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Biomedical subjects

C E Riva

Publications and source records attributed to C E Riva.

At least 37 records · Page 2Linked to original sources
PubMed1998

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.

Adolescent↗
PubMed1998

Vasomotion and spontaneous low-frequency oscillations in blood flow and nitric oxide in cat optic nerve head.

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.

Animals↗
PubMed1998

[Flicker stimulation induces retinal vasodilation in man].

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.

Adolescent↗
PubMed1998

[Blood flow measurement in the optic nerve head during isometric exercise].

PURPOSE: Autoregulation of optic nerve head blood flow (Fonh) in response to decreases in perfusion pressure has been demonstrated in animals and humans. The aim of this study was to determine change in Fonh when systemic blood pressure is increased. METHODS: Blood flow parameters, i.e. relative mean velocity, number, and flux of red blood cells in the ONH tissue (Velonh, Volonh and Fonh, respectively) were measured by laser Doppler flowmetry in one eye of 13 normal subjects (aged 16 to 58 years), at baseline, during, and after isometric exercises consisting of squatting. Brachial artery blood pressure was measured by sphygmomanometry. IOP was measured at baseline and at the end of squatting. RESULTS: During squatting mean arterial pressure increased from 103 +/- 6 mm Hg to 139 +/- 58 mm Hg (average +/- 95% confidence interval), IOP increased from 13 +/- 0.5 to 17 +/- 1 mm Hg. An average increase in PPm from 56 +/- 4 to 80 +/- 7 mm Hg induced no significant (p > 0.05) change in the blood flow parameters. The sensitivity (detection threshold) of the blood flow changes was 8%. CONCLUSION: This study shows for the first time in human autoregulation of Fonh when PPm is increased by increasing the systemic blood pressure.

Adolescent↗
PubMed1998

Noninvasive measurement of oxygen tension in the optic nerve head.

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.

Blood Flow Velocity↗
PubMed1997

Photographic and angiographic characterization of the retina of Kenyan children with severe malaria.

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.

Blood-Retinal Barrier↗
PubMed1997

Autoregulation of human optic nerve head blood flow in response to acute changes in ocular perfusion pressure.

BACKGROUND: Studies in animals have demonstrated that optic nerve head (ONH) blood flow (F(onh)) is autoregulated, but there is a lack of evidence for such a process in humans. Therefore, we investigated the relationship between F(onh) and mean ocular perfusion pressure (PPm) in normal volunteers when PPm is decreased through elevation of the intraocular pressure (IOP). METHODS: Laser Doppler flowmetry (LDF) was used to measure relative mean velocity (Velohn), volume (Volonh) and F(onh) of blood at sites of the ONH away from visible vessels, while PPm was decreased in two ways: (1) rapidly, by IOP increments of 15 s duration, and (2) slowly, by IOP increments of 2 min duration, both by scleral suction cup in one eye of each of nine subjects. RESULTS: A rapid and large decrease of PPm of more than 100% induced a decrease of more than 80% in F(onh). With the slower decrease in PPm, F(onh) remained constant down to a PPm of approximately 22 mm Hg (IOP = 40 mm Hg) and then decreased, predominantly due to a decrease in Velohn. Immediately after removal of the suction cup, F(onh) increased transiently by 44% above baseline. CONCLUSIONS: This study demonstrates efficient blood flow autoregulation in the OHN, which is probably brought about by an increase in vascular capacitance. The magnitude of the reactive hyperaemia agrees with the compensatory decrease in ONH vascular resistance during IOP elevation. The time scale of the autoregulatory process and the dependence of the hyperaemia upon duration of IOP elevation suggest a metabolic mechanism of autoregulation.

Adult↗
PubMed1997

[Measuring leukocyte velocity in macular capillaries using a miniaturized blue field simulator: effect of aperture of the pupil].

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.

Adult↗
PubMed1997

[Effect of changes in ocular perfusion pressure on choroid ischemia in man].

BACKGROUND: The effect of changes in ocular perfusion pressure (PPm) on the choroidal blood flow (ChBF) in man was studied with the laser Doppler flowmetry (LDF) technique. MATERIALS AND METHODS: We changed the PPm by increasing the intraocular pressure (IOP) or by increasing the blood pressure (BP) with isometric exercises. RESULTS: We observed that a) ChBF was not significantly different from baseline up to an IOP of 27 mm Hg and b) ChBF remained constant even if PPm increased by as much as 72%. CONCLUSION: Our results suggest that ChBF is autoregulated in response to an increase in IOP up to about 27 mm Hg. ChBF remains also constant in spite of an increase in systemic BP, probably due to a vasoconstriction induced by increased sympathetic activity.

Adult↗
PubMed1997

Continuous measurements of intra-vascular pO2 in the pig optic nerve head.

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.

Animals↗
PubMed1997

Diffuse luminance flicker increases retinal vessel diameter in humans.

PURPOSE: To determine retinal vessel diameter variations in response to neuronal activity induced by diffuse luminance flicker. METHODS: The diameter of retinal arteries and veins was measured in 9 normal subjects by computer analysis of fundus pictures taken in monochromatic light under normal conditions of illumination and after 1 min of sinusoidally varying diffuse luminance flicker at 10 Hz. RESULTS: The diameter immediately after flicker was significantly larger than the pre-stimulus diameter by 4.2 +/- 2.2% for the retinal arteries and 2.7 +/- 1.7% for the retinal veins (mean +/- SD). Six is 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 increase most probably reflects an increase in retinal blood flow previously evidenced in humans by the blue field simulation technique. The technique needs to be optimized in terms of flicker parameters, to determine whether flicker-evoked retinal diameter changes could represent a useful clinical measure of the capability of the retinal vascular system to vasodilate.

Adolescent↗
PubMed1997

Effect of acute decreases of perfusion pressure on choroidal blood flow in humans.

PURPOSE: To investigate the relationship between choroidal blood velocity (ChBVel), blood volume (ChBVol) and blood flow (ChBF) in the foveal region of the human ocular fundus and ocular perfusion pressure and to determine whether the choroidal circulation has some autoregulatory capacity. METHODS: Measurements of ChBVel, ChBVol and ChBF were obtained by laser Doppler flowmetry in healthy subjects (age range, 21 to 57 years) with normal eye examination results. Measurements were performed at normal intraocular pressure (IOP) and during successive step increases in IOP induced by scleral suction. In experiment 1, in six eyes (five subjects), the IOP was increased rapidly, in steps of 50 to 100 mm Hg of suction pressure, which each lasted approximately 10 seconds to a level above diastolic ophthalmic artery blood pressure (IOP = approximately 72 mm Hg). In experiment 2, in 14 eyes (seven subjects), the IOP was increased slowly in four successive steps at 2-minute intervals to a level of approximately 42 mm Hg. We also determined the pulsatility of the flow parameters during the heart cycle, pulsatility = 1 - diast value/syst value. RESULTS: For both rates of suction cup increase, the relationship between ChBFm (mean ChBF over the heart cycle) and mean perfusion pressure was not linear. At high pressure, ChBFm was less affected by decreases in the pressure than expected from a passive vascular system. In some cases, no change in ChBFm was detectable, although significant changes in PChBF occurred. Further decreases in perfusion pressure resulted in a proportional decrease in ChBFm. On release of suction, a significant increase in ChBFm over baseline value was detectable in experiment 1. CONCLUSIONS: The relationship between ChBFm and ocular mean perfusion pressure appears to be bilinear and reveals some autoregulation for moderate step decreases in perfusion pressure. The temporal characteristics of the ChBFm-response suggest a neural or passive hemodynamical process rather than a myogenic or metabolic compensatory mechanism.

Adult↗
PubMed1997

[Autoregulation in ischemia of the optic nerve in the human].

BACKGROUND: Autoregulation is defined as the maintenance of constant blood flow in a vascular system in spite of changes in perfusion pressure (PPm). MATERIALS AND METHODS: PPm was decreased by increasing the intraocular pressure (IOP) with a suction cup and optic nerve blood flow was measured with the laser Doppler flowmetry technique (LDF) in 9 normal volunteers. RESULTS: The blood flow was autoregulated down to a PPm of 13 mm Hg (IOP = 47 mm Hg). CONCLUSIONS: These results confirm previous studies in cats and monkeys. The mechanism of autoregulation is probably a decrease in resistance due to capillary recruitment.

Adult↗
PubMed1997

Choroidal blood flow during isometric exercises.

PURPOSE: To investigate the response of choroidal blood flow in the foveal region of the human eye to increases in mean perfusion pressure (PPm = mean ophthalmic artery pressure - intraocular pressure; IOP) induced by isometric exercises. METHODS: Using laser-Doppler flowmetry, changes in velocity (ChBVel), number (ChBVol), and flux (ChBF) of red blood cells in the choroidal vascular system in the foveal region of the fundus were measured in both eyes of 11 normal subjects (ages 18 to 57 years) during isometric exercises. RESULTS: During 90 seconds of squatting, PPm increased by an average of 67%, from 46 to 77 mm Hg. This resulted in a significant increase of 12% in ChBFm (the mean of ChBF during the heart cycle), mainly caused by an increase in ChBVelm. A further increase in PPm to a value approximately 85% above baseline resulted in a 40% increase in ChBFm. A significant negative correlation was found between the changes in ChBVelm and ChBVolm, during squatting. CONCLUSIONS: Previous studies have demonstrated that during isometric exercise, blood pressures in the ophthalmic and brachial arteries rise in parallel. These observations and the current results indicate that an increase in PPm up to 67% induces an increase in choroidal vascular resistance that limits the increase in choroidal blood flow to approximately 12%. This regulatory process fails when PPm is further increased.

Adolescent↗
PubMed1996

Nitric oxide has a vasodilatory role in cat optic nerve head during flicker stimuli.

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.

Animals↗
PubMed1996

Effect of decreased ocular perfusion pressure on blood flow and the flicker-induced flow response in the cat optic nerve head.

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.

Animals↗
PubMed1996

Effects of endothelin-1 on optic nerve head blood flow in cats.

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.

Animals↗