviation. CONCLUSION: The results suggest a significant alteration of both ChBVel and ChBF in EMG, which does not appear to be associated with the severity of central retinal dysfunction. These findings may contribute to a better understanding of the pathophysiology of early glaucomatous damage in EMG and have implications for the treatment of this pathologic condition.

The effects of strict diabetic control on retinal haemodynamics were studied to elucidate whether such effects are associated with retinopathy changes. In 28 patients with poorly controlled insulin dependent diabetes mellitus and non-proliferative retinopathy, retinal haemodynamics were investigated at baseline, 5 days, 2 months, and 6 months after the institution of strict diabetic control using the bidirectional laser Doppler velocimetry technique and monochromatic fundus photography. Changes in retinal blood flow measured in a major retinal vein (Q) on the fifth day of strict diabetic control correlated significantly with changes in retinopathy level observed at the end of the 6 months of this study (rank correlation 0.65, p < 0.01). On the fifth day of strict diabetic control, 16 out of 20 eyes that showed no progression (NP) of retinopathy at the end of the study had decreases in Q, whereas six out of eight eyes that showed progression (P) had increases in Q. The difference in these changes in Q between P and NP eyes was statistically significant (one way analysis of variance, p = 0.001). No significant changes in Q were detected at 2 months or 6 months. Following the institution of strict diabetic control, no significant changes in time were detected in the regulatory response to 100% oxygen breathing characterised as the percentage decrease in Q at 4-6 minutes of oxygen breathing (analysis of variance, p = 0.36). Changes in Q following institution of strict diabetic control are associated with progression of retinopathy. Measurements described in this study may help identify diabetic patients at risk of progression when their metabolic control is improved.

Background: The relationship between the flicker-evoked changes in optic nerve blood flow (F onh ) and neural retinal activity was investigated by laser Doppler flowmetry and electroretinogram (ERG), respectively. Material and methods: In five normal subjects F onh was continuously recorded before, during and after exposure to green flicker modulation (30° field at the posterior pole), at different levels of mean illuminance between 0.9 and 13.5 lux. During flicker stimulation, ERGs were simultaneously recorded with F onh . The flicker-evoked changes in F onh (RF onh ) and the amplitudes of the first (1F) and second (2F) harmonic component of the ERG were measured. Results: By increasing mean flicker illuminance, RF onh and ERG 2F, but not 1F amplitude first increased and then saturated beyond 10 lux. RF onh and the corresponding 2F amplitudes, recorded at the various mean illuminances, showed a significant positive correlation (r = 0.79, p < 0.01). Conclusions: Under specific experimental conditions, changes in human RF onh are quantitatively correlated with those of the flicker ERG 2F amplitudes. Since the 2F component reflects, unlike 1F, a strong contribution from inner retina, the present findings support the presence of an association between vasoactivity and inner retinal activity changes in the human eye. This finding may provide a new approach to the study of neuro-ophthalmic disorders.

Laser Doppler flowmetry (LDF) was performed on a simulated blood vessel in a model eye through optic nerve tissue sections in order to ascertain the ability to detect flow through them. LDF was performed using either near-infra-red or green laser light. Tissue section thickness ranged from 50 μm to 1000 μm. As expected, we found that our ability to detect flow with LDF decreased as we increased the thickness of optic nerve sections interposed between the LDF apparatus and the simulated blood vessel. We also found that the sampled depth of LDF increased with increasing separation of the optical detection fibre from the centre of the illuminated tissue volume. With adequate separation, we were able to detect flow with LDF through tissue sections of 1000 μm thickness using either near-infra-red or green laser light.

Recessed cathode O2 microelectrodes were used to measure spatially detailed oxygen tension (PO2) gradients in the vitreous humor near the cat retina. Measurement sites (n = 41 in 8 cats) included single arterioles and venules and parallel vessel pairs. Mean vitreous PO2 was 37.9 +/- 1.5 (SE) Torr. Close to the retinal surface (approximately 200 microns), PO2 was found to be both higher and lower than the vitreous PO2, depending on the proximity of the microelectrode tip to retinal vessels. Both positive (inward) and negative (outward) O2 fluxes (JO2) were measured, consistent with the anatomy and expected boundary conditions in the eye. The PO2 at the closest approach above arterioles was 55.2 +/- 2.3 Torr, significantly higher than in the vitreous (P < 0.0001). All arterioles had outward JO2 with an overall mean of -2.58.10(-6) ml O2/sec/cm2. Some of the venules were also losing O2, but at much lower rates than arterioles. Several venules were gaining O2. Countercurrent transport (A-V shunting) was also seen between vessel pairs. Our experimental results allow theoretical predictions to be made for the axial drop in blood PO2 along an arteriole as a function of blood flow.

Clinical use of laser Doppler velocimetry of absolute red blood cell velocity V(max) in the center of major retinal vessels becomes possible with recent advances in microcomputer technology- Speed and automation of Doppler photocurrent analysis and identification of proper laser beam positioning are the major requirements in making direct quantitative assessment of retinal hemodynamics a routine diagnostic tool. We discuss our efforts toward achieving this goal and illustrate our current capabilities with examples of changes in retinal blood flow in response to physiologic maneuvers. In veins, V(max) can now be determined on-line. In arteries, current computing speed only supports intermittent on-line data acquisition.

A new bidirectional laser Doppler velocimeter (LDV) is described for absolute measurement of the speed of red blood cells flowing in individual retinal vessels. The basic component of the instrument is a standard retinal camera that eliminates the need for a contact lens. The laser beam is delivered to the eye through the fundus illumination optical system of the camera. Target fixation is done with the eye under examination. The measurements are independent from the ocular refraction; only the axial length of the eye need be determined. The instrument markedly simplifies the technique of retinal blood flow measurement.

PURPOSE To report on a novel approach to measure the velocity of red blood cells (RBCs) at different retinal vessel depths. METHOD The technique is an extension of conventional laser Doppler velocimetry using light sources of various coherence lengths (CL). Light scattered by the moving RBCs interferes with that reflected from the anterior vessel wall only if the optical path difference is shorter than CL. Therefore, using low coherence light sources, localized measurements of RBCs velocity can be performed. RESULTS Measurements of RBCs velocity at different depths in a main retinal vein (diameter: 152 microns) of a volunteer has been performed using 4 different light sources with CLs of 14 microns, 21 microns, 32 microns and > m. Measured values are in good agreement with theoretically predicted values. CONCLUSION This new approach permits to measure RBCs velocity at different depths of retinal vessels in the human retina.

Sustained pattern stimulation (SPS) induces habituation in the normal pattern electroretinogram (PERG). In this study, the authors evaluated whether sustained flicker stimulation (SFS) induces habituation in the normal flicker ERG (FERG).FERGs were elicited in normal volunteers by an 8Hz flicker stimulus, presented continuously over 3 min after 20 min of light adaptation. One stimulus temporal period was sampled and averaged in packets (n=20) of 60 events, each of 8s duration. Amplitudes and phases of the response 1st and 2nd harmonics (1F and 2F, respectively) were measured. FERG results were compared with those obtained by recording PERGs with a similar SPS paradigm.During SFS, FERG 2F showed a modest increase in amplitude (about 25%, p0.05). No changes were observed for the 1F amplitude and for the phase of both components. In contrast, PERG amplitude showed SPS-induced habituation, described by an exponential decay with a time constant of approximately 20s.The normal FERG, unlike PERG, does not show habituation, suggesting that the adaptive changes of retinal neurons underlying FERG are different from those of PERG generators.Our findings may have implications for diagnosis and/or pathophysiology of retinal disorders involving the inner retina.

Optimal retinal neuronal cell function requires an appropriate, tightly regulated environment, provided by cellular barriers, which separate functional compartments, maintain their homeostasis, and control metabolic substrate transport. Correctly regulated hemodynamics and delivery of oxygen and metabolic substrates, as well as intact blood-retinal barriers are necessary requirements for the maintenance of retinal structure and function. Retinal blood flow is autoregulated by the interaction of myogenic and metabolic mechanisms through the release of vasoactive substances by the vascular endothelium and retinal tissue surrounding the arteriolar wall. Autoregulation is achieved by adaptation of the vascular tone of the resistance vessels (arterioles, capillaries) to changes in the perfusion pressure or metabolic needs of the tissue. This adaptation occurs through the interaction of multiple mechanisms affecting the arteriolar smooth muscle cells and capillary pericytes. Mechanical stretch and increases in arteriolar transmural pressure induce the endothelial cells to release contracting factors affecting the tone of arteriolar smooth muscle cells and pericytes. Close interaction between nitric oxide (NO), lactate, arachidonic acid metabolites, released by the neuronal and glial cells during neural activity and energy-generating reactions of the retina strive to optimize blood flow according to the metabolic needs of the tissue. NO, which plays a central role in neurovascular coupling, may exert its effect, by modulating glial cell function involved in such vasomotor responses. During the evolution of ischemic microangiopathies, impairment of structure and function of the retinal neural tissue and endothelium affect the interaction of these metabolic pathways, leading to a disturbed blood flow regulation. The resulting ischemia, tissue hypoxia and alterations in the blood barrier trigger the formation of macular edema and neovascularization. Hypoxia-related VEGF expression correlates with the formation of neovessels. The relief from hypoxia results in arteriolar constriction, decreases the hydrostatic pressure in the capillaries and venules, and relieves endothelial stretching. The reestablished oxygenation of the inner retina downregulates VEGF expression and thus inhibits neovascularization and macular edema. Correct control of the multiple pathways, such as retinal blood flow, tissue oxygenation and metabolic substrate support, aiming at restoring retinal cell metabolic interactions, may be effective in preventing damage occurring during the evolution of ischemic microangiopathies.

Using laser Doppler flowmetry, slow variations in velocity, volume, and flux of red blood cells in the optic nerve head (ONH), choroid, and retina of the anesthetized minipig have been demonstrated. The variations of velocity and volume were highly regular and vigorous in the ONH and had frequencies ranging from 2.5 to 4.5 cycles/min. The flux variations were smaller or absent due to a phase shift of approximately 180 degrees between the volume and velocity changes. The volume fluctuations were synchronized to those of the PO2 measured in the vitreous, at approximately 50 microns from the surface of the ONH tissue. The fluctuations were less regular in the choroidal and the retinal vessels and their frequencies were higher than those in the ONH. The lack of correlation between the fluctuations in the ONH and those in the retinal and choroidal vessels points toward a local mechanism. The changes in blood volume in the ONH, the phase shift between volume and velocity changes, and the predominance of venules at the surface of the ONH are three factors suggesting that this mechanism involves a change in the diameter of the venules rather than in the arterioles.

The effect of strict glycemic control on retinal volumetric blood flow rate (Q) was investigated in 13 insulin-dependent diabetic patients with laser Doppler velocimetry and monochromatic fundus photography. Strict glycemic control was achieved by glucose monitoring and four daily insulin injections. Q was determined in a major retinal vein at baseline and then 5 days, 2 mo, and 6 mo after the institution of strict control. Level of retinopathy was assessed from stereocolor fundus photographs taken at baseline and 6 mo. After 6 mo of strict diabetic control, five eyes demonstrated progression (P) by one or more retinopathy levels, and eight eyes showed no progression (NP). At 5 days, there was a significant decrease in Q of 1.4 +/- 0.9 microliters/min (P less than 0.005) in NP eyes and a nonsignificant increase in Q of 1.2 +/- 1.7 microliters/min in P eyes. Changes in Q from baseline observed at 5 days were strongly correlated with changes in retinopathy level at 6 mo (r = 0.79, P less than 0.005). No significant changes in Q from baseline were observed at 2 and 6 mo. A lack of decrease in Q at 5 days was associated with the progression of retinopathy that occurs in some patients after the institution of strict glycemic control and may serve as a predictor for progression of retinopathy.

BACKGROUND The measurement of oxygen concentration in the microvasculature of a tissue is important for the understanding of oxygen delivery. MATERIAL AND METHODS A technique was developed to measure in a continuous manner the partial pressure of oxygen (pO2) in the optic nerve microvasculature of the minipig. This technique is based on the quenching by oxygen of the phosphorescence emitted by a dye that is injected intravenously. It provides measurements of the intra-vascular pO2 and was applied simultaneously with measurements of the extra-vascular pO2 with microelectrodes and blood flow by laser Doppler flowmetry. RESULTS We demonstrate for the first that the optic nerve head intra-vascular pO2 increases during dark adaptation. Furthermore no change in extra-vascular pO2 and blood flow could be detected. CONCLUSIONS Based on Krogh's model, these results suggest that oxygen consumption by the optic nerve head tissue increases during dark adaptation.

BACKGROUND In this paper we present a miniaturized blue field simulator (BFS) which allows the measurement of the velocity (V) pulsatility (P) and number (D) of leukocytes in the macular retinal capillaries. A study on the effect of the aperture size of the blue light source at the subject's pupil on the measured flow parameters was performed. METHODS A blue field entoptoscope with a small halogen lamp was used to induce the perception of the "flying corpuscles" and a flat color screen was selected to display a computer simulation of this entoptic phenomenon. The aperture of the blue light source at the pupil was varied by a diaphragm placed at a conjugate pupil plane while the size and retinal irradiance of the blue field stimulus at the fundus was held constant. RESULTS The results show significant correlations between log pupil area and both V (-18% per log unit) and D (+42% per log unit). When the retinal illuminance is expressed in Log Trolands, V shows no dependency, but D is strongly correlated (+40% per log Troland). CONCLUSIONS When applying the BFS technique, the size of the blue light stimulus at the pupil, if undilated, must be well controlled to minimize instrument-related variations of the blood flow measurements.

We have previously shown from direct, in vivo measurements of NO in cats with recessed electrochemical microsensors that NO mediates increases in ONH blood flow during functional activation of the eye by flickering light. We have also reported that there are low frequency (< 15 cycles/min) spontaneous oscillations in NO that appear to be passively coupled to oscillations in blood flow at similar frequencies in the cat ONH. In this paper, we describe similarities between in vivo measurements of NO in the ONH of the cat eye and in the cortex of the rat brain. These data are consistent with a role for NO in the coupling of blood flow with increases in neuronal activity, autoregulation of blood flow, hyperemia, and vasodilation during hypoxia and hypercapnia.

Blood flow studies in the brain, heart, and other organs suggest that there could be interaction between nitric oxide (NO) and adenosine. This possibility was investigated in the optic nerve head (ONH) during photic stimulation of the dark-adapted cat eye. Functional activation of ONH blood flow was measured by laser Doppler flowmetry, simultaneously with NO and PO 2 using double-barrel recessed electrochemical sensors. Photic stimulation (diffuse luminance flickering light at 30 Hz) increased ONH blood flow to 127.4 ± 4.7% (mean ± SEM) of baseline with a transient increase in NO by 79.8 ± 12.8 nM, while P O 2 decreased from 24.5 ± 2.7 to 22.7 ± 2.4 Torr (control responses, 15 trials, 10 cats). Adenosine (3 mg/kg iv) increased baseline ONH blood flow to 113.8 ± 8.4% of control within 5 min. Functional activation of ONH blood flow was enhanced during photic stimulation, reaching a maximum of 155.8 ± 8.1% within 5 min, and remained enhanced for 30 to 45 min. NO responses during photic stimulation were not different from control responses. Treatment with a nonspecific NO synthase inhibitor ( N G -nitro- l -arginine methyl ester, 40 mg/kg iv, 5 cats) did not alter the increase in resting ONH blood flow or the enhanced functional activation after adenosine. We conclude that there is no interaction between NO and adenosine during functional activation of cat ONH blood flow by photic stimulation.

Background Development of a reflectance oximeter to measure non-invasively the changes in oxygen saturation (SO 2 ) in the vessels of the human eye fundus, with the goal of obtaining a better understanding of the retinal circulation in health and disease. Material and methods For retinal oximetry, an instrument must image the retina at multiple wavelengths (lambda) and measure the apparent optical density (OD lambda ) of retinal vessels. The new IRO oximeter acquires two-dimensional images of the retina at four different wavelengths simultaneously. The hemoglobin oxygen saturation is calculated using the spectral images at 569 nm (not sensitive to the oxygenation status of hemoglobin) and at 600 nm (sensitive to the oxygenation status). Results To test the method, two subjects were asked to breathe first room air and then pure oxygen. During hyperoxia (100 % O 2 breathing), the apparent optical density ratio (OD 600 /OD 569 ) decreases significantly, and consequently, the SO 2 in the veins increases. Conclusions The optical density ratio method could be used for relative oxygen saturation measurements, for example, for determining the variation of the SO 2 under various physiological conditions, before and after an intervention or for monitoring retinal diseases such as diabetic retinopathy.

The technique of phosphorescence quenching by O 2 (PQ) allows the non-invasive measurement of the partial pressure of oxygen in blood (pO 2 blood ). This technique and its application to the investigation of the pO 2 blood in the microvasculature of the retina and optic nerve head (ONH) of two animal species is described. Using the imaging mode of PQ, 2-dimensional pO 2 blood maps were obtained to investigate the response of the pO 2 blood to various physiological stimuli in miniature pigs and the effect of experimental glaucoma in monkeys. Applied in its focal mode, PQ allows measurements of the pO 2 blood with a time resolution of 1 second and is adequate to investigate the pO 2 blood time course during light stimulation.

Laser Doppler flowmetry (LDF) has been applied to investigate the effect of increased retinal neural activity induced by diffuse luminance flicker stimulation and dark/light transition on blood flow in the optic nerve head (ONH). We describe the theoretical basis, instrumentation and software for LDF measurements of the blood flow response. The characteristics of this response strongly suggest a coupling between blood flow and neural activity at the ONH. Some of the mechanisms underlying the neurovascular coupling are discussed and problems that remain to be studied to better understand this coupling in normal and pathological conditions are mentioned.

Low coherence laser Doppler interferometry (LDI) allows high precision measurements of the axial length of the eye and of the thickness of the individual layers of the ocular fundus. Here, we used LDI to monitor diurnal changes in these dimensions in eyes of newly hatched chicks and one-year-old chickens with normal or altered visual input. In chicks and chickens with normal visual experience, axial eye length displays diurnal fluctuations increasing during the light phase. Choroidal thickness also exhibits a diurnal pattern, shrinking during the day and expanding during the night. Retinal thickness does not vary. Based on the pressure compliance of the enucleated chick eye, the diurnal intraocular pressure (IOP) fluctuation could contribute both to the increase in axial length and to daytime choroidal shrinkage. Following deprivation of form vision by unilateral goggle wear, occluded chick eyes demonstrate enhanced axial elongation. Diurnal fluctuations in axial length but not in choroidal thickness are temporarily disrupted. The retina of form deprived eyes thins approximately 10% in five days. In contralateral eyes, the diurnal patterns of both axial length and choroidal thickness fluctuations are also disrupted. Following occluder removal in chicks, choroidal thickness increases for several days during both the light and dark phase, leading to its overall expansion. Retinal thickness returns to baseline. When deprived of form vision for five days, the eyes of year-old chickens do not exhibit measurable axial elongation. Diurnal patterns of fluctuation in axial length and choroidal thickness are however disrupted. After goggle removal, axial length fluctuation is restored to normal, but the diurnal choroidal thickness pattern is inverted. In contralateral eyes, choroidal thickness exhibits normal diurnal fluctuations both during and after form vision deprivation. In conclusion, axial length and choroidal thickness fluctuations are influenced by visual experience in both newborn chicks and one-year-old chickens. In selected instances a binocular interaction regarding axial length and choroidal thickness changes is suggested, the effect weakening with age.

The purpose of this study was to determine whether spontaneous oscillations in blood flow (relative red blood cell flux) measured by laser Doppler flowmetry (LDF) in the cat optic nerve head were related to fluctuations in nitric oxide (NO) measured with electrochemical sensors (n = 16 cats). Power spectral densities for the magnitude and frequency of LDF and NO fluctuations were determined by discrete Fourier transform analysis. Complex behavior was found for both LDF and NO oscillations with broad spectra containing peaks at multiple frequencies. Most of the power was in the low-frequency range (10 cycles/min). Spectra were also obtained after administering NO synthase inhibitors (l-nitroarginine, L-NA, n = 6 cats; l-nitroarginine methyl ester, L-NAME, n = 5 cats). Both inhibitors caused a decrease in blood flow, basal NO levels, and amplitude of NO fluctuations. There was little change in amplitude for blood flow oscillations, with some enhancement at the lowest frequencies. We conclude that NO is not required for vasomotion and that spontaneous, low-frequency NO fluctuations observed in the cat optic nerve head are a passive phenomenon caused by natural variations in shear stresses.

Understanding the pathophysiology of the optic nerve head (ONH) requires precise knowledge of the oxygenation of this tissue. Previous investigations used methods based on oxygen sensitive microelectrodes to measure the partial pressure of oxygen in the ONH tissue. These microelectrodes are inserted into the eye through the pars plana. New methods are now being developed to measure the intravascular PO2 in the ONH. They are based on the quenching of phosphorescence of a porphyrin dye by oxygen. Although these methods require an intravenous injection of this dye, the eye is left untouched. This article reviews these noninvasive methods.

Mechanisms coupling blood flow with changes in neuronal activity (functional hyperemia) in the brain, eye, and other organs are not completely understood. Nitric oxide (NO) has been suspected to play an important role, especially in the brain (Iadecola, 1993). We have shown that optic nerve head (ONH) blood flow increases during stimulation of the cat eye by flickering light, depending on the frequency and luminance of the stimulus, using a microscope-mounted infrared laser Doppler system to measure relative changes in ONH blood flow simultaneously with changes in extracellular K+ measured with ion-selective microelectrodes in the vitreous humor immediately above the ONH (Buerk et al., 1995). We have also measured changes in ONH blood flow simultaneously with NO near the ONH using recessed electrochemical microsensors (Buerk et al., 1996). We found that NO increased during neuronal activation by flickering light, and that the functional hyperemia in ONH blood flow and NO increases were significantly attenuated after inhibiting NO synthase. In this report, we describe simultaneous ONH blood flow, NO and PO2 measurements during increased neuronal activity with flickering light. Preliminary results are consistent with coupling of functional hyperemia with increased O2 consumption, but multiple interactions with NO need to be taken into account.

AIMS--The purpose of this study was to investigate the effect of instituting strict diabetic glycaemic control on the retinal macular microcirculation and to compare this effect with that observed in the main retinal veins. METHODS--In 28 insulin dependent diabetic patients with poor glycaemic control a regimen of strict diabetic control, consisting of four daily insulin injections was instituted and maintained for 6 months. Retinal haemodynamics were investigated in the macular microcirculation by the blue field simulation technique and in the major retinal veins by a combination of bidirectional laser Doppler velocimetry and monochromatic fundus photography. Progression of diabetic retinopathy was assessed from fundus photographs taken at baseline and at the end of the study. RESULTS--Institution of strict diabetic control resulted in a significant increase in leucocyte velocity in the macular circulation (p = 0.013). No significant difference in this increase was observed between eyes that showed progression (n = 8) and no progression (n = 20) of retinopathy during the study. Significant correlations were found between relative changes over time of blood flow measured in the main retinal veins and relative changes of leucocyte velocity determined in the macular microcirculation at 2 months (p = 0.008) and 6 months (p = 0.001) but not at 5 days (p = 0.49). In the eight eyes that showed progression of retinopathy, the product of leucocyte velocity and density at baseline was significantly higher than normal (p < 0.05). During the length of this study, this product was also significantly higher in the eight eyes that showed retinopathy progression than in the 20 eyes that did not show progression (p = 0.005). CONCLUSION--Our results suggest that increased flow in the macular microcirculation may be associated with progression of retinopathy, thus supporting the hypothesis that increased blood flow may play a role in the development of diabetic microangiopathy. Although there are correlations between the changes detected in the macular microcirculation and those measured in the main retinal vessels, there are also differences which need to be further investigated in order to better understand pathogenetic mechanisms.

Laser Doppler flowmetry (LDF) is a technique that measures relative average velocity, number and flux (number times velocity) of red blood cells in a tissue. In this paper, we demonstrate its application in the optic nerve head tissue, describe the laser delivery and light scattering detection schemes and investigate the effect of the distance between the sites of illumination and detection. We also provide evidence that the flow measured by LDF varies linearly with actual blood flow in the optic nerve and examine the question of the depth of the sampled volume. Experiments in anesthetized cats illustrate potential applications which make use of the high temporal resolution of LDF. These include the response of blood flow to changes in the composition of the breathing gases and changes induced by neuronal stimulation with multiple and single flashes.

PURPOSE To investigate whether light and dark exposures induce a response of choroidal blood flow (ChBF) in the foveal region in humans. METHODS In a group of healthy volunteers (age 25-60 years) ChBF was measured using a new confocal laser Doppler flowmeter (probing laser at 785 nm, power at the cornea = 90 microW). ChBF was recorded at room light, in darkness, at room light following dark adaptation, and during strong light exposure following room light. RESULTS While ChBF was stable during room light condition, it decreased significantly by 15% (p < 0.01) during darkness. After 6 min of room light following darkness, ChBF was back to baseline. Strong diffuse, green light exposure over a field of 45 degrees had no detectable effect on ChBF. In all the experiments, no significant change of blood pressure was detected. CONCLUSIONS Our findings did not confirm the presence of an active process of ChBF regulation in response to strong light exposure in humans. They demonstrate, however, a reversible decrease in ChBF occurring after a transition from room light to darkness.

PURPOSE: Laser Doppler measurements performed immediately after the transition from dark adaptation (DA) to light led to the hypothesis that retinal blood flow, Fret, is increased during DA, but the use of visible lasers had prevented measurements during DA. Our aim was to test this hypothesis by measuring Fret during and after DA. MATERIAL AND METHODS: Fret in retinal vessels at the optic disk surface was recorded quasi-continuously in one eye of 6 normal subjects (age 27 to 60 years) using a laser Doppler flowmeter in the near-infrared (810 nm). Measurements were performed during light (baseline), various periods of DA and again during light. DA lasted between 2 and 32 min. RESULTS: Average Fret for the 6 subjects did not change significantly (-2.7 +/- 8% sd, p > 0.05) during the various periods of DA, as determined from linear regressions of the flux versus time. Following the transition from DA to light, there was, in most cases, a rapid transient increase of the flux, which reached an average value of 37 +/- 10% above the pre-transition value and peaked at 30-60 sec after the transition. CONCLUSIONS: These results do not support the hypothesis that Fret in normal volunteers is increased during DA. Rather, they strongly suggest that the transient increase in flux observed after DA is induced by the transition from dark to light (FNSRS #3200-043157 et CNR, It. #95.01715.CT04).

The effect of diffuse luminance flicker stimulation of a large area (≈ 30° diameter) on red blood cell flux ( F ) in the optic nerve head was measured in the anesthetized cat. F increased markedly during sustained flicker. The F -response to the initiation and cessation of the stimulation was found to occur within a few seconds. Upon sustained stimulation, the increase in F reached a plateau within approximately 2 min. Its level depended upon the intensity, frequency and wavelength of the stimulation and the state of adaptation of the retina. This stimulus offers a new and powerful means of investigating blood flow regulation in the optic nerve head (ONH).

Transretinal PO2 profiles were recorded with O2-sensitive microelectrodes in the normal retina and in ischemic retinal foci induced by the occlusion of a retinal branch vein with argon laser photocoagulation in anesthetized miniature pigs. In the normal retina there are two PO2 gradients: one from the inner retina and the other from the choroid, both directed toward the middle of the retina. Both PO2 gradients persisted during hyperoxia. Thus, even in hyperoxia, the choroid does not supply the whole thickness of the normal retina with O2. Preretinal and transretinal PO2 measurements in ischemic inner retinal foci showed the existence of two PO2 gradients in steady-state systemic normoxia, as did those in the normal retina. This finding indicates that even in ischemia the choroid does not supply O2 to the inner retina; as a result, tissue hypoxia is maintained. During systemic hyperoxia, the intraretinal PO2 measurements in the ischemic foci showed only one gradient going from the choroid toward the inner retina. This gradient indicates that under these conditions, the choroid can supply O2 to the entire thickness of the ischemic retina. Extending a previously formulated hypothesis, we propose that in the ischemic retina as opposed to the normal retina, hyperoxia does not induce an increase in the O2 consumption of the outer retina. This suggestion could explain the rise in PO2 in the inner ischemic retina during hyperoxia.

We report on what we believe is a novel approach to measuring the velocity of red blood cells (RBC’s) at different depths of retinal vessels by use of low-coherence sources. The technique, variable coherence optical Doppler velocimetry (VCODV), performs Doppler shift measurements through autodyne mixing between the light scattered by the RBC’s and by the vessel front wall (reference). Only the light from RBC’s moving at a depth less than half the coherence length (CL) mixes efficiently with the reference. Measurements of the Doppler shifts from RBC’s with sources of four different CL’s in a 152-µm vein of a volunteer confirmed the feasibility of VCODV. This approach has the potential to monitor in vivo retinal RBC velocity gradient at the vessel wall and the velocity profile within the blood vessel in the condition of symmetric blood flow profiles.