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Biomedical subjects

G T Feke

Publications and source records attributed to G T Feke.

45 records · Page 3Linked to original sources
PubMed1983

Response of human retinal blood flow to light and dark.

The laser Doppler technique was used to measure alterations in retinal blood flow in human subjects under dark-adapted conditions compared to light-adapted conditions. Retinal blood flow increased 40 to 70% after the transition from light to dark. We show quantitatively that the measured increases in retinal blood flow provide the appropriate increases of available oxygen for the inner retinal tissue necessary to compensate for increased oxygen consumption in the dark by the photoreceptor-RPE complex. In addition, we show that measured increases or decreases in human retinal blood flow are quantitatively linked to two intrinsic characteristics of the rod photoreceptor: its metabolic response to known, graded light levels, and its rhodopsin-mediated wavelength-dependent light-absorption properties. We also discuss how light-activated blood flow changes may provide a physiologic test for the autoregulatory capacity of the retinal vasculature.

Adaptation, Physiological↗
PubMed1982

Prolongation of the retinal mean circulation time in diabetes.

Retinal mean circulation time (MCT) and vascular sizes were measured in 21 normal individuals and 32 individuals with diabetes, and segmental blood flow (SBF) was calculated. The MCT was similar in the normal individuals (4.0 +/- 1.1 s) and the individuals with diabetes (4.2 +/- 1.9 s) when seven individuals with diabetes with prolonged but not quantifiable MCT were excluded. Including them by nonparametric statistics revealed that MCT was significantly longer in individuals with diabetes with proliferative retinopathy than in normal individuals or in individuals with diabetes with nonproliferative retinopathy. The prolonged MCT correlated significantly with advanced retinopathy as judged by leakage, neovascularization, and the need for photocoagulation therapy. Reduced SBF may account for the prolonged MCT, since the increase in vascular sizes observed failed to do so. However, pathologic vascular changes may alter the relation between SBF and MCT. Irrespective of implications about SBF, prolonged MCT, which indicates marked circulatory disturbance, represents an important new observation in diabetic retinopathy.

Adult↗
PubMed1982

Laser Doppler measurements of the effect of panretinal photocoagulation on retinal blood flow.

The laser Doppler technique was used to measure the pulsatile characteristics of the retinal arterial blood flow in diabetic patients with severe retinopathy immediately before argon laser panretinal photocoagulation (PRP) and again one to two months after PRP. In each measurement the relative variation of the maximum red blood cell velocity, Vmax, in a major branch retinal artery was determined throughout the cardiac cycle. Flow pulsatility was defined as the ratio of Vmax at maximum systole to Vmax at minimum diastole. Following PRP, each eye showed a decrease in flow pulsatility as well as a decrease in retinal arterial and venous diameters. The measurements are consistent with an autoregulatory response of the retinal circulation to increased inner retinal oxygen concentration following PRP. Laser Doppler measurements may be a useful means of assessing the effectiveness of a particular PRP treatment.

Adult↗
PubMed1979

Mean circulation time of fluorescein in retinal vascular segments.

Mean circulation time (MCT) of sodium fluorescein in retinal vascular segments was determined in healthy human volunteers, using the dye dilution technique. The fluorescein dilution curves were recorded with a two-point fluorophotometer. Average MCT was 3.9 seconds. The accuracy of the method was determined from repeated measurements in owl monkeys and human volunteers. No significant linear correlation was found between the MCT and the cross section of the vessels feeding and those draining the segment. The application of this technique to clinical work is discussed, and is illustrated by three clinical cases.

Adult↗
PubMed1978

Fluorescein dye-dilution technique and retinal circulation.

Using theoretical models for the flow of fluourescein dye in retinal arteries and veins, we have determined the effects of optical absorption in blood of the incident excitation light and the emitted fluorescent light on the time course of measured fluorescence intensity, I(t). Our results indicate that I(t) curves recorded from arteries adequately represent the mean time course of the fluorescein concentration (C(t)), when either a circular or rectangular light-collecting aperture is used. I(t) curves recorded from veins adequately represent C(t), but only when a circular aperture of approximately the same diameter as that of the vessel is used. A two-point fluorophotometer, which provides simultaneous, on-line measurements of arterial and venous I(t) curves is described. Typical recordings obtained with the instrument are shown and the method employed to analyze the curves quantitatively is described in detail. This method, which consists of fitting the first passage of the fluorescence intensity curve with a log-normal function, provides results that are more accurate than those obtained using the standard exponential extrapolation method.

Dye Dilution Technique↗
PubMed1978

Laser Doppler measurements of blood velocity in human retinal vessels.

We have measured Doppler shift frequency spectra (DSFS) of laser light scattered from red blood cells (RBC's) flowing through individual human retinal vessels. Each spectrum exhibits a cutoff frequency which is directly related to the maximum RBC velocity at the measurement site. Meaningful DSFS can be measured in times as short as 0.1 s. DSFS obtained from suspensions of polystyrene spheres and RBC's flowing through glass capillary tubes are used to illustrate the basic principles of the technique and to aid in the interpretation of the in vivo measurements. Our measurements show that maximum RBC velocities in retinal arteries vary markedly during each cardiac cycle.

Blood Flow Velocity↗