Effect of visual stimulation on blood oxygenation in the optic nerve head of miniature pigs: a pilot study.

BACKGROUND: Visual stimulation is increasingly used to investigate the coupling between neuronal activity, blood flow and metabolism in the neural tissue of the ocular fundus. In an attempt to clarify whether the oxygen metabolism is involved in this coupling, we investigated the changes in the partial pressure of oxygen of venous blood (pO (2,blood)) in the optic nerve head of pigs in response to two different visual stimuli. MATERIALS AND METHODS: In 3 miniature pigs, the pO (2,blood) was measured in the optic disk rim using the technique of phosphorescence quenching by oxygen. This parameter was recorded every 8 seconds during a dark-to-light transition and during diffuse luminance flicker (field of 30 degrees centered at the optic disk, temporal frequencies of 2 to 80 Hz). RESULTS: The venous pO (2,blood) level (mean +/- standard deviation) did not change between dark- and light-adapted conditions (26.2 +/- 5.3 and 26.0 +/- 6.2 mm Hg, respectively), nor did we observe any transient change of pO (2,blood) during the light adaptation phase. On the other hand, the venous pO (2,blood) increased, on average, relative to its level during continuous light conditions (24.5 +/- 1.9 mm Hg) by at least 6 % for all flickering frequencies, with a maximum response of 14 % at 15 Hz. CONCLUSIONS: The phosphorescence quenching technique can reveal changes in venous pO (2,blood) induced by visual stimulation. Our results show that the pO (2,blood) in the optic nerve head of miniature pigs does not change with the light adaptation state of the retina, but increases during flicker stimulation with a band-pass type response. The previously reported increase of the ONH blood flow in response to flicker stimulation could lead to this increase of pO (2,blood).

[Effect of squatting on sub-foveal blood flow defect in pseudophakic eyes operated by cerclage]. / Effet du squatting sur le débit sanguin sous-fovéolaire dans les yeux pseudophaques opérés par cerclage.

UNLABELLED: ZIEL: To investigate the relationship between velocity (Velch), blood volume (Volch) and blood flow (Fch), and the mean ocular perfusion pressure (PPm) in the foveal region, and to determine how the regulatory capacity of the choroidal circulation is affected after an encircling buckle procedure. METHODS: We investigated both pseudophakic eyes of 6 patients (age range 56-79 years) in a masked study. Subjects presenting eye diseases (glaucoma, uveitis, diabetic retinopathy) as well as systemic diseases were excluded from the study. All subjects had in one eye a successful management of retinal detachment with an encircling buckling; the second eye was considered as control. Measurements of Velch, Volch and Fch were obtained by Laser Doppler Flowmetry (LDF) at baseline and during isometric exercise (squatting). RESULTS: In the operated eyes, Velch and Fch increased significantly (ANOVA, p < 0.05) during the PPm raise, which was not the case for Volch (ANOVA, p > 0.05). In the control eyes, all hemodynamic parameters remained unaffected by the PPm increase (ANOVA, p > 0.05). The response of Velch and Fch was significantly different (ANCOVA, p < 0.002) between operated and control eyes. CONCLUSION: These results suggest that encircling buckle does affect subfoveal choroidal blood flow regulation, which may explain a possible macular dysfunction in the operated eyes.

Iris blood flow response to acute decreases in ocular perfusion pressure: a laser Doppler flowmetry study in humans.

The relationship between blood flow and ocular perfusion pressure in the iris vascular bed of the human eye has not been established yet. Consequently, it is not known whether the iris circulation has some autoregulatory capacity. The aim of the present study was to investigate this relationship in the particular case where the perfusion pressure was decreased by increasing the intraocular pressure. Using laser Doppler flowmetry, mean velocity, volume and flow of blood through the iris were measured in normal subjects during acute decreases of the mean ocular perfusion pressure induced by raising the intraocular pressure with a scleral suction cup. Two experimental paradigms were applied. In the first, the suction pressure was rapidly increased from baseline, in steps of 50-100 mmHg, to a level where the intraocular pressure was above the mean ophthalmic artery blood pressure. In the second, the suction pressure was increased from baseline in four successive steps of 50 mmHg each. The suction pressure was kept constant for 2 min at each step. With the first paradigm (nine eyes), a 72% decrease in perfusion pressure induced a 63% decrease of iris blood flow due mainly to a drop in blood velocity. Immediately after suction release, blood flow increased by 62% above baseline and then returned to its baseline value within 2 min. With the second paradigm (six eyes), a 28% decrease in perfusion pressure resulted in a 30% flow reduction, which was due to significant decreases (P<0. 001) of both blood volume and velocity. Combining the results of both paradigms, we observed a significant linear correlation between iris blood flow and perfusion pressure (R =0.964, P<0.001). These results demonstrate that a decrease of the perfusion pressure due to an increase of the intraocular pressure induces a decrease of the iris blood flow. No evidence of an autoregulatory process in the iris vascular bed could be demonstrated. A reactive hyperemia was observed in response to a sudden increase in perfusion pressure occurring after a period of decreased blood flow.

[Effect of decreased ocular perfusion pressure on iris blood flow measured by laser Doppler flowmetry]. / Effet d'une diminution de la pression de perfusion oculaire sur le flux sanguin dans l'iris mesuré par fluxmétrie laser Doppler.

PURPOSE: To determine whether iris blood flow (IBF) is regulated in response to an acute decrease in mean ocular perfusion pressure (PPm = MOAP-IOP, MOAP = mean ophthalmic arterial pressure) induced by increasing the intraocular pressure (IOP). METHODS: Iris blood flow was measured using a slit lamp incorporating a laser Doppler flowmetry (LDF) module. The study was conducted on 12 normal volunteers (14 to 59 years old). IOP was raised using a scleral suction cup. In Exp. #1, the suction pressure was successively raised in steps of 50 to 100 mm Hg, each lasting about 10 sec, until IOP reached the MOAP level. In Exp. #2, the suction was raised to 200 mm Hg in 4 successive steps of 2 min duration. RESULTS: In Exp. #1, no significant change of IBF was observed for small decreases of PPm (< 23%); greater decreases of PPm resulted in a linear IBF decrease (p < 0.01). In Exp. #2, such a IBF versus PPm decrease was also observed (p < 0.001). Immediately after release of suction, a significant, transient IBF increase of 79% above baseline level was observed. CONCLUSION: These results suggest that some IBF regulation occurs for small PPm decreases (< 23%); no IBF compensatory mechanism appears to operate for further decreases of PPm (> 23%).

Blood flow in the human iris measured by laser Doppler flowmetry.

A new instrument based on laser Doppler flowmetry (LDF) has been developed to determine noninvasively the relative flux of red blood cells (RBCs) through the microcirculatory network of the iris of the human eye. The probing laser, photodetector, and target fixation devices required by this method were adapted to a slit lamp. Electronic processing and computer analysis of the Doppler signal allow determination of relative velocity, number, and flux of RBCs in the iris, as well as the pulsatility of these flow parameters during the heart cycle. Based on measurements in one eye of eight normal volunteers, the sensitivity of the technique, i.e., the minimum change detectable at the P < 0.05 level, was 4% for the flux. The decrease in blood flow in response to decreases of the ocular perfusion pressure demonstrates the capability of the technique to detect flow changes and its suitability for investigating the physiology and the pharmacology of iris circulation.

Blood flow in the human optic nerve head during isometric exercise.

Investigating blood flow autoregulation in the optic nerve is important to understand the physiopathology of various ocular diseases such as glaucoma. This investigation requires that one establishes the relationship between optic nerve blood flow and perfusion pressure. Previous work has documented the effect of lowering the perfusion pressure on optic nerve blood flow. The purpose of the present study was to investigate the effect of elevated perfusion pressure on blood flow in this tissue. Laser Doppler flowmetry was applied to measure relative mean velocity, volume and flux of red blood cells in the tissue of the optic nerve head. These parameters were measured in 13 subjects during isometric exercise consisting of squatting. In the range of perfusion pressures from 56+/-4 to 80+/-5 mmHg (30+/-8%), there was no significant variation of mean velocity, volume and flux of red blood cells, but vascular resistance increased by about 50%. Intraocular pressure was increased significantly above baseline at the end of squatting and decreased during recovery. The results suggest that the maintenance of constant blood flow is achieved by an increase in vascular resistance taking place either at the arterioles feeding or at the veins draining the blood from the ONH or at the ophthalmic artery and/or vessels between this artery and the site of LDF measurements. Combining the results of this study with those of a previous one where perfusion pressure was decreased by increasing the intraocular pressure, we show the entire relationship between perfusion pressure and optic nerve blood flow in man.