Relationship between retinal blood flow and arterial oxygen

Abstract

Key pointsr Vascular reactivity, the response of the vessels to a vasoactive stimulus such as hypoxia and hyperoxia, can be used to assess the vascular range of adjustment in which the vessels are able to compensate for changes in P O 2 .r Previous studies in the retina have not accurately quantified retinal vascular responses and precisely targeted multiple P aO 2 stimuli at the same time as controlling the level of carbon dioxide, thus precluding them from modelling the relationship between retinal blood flow and oxygen.r The present study modelled the relationship between retinal blood flow and P aO 2 , showing them to be a combined linear and hyperbolic function.r This model demonstrates that the resting tonus of the vessels is at the mid-point and that they have great vascular range of adjustment, compensating for decreases in oxygen above a P ETCO 2 of 32-37 mmHg but being limited below this threshold.Abstract Retinal blood flow (RBF) increases in response to a reduction in oxygen (hypoxia) but decreases in response to increased oxygen (hyperoxia). However, the relationship between blood flow and the arterial partial pressure of oxygen has not been quantified and modelled in the retina, particularly in the vascular reserve and resting tonus of the vessels. The present study aimed to determine the limitations of the retinal vasculature by modelling the relationship between RBF and oxygen. Retinal vascular responses were measured in 13 subjects for eight different blood gas conditions, with the end-tidal partial pressure of oxygen (P ETCO 2 ) ranging from 40-500 mmHg. Retinal vascular response measurements were repeated twice; using the Canon laser blood flowmeter (Canon Inc., Tokyo, Japan) during the first visit and using Doppler spectral domain optical coherence tomography during the second visit. We determined that the relationship between RBF and P aO 2 can be modelled as a combination of hyperbolic and linear functions. We concluded that RBF compensated for decreases in arterial oxygen content for all stages of hypoxia used in the present study but can no longer compensate below a P ETCO 2 of 32-37 mmHg. These vessels have a great vascular range of adjustment, increasing diameter (8.5% arteriolar and 21% total venous area) with hypoxia (40 mmHg P ETCO 2 ; P < 0.001) and decreasing diameter (6.9% arteriolar and 23% total venous area) with hyperoxia (500 mmHg P ETCO 2 ; P < 0.001) to the same extent. This indicates that the resting tonus is near the mid-point of the adjustment ranges at resting P aO 2 where sensitivity is maximum.

OCT Angiography Changes in the 3 Parafoveal Retinal Plexuses in Response to Hyperoxia

“…Decorrelation is not linearly related to blood velocity—laboratory measurements using the current OCTA system showed that the decorrelation-velocity curve begins to flatten around 2 millimeters per second and reaches saturation at approximately 6 millimeters per second. 44 Thus, the percentage reduction in flow index was expected to be smaller than the change in volumetric blood flow as measured by laser Doppler 11 or Doppler OCT techniques. 19 …”

“…5–10 Physiologic variations in oxygen delivery to the retina were found to induce multiple hemodynamic changes. 11,12 The ability to characterize this response can be useful to investigate the retinal vascular physiology as well as pathophysiology of those diseases.…”

Retinal perfusion changes in radiation retinopathy

“…The repeatability of these two techniques has been analysed under varying blood gas challenges and the results were previously reported (Cheng et al. ; Rose et al. ).…”

Retinal oxygen: from animals to humans

“…In contrast to the choroid, the retinal circulation does regulate in response to metabolic demand, and blood flow increases during hypoxia (Cheng et al, 2016; Eperon et al, 1975; Hickam and Frayser, 1966; Palkovits et al, 2014c; Wanek et al, 2013; Yi et al, 2015b). As a result, inner retinal PO 2 is quite resistant to hypoxia, which can be appreciated from Figure 12.…”

“…The retinal circulation constricts during hyperoxia (Cheng et al, 2016; Gilmore et al, 2005; Grunwald et al, 1984a; Pournaras et al, 2008; Riva et al, 1986; Werkmeister et al, 2015). This tends to keep inner retinal PO 2 close to levels during air breathing, but the regulation is not perfect in cats, at least partly because the diffusion of O 2 from the choroid is unavoidable.…”