The calibre changes of retinal vessels subject to prolonged hyperoxia

The constriction of retinal vessels occurring in response to breathing 100% oxygen was studied in 10 normal subjects over a period of 30 min. Brachial blood pressure, radial pulse and end tidal CO2 were recorded throughout. The mean arteriolar constriction was 15.3% +/- 4.6 and venous constriction 21.8% +/- 4.3. Time constants (the time for 95% of the maximal response to have occurred) were 2.25 min +/- 0.75 and 2.37 min +/- 0.57 for arterioles and veins respectively. A highly variable late vascular response was recorded. A small proportion of arterial vessels showing a response were nonuniform while a larger proportion of veins showed a significant dilatation.