Reduced Oxygen Extraction in the Retinal Periphery When the Arterial Blood Pressure Is Increased by Isometric Exercise in Normal Persons

Potential measurement error from vessel reflex and multiple light paths in dual-wavelength retinal oximetry

PURPOSE

This study aims to characterize the dependence of measured retinal arterial and venous saturation on vessel diameter and central reflex in retinal oximetry, with an ultimate goal of identifying potential causes and suggesting approaches to improve measurement accuracy.

METHODS

In 10 subjects, oxygen saturation, vessel diameter and optical density are obtained using Oxymap Analyzer software without diameter correction. Diameter dependence of saturation is characterized using linear regression between measured values of saturation and diameter. Occurrences of negative values of vessel optical densities (ODs) associated with central vessel reflex are acquired from Oxymap Analyzer. A conceptual model is used to calculate the ratio of optical densities (ODRs) according to retinal reflectance properties and single and double-pass light transmission across fixed path lengths. Model-predicted values are compared with measured oximetry values at different vessel diameters.

RESULTS

Venous saturation shows an inverse relationship with vessel diameter (D) across subjects, with a mean slope of -0.180 (SE = 0.022) %/μm (20 < D < 180 μm) and a more rapid saturation increase at small vessel diameters reaching to over 80%. Arterial saturation yields smaller positive and negative slopes in individual subjects, with an average of -0.007 (SE = 0.021) %/μm (20 < D < 200 μm) across all subjects. Measurements where vessel brightness exceeds that of the retinal background result in negative values of optical density, causing an artifactual increase in saturation. Optimization of model reflectance values produces a good fit of the conceptual model to measured ODRs.

CONCLUSION

Measurement artefacts in retinal oximetry are caused by strong central vessel reflections, and apparent diameter sensitivity may result from single and double-pass transmission in vessels. Improvement in correction for vessel diameter is indicated for arteries however further study is necessary for venous corrections.

Increased Oxygen Saturation in Retinal Venules During Isometric Exercise Is Accompanied With Increased Peripheral Blood Flow in Normal Persons

Purpose

A recent study has shown that an increase in the arterial blood pressure of approximately 10 mm Hg in healthy persons can increase the oxygen saturation in venules from the retinal periphery but not from the macular area. The purpose of the present study was to investigate whether a higher increase in blood pressure has further effects on oxygen saturations and whether this is accompanied with changes in retinal blood flow.

Methods

In 30 healthy persons, oxygen saturation, diameter, and blood flow were measured in arterioles to and venules from the retinal periphery and the macular area. The experiments were performed before and during an experimental increase in arterial blood pressure of (mean ± SD) 18.3 ± 6.2 mm Hg.

Results

A higher number of venules than arterioles branching from the temporal vascular arcades to the macular area was balanced by a smaller diameter of the venules. Isometric exercise induced significant contraction of both peripheral and macular arterioles (P < 0.01 for both comparisons) and significant increase in oxygen saturation in both peripheral and macular venules (P < 0.001 for both comparisons). This was accompanied with a significant increase in the blood flow in the peripheral arterioles and venules (P = 0.4 for both comparisons), but not in their macular counterparts (P > 0.06 for both comparisons).

Conclusions

Increased systemic blood pressure leading to arterial contraction and increased venous oxygen saturation in the retina in normal persons can increase peripheral blood flow without significant effects on macular blood flow. This may contribute to explaining regional differences in the response pattern of retinal vascular disease.

The assessment of acute chorioretinal changes due to intensive physical exercise in young adults

Purpose

There is abundant evidence on the benefits of physical activity on cardiovascular health. However, there are only few data on the acute effects of physical exercise on the retina and choroid. Our aim was the in vivo examination of chorioretinal alterations following short intense physical activity by spectral domain optical coherence tomography (SD-OCT).

Methods

Twenty-one eyes of 21 healthy, young subjects (mean age 22.5 ± 4.1 years, 15 males and 6 females) were recruited. Macular scanning with a SD-OCT was performed before and following a vita maxima-type physical strain exercise on a rowing ergometer until complete fatigue. Follow-up OCT scans were performed 1, 5, 15, 30 and 60 minutes following the exercise. The OCT images were exported and analyzed using our custom-built OCTRIMA 3D software and the thickness of 7 retinal layers was calculated, along with semi-automated measurement of the choroidal thickness. One-way ANOVA analysis was performed followed by Dunnett post hoc test for the thickness change compared to baseline and the correlation between performance and thickness change has also been calculated. The level of significance was set at 0.001.

Results

We observed a significant thinning of the total retina 1 minute post-exercise (-7.3 ± 0.6 μm, p < 0.001) which was followed by a significant thickening by 5 and 15 minutes (+3.6 ± 0.6 μm and +4.0 ± 0.6 μm, respectively, both p <0.001). Post-exercise retinal thickness returned to baseline by 30 minutes. This trend was present throughout the most layers of the retina, with significant changes in the ganglion cell–inner plexiform layer complex, (-1.3 ± 0.1 μm, +0.6 ± 0.1 μm and +0.7 ± 0.1 μm, respectively, p <0.001 for all), in the inner nuclear layer at 1 and 5 minutes (-0.8 ± 0.1 μm and +0.8 ± 0.1 μm, respectively, p <0.001 for both), in the outer nuclear layer–photoreceptor inner segment complex at 5 minute (+2.3 ± 0.4 μm, p <0.001 for all) and in the interdigitation zone–retinal pigment epithelium complex at 1 and 15 minutes (-3.3 ± 0.4 μm and +1.8 ± 0.4 μm, respectively, p <0.001 for both). There was no significant change in choroidal thickness; however, we could detect a tendency towards thinning at 1, 15, and 30 minutes following exercise. The observed changes in thickness change did not correlate with performance. Similar trends were observed in both professional and amateur sportsmen (n = 15 and n = 6, respectively). The absolute changes in choroidal thickness did not show any correlation with the thickness changes of the intraretinal layers.

Conclusions

Our study implies that in young adults, intense physical exercise has an acute effect on the granular layers of the retina, resulting in thinning followed by rebound thickening before normalization. We could not identify any clear correlation with either choroidal changes or performance that might explain our observations, and hence the exact mechanism warrants further clarification. We believe that a combination of vascular and mechanic changes is behind the observed trends.