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Abstract

BACKGROUND

Mild hypoxia is well known to impair scotopic (night) vision while moderate to severe hypoxia is required to compromise photopic (day) vision. However, the severity of hypoxia required to affect mesopic (twilight) vision, when both rod and cone photoreceptors contribute to visual perception, is unknown. This question is relevant to night flying at low altitude when mild hypobaric hypoxia might influence the visual performance of healthy aircrew operating in dimly illuminated cockpits and flight decks. Comparative studies indicate that increased rod oxygen consumption in dim light promotes outer retinal hypoxia under normal respiratory conditions. This might increase the susceptibility of the outer retina to the deleterious effects of exogenous hypoxia. This thesis aims to identify and quantify effects of respiratory disturbance on visual performance across the low photopic to mid-mesopic range. As well as mild to moderate hypoxia, the effects of hype roxi a and hypocapnia (hyperventilation) are considered.

METHODS

Cardio-respiratory status was monitored closely in all experiments, including breath-by-breath mass spectrometry for respired partial pressures of oxygen and carbon dioxide. An initial dark adaptation study in a hypobaric chamber assessed the effects of hypoxia, hyperoxia and hypocapnia on threshold sensitivity to dim flash stimuli, enabling definition of a mesopic adaptation procedure that was independent of respiratory condition. Hyperventilation increases flicker sensitivity, so the potential for hypocapnia to lower mesopic flicker thresholds and thereby confound subsequent studies was assessed under progressive hypocapnia. Subsequent experiments examined the visual perception of 12 healthy volunteers (6 male and 6 female) under low photopic, upper mesopic and mid-mesopic viewing conditions while breathing gas mixtures to establish normoxic, mildly hypoxic or hyperoxic respiratory states. Visual parameters comprised spatial contrast sensitivity, threshold chromatic sensitivity, visual processing speed, temporal contrast sensitivity and low contrast acuity, the last incorporating assessment of pupil size at each light level and respiratory condition. Visual stimuli were display-based and comprised foveal Gaussian Gabor patch gratings; the City University Colour Assessment and Diagnosis Test; the Useful Field of View® Test; threshold Frequency Doubling Technology perimetry; and the City University Contrast Acuity Assessment Test. Breathing gases were masked from the subjects and exposure orders were balanced and randomised between males and females. In some experiments hypocapnia was induced by voluntary hyperventilation. Vision testing was conducted binocularly and monocularly where practicable. Repeated measures designs favoured initial statistical analyses using balanced Analysis of Variance followed by various parametric and non-parametric post hoc analyses.

RESULTS

The achievement of scotopic sensitivity during dark adaptation is oxygen dependent, being delayed progressively by worsening hypoxia but hastened by hyperoxia and hypocapnia, such that rod photoreceptors are functionally hypoxic, in the dark, under normal respiratory conditions. Mesopic flicker sensitivity is highly correlated with the severity of hypocapnia but the magnitude of the effect is slight and unlikely to be meaningful. At the fovea, spatial contrast sensitivity is resistant to respiratory disturbance but threshold chromatic sensitivity is increasingly vulnerable to hypoxia as light level decreases in the mesopic range. Both spatial contrast sensitivity and chromatic sensitivity exhibit clear binocular summation. Mild hypoxia tends to delay visual processing speed and may be relevant to the extraction of visual information from complex scenes. Effects of hypoxia to impair and hyperoxia to enhance temporal contrast sensitivity vary with light level and retinal eccentricity but also support a progressive effect of hypoxia with decreasing mesopic luminance. Low contrast acuity is compromised by mild hypoxia at photopic and mesopic luminance but is enhanced by hyperoxia, relative to normoxia, in the mesopic range, implying further oxygen-dependent functional impairment. Mild hypoxia consistently induces pupillary miosis in the low photopic to mid-mesopic range.

CONCLUSIONS

Mild hypoxia compromises numerous visual attributes and its effects may be promoted by endogenous, rod-driven, outer retinal hypoxia with decreasing mesopic luminance. In dim light hyperoxia enhances some aspects of visual sensitivity relative to performance breathing air, implying oxygen-limited functional impairment under normal respiratory conditions, presumably due to increasing rod oxygen consumption in dim light. These effects have implications for the use by aircrew of supplementary oxygen at modest altitudes. Hypocapnia enhances visual sensitivity but is unlikely to be meaningful except, perhaps, in the scotopic range. The outer retina is functionally hypoxic in the mesopic range under normal respiratory conditions and this may have implications for the aetiology of retinal pathology. Effects of respiratory disturbance on pupil size warrant further consideration.

Publication Type:	Thesis (Doctoral)
Subjects:	R Medicine > RE Ophthalmology
Departments:	Doctoral Theses School of Health & Psychological Sciences > Optometry & Visual Sciences School of Health & Psychological Sciences > School of Health & Psychological Sciences Doctoral Theses

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