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The retina provides the opportunity for non-invasive observation of human microcirculation in vivo. The study of retinal circulation during exposure to hyperbaric oxygen (HO) treatment is important for further understanding of normal physiological changes and also of a number of pathological conditions, both locally in the eye and extending to brain metabolism and brain oedema.
Short term HO has a large influence on the cardiovascular system. Systolic blood pressure increases and diastolic pressure falls. The mean pressure is constant. Peripheral resistance increases by about 30%. Heart frequency and cardiac output fall by approximately 20%. 1, 2 Retinal blood vessels constrict when the concentration of inspired oxygen rises. 3, 4 After breathing oxygen at 2.36 atmospheres absolute pressure (ATA) for 5 minutes, arterioles constricted by 22.6% 5 and by 42.3% at 2.0 ATA, 3, 6 and venoles constricted by 26.9% at 2.36 ATA 5 and by 30% at 2.0 ATA. 3 Constriction is even greater with higher oxygen pressure: when oxygen is inspired at 3.7 ATA for 5 minutes, arterioles constricted by 25.9% and venoles by 32.4%. 5 The retinal venoles develop the same colour as the retinal arterioles, indicating a significant increase in retinal venous blood oxygen saturation: oxygen breathing at 2.36 ATA for 5 minutes increased venous oxygen saturation from 58% to 94% and retinal mean circulation time (t) increased by 53% from 4.5 to 6.9 seconds. 5 If we consider the fact that main circulation time is proportional to the ratio of retinal blood volume to blood flow rate (t = volume/flow), the calculated blood flow decreased to approximately 30% of the control value after 5 minutes on oxygen at 2.36 ATA. 5
Similar results were observed when cerebral blood flow (CBF) was measured: after 30 minutes of O2 exposure at 3.0 and 4.0 ATA, CBF decreased by 26-39% and 37-43%, respectively. 7
Likewise after 10 minutes exposure to 2.0, 3.5, and 5.0 ATA, CBF decreased 20-25%. 8 It is well known that decreased CBF is correlated with seizures. 8- 10 If we assume that the retinal blood flow changes are similar to changes in CBF, then it might be possible to extrapolate the changes of retinal vessel diameter to predict seizures.
The widespread use of hyperbaric oxygenation raises the issue...