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Introduction: We investigated the effects of head-down tilt (HDT), which simulates microgravity during spaceflights, on the choroidal pulsatile ocular blood flow (POBF). This investigation is important because alterations in the choroidal blood flow can affect the function of retinal rods and cones that rely totally on the choroid for metabolites., Methods: Nineteen healthy adults between 20 and 38 yr of age participated in this study. The POBF was compared for: 1) baseline, wherein subjects were declined 30 degrees from vertical; 2) microgravity simulation where subjects were in a 7 degrees HDT for 2 min; 3) 90 min of the 7 degrees HDT; and 4) recovery, i.e., back at 30 degrees for 2 min., Results: The group averaged POBF (Mean +/- SEM values: 828.43 +/- 48.88 microL x min(-1)) decreased immediately during the 2-min microgravity simulation (582.18 +/- 43.62 microL x min(-1)), remained at that inferior level at the 90-min mark of HDT (542.26 +/- 45.35 microL x min(-1)), and came back toward baseline POBF during the recovery period (760.11 +/- 46.03 microL x min(-1)) (p = 0.0001)., Discussion: The results show that simulated-microgravity of relatively short duration induces retinal hypoperfusion throughout the microgravity interval through the reduction in the POBF. This finding may have important implications regarding visual performance in space crewmembers placed in prolonged microgravity environments.

The objective of the present study was to reveal an interaction between choroidal blood flow (ChBF) and light-induced photoreceptor activity, a physiological coupling that has been already demonstrated for retinal blood flow but rejected for ChBF. Ten healthy adults volunteered for this study. A real-time recording near-infrared laser-Doppler flowmeter was used to quantify the subfoveal ChBF while the luminance of blue flicker between 1 and 64 Hz was first increased and then decreased by 4.0 log units in 1.0-log unit steps between 0.0375 and 375 cd/m2. In separate testing, flash electroretinograms (ERGs) provided electrophysiological indexes of the relative response of short-wave cones (s-cones) and rods to blue light stimulation. Group-averaged, normalized ChBF measurements revealed that it was modulated by approximately 9% by flicker frequency. Increasing the blue flicker luminance from low to high attenuated the subfoveal ChBF, volume, and velocity by approximately 32%, approximately 30%, and approximately 5%, respectively. Decreasing the luminance from high to low over the same range had no effect on the subfoveal choroidal hemodynamics. The markedly different effects of reversed directions of change in blue flicker luminance on the subfoveal ChBF were linked to transitions between rod-dominated and s-cone-dominated retinal responses. Collectively, these findings indicate that the blue light-induced photoreceptor response is associated with a differential distribution of the ChBF across the ocular fundus according to the degree and type of retinal photoreceptor stimulated.

Purpose: To investigate the effects of transient mild systemic hypoxia on the pulsatile ocular blood flow (POBF) in the healthy young adult., Methods: Two measurements of the intraocular pressure (IOP) pulses, used to derive the POBF, were recorded from 19 subjects before, during, and after breathing 12% oxygen in nitrogen. Many physiological variables were also assessed throughout testing., Results: Transient mild systemic hypoxia decreased the hemoglobin oxygen saturation and end-tidal carbon dioxide, increased the heart rate, but did not change the respiratory rate, systemic blood pressure, or ocular perfusion pressure. Mild systemic hypoxia decreased the intraocular pulse volume and the systolic and diastolic times but did not alter the pulse amplitude or the POBF. The IOP was not altered during mild systemic hypoxia., Conclusions: The absence of change in the POBF during transient mild systemic hypoxia indicated that the global pulsatile choroidal blood flow was not vulnerable to the effects of this transient mild systemic hypoxic stress in the healthy young adult.

Purpose: The high metabolic rate of the human retina is supported by the choroidal vasculature. Knowledge of the normal choroidal blood flow (ChBF) responses to various physiological stimuli is therefore highly important if the pathophysiology of ocular diseases involving the choroid is to be understood better. In the present study, the hemodynamic responses of the subfoveal ChBF were examined during and after an exercise-induced increase in the ocular perfusion pressure (OPP)., Methods: Twenty-six healthy volunteers, 19 to 55 years of age participated in this two-phase study. Each subject increased resting OPP through stationary biking at a heart rate (HR) of 140 beats per minute (bpm) over 20 minutes. The ChBF was measured by laser Doppler flowmetry (LDF), the systemic BP by electronic sphygmomanometry, and the resting intraocular pressure (IOP) by applanation tonometry., Results: The OPP increased by approximately 43% at the onset of biking, and then decreased biphasically to approximately 12% above resting value by the end of biking. The ChBF remained within 10% of its basal value throughout biking. Immediately after biking, the OPP decreased twice as much as the ChBF in the same time frame., Conclusions: The dissociation between the OPP and the ChBF during biking and recovery suggests that some mechanism keeps the ChBF close to its basal value, an observation that indicates blood flow regulation.

Purpose: Previous studies in humans have demonstrated that oxygen (O2) reduces retinal vessel caliber and blood flow, whereas carbon dioxide (CO2) usually has opposite effects. The influence of O2 and CO2 on choroidal circulation is not fully understood, however. This study was conducted to determine the effects of systemic hyperoxia and hypercapnia on global choroidal hemodynamics, as evaluated by pulsatile ocular blood flow (POBF) tonography., Methods: In this experiment, 16 and 22 healthy volunteers breathed 100% O2 and carbogen, respectively. POBF and intraocular pressure (IOP) were measured twice with a UK OBF tonograph for each of the following conditions: in ambient room air breathing, after breathing pure O2 or carbogen through a face mask, and in ambient room air 10 minutes after mask removal. Heart rate (HR), hemoglobin oxygen saturation level (SaO2), and systemic arterial blood pressure (BP) were monitored throughout testing. The end-tidal CO2 (EtCO2) level and respiratory rate (RR) were also recorded during carbogen breathing., Results: Results revealed that HR was reduced (P<0.004) and SaO2 was increased (P = 0.0001) by both oxygen and carbogen breathing. Systemic arterial BP remained stable throughout the experiment. EtCO2 was increased during carbogen breathing (P = 0.0001), whereas RR was reduced (P = 0.0175). IOP was significantly decreased during both phases of the experiment (P = 0.0001). Finally, POBF was not altered by pure O2 breathing, but it increased on average by 7.7% during carbogen breathing (P = 0.0222)., Conclusions: The data obtained with POBF tonography indicate that the choroid reacts to increased blood CO2 concentration, but not to systemic hyperoxia, in a manner similar to that in retinal and brain vessels.

We quantified the changes in macular and choroidal blood flow during body inversion to elaborate their hemodynamic attributes and relate them to our previously reported effects of inversion on visual neural function. Ten healthy subjects took part in the study. The intraocular pressure (IOP) was altered through graded body tilts. Macular and choroidal pulsatile ocular blood flow (POBF) were measured with an Oculix BFS 2000 and a Langham OBF system, respectively. Body inversion caused a systematic decrease in the POBF for all subjects. Systematic changes in macular leukocyte velocity and density were also found with inversion. Six of ten subjects showed a reciprocal relationship between the leukocyte density and velocity as a function of body orientation, while parallel changes in the leukocyte density and velocity were seen during inversion for the remaining subjects. Our results may reflect individual strategies for autoregulation within the macular circulation and an absence of autoregulation in the pulsatile component of the choroidal circulation.

Vision loss in glaucoma may be due to the compressive effects of the intraocular pressure (IOP) on the ganglion cell axons, impaired blood flow to the optic nerve, or some combination of these two factors. While reducing the IOP may preserve vision in patients with elevated IOP, not all patients experience longterm benefits from this therapeutic approach. The survival of ganglion cells may be more dependent upon the degree of vascular perfusion of the optic nerve head (ONH). In this report we present some preliminary data on a new noninvasive videographic technique for elaborating the capacity of the ONH vasculature to maintain perfusion constancy in the presence of transient elevations of the IOP and reductions of the ocular perfusion pressure (OPP). Shortterm, stepwise reductions of the OPP in the test eye of visually normal subjects (n = 5) systematically altered both the chromaticity (hue, saturation, brightness) of the ONH and the pulsatile ocular blood flow (POBF) in the choroid. Similar transient reductions in the OPP were seen to impair normal retinal physiology, as indicated by a significant attenuation of the bilateral pattern-reversal electroretinograms (pERGs) in visually normal subjects (n = 7). This technique was also used to reveal spontaneous rhythmical variations in the ONH chromaticity which were linked to the cardiac pulse rate. This so-called "chromatic pulse" of the ONH offers potential as a useful clinical index for evaluating the vascular perfusion of the ONH. The diagnostic and prognostic potential of dynamic digital imaging for the detection of abnormal hemodynamics in the ONH is discussed.