PURPOSE. This study determined whether cerebral blood flow (CBF), as measured at the middle cerebral artery by a transcranial Doppler, increased when subjects were required to complete a visual field test. METHOD. Subjects (n=7) were tested while breathing either 21% O2 (normoxia), or 15% O2 to further determine any interaction with hypoxia. Data were collected during a 15 minute baseline period (21% O2) and a 15 minute experimental period (15% O2). Threshold visual field testing was carried out during the last 5 minutes of each 15 minute period using the Humphrey System''s Frequency Doubling Technology (FDT) Visual Field Instrument.
RESULTS. Data presented here are a subset of a larger study to determine the cardiovascular and visual system changes that occur with hypoxia. Average oxygen saturation level (SaO2) at 21% oxygen was 98% compared to a saturation of 94% at 15% oxygen. Results showed significant differences in percent change from average flow velocity of a 5 minute matched oxygen condition prior to the visual test. During the test both 21% and 15% O2 mean (p=0.003), minimum (p=0.005), and peak (p=0.01) cerebral flow velocities (cm/s) increased by 7.4±2.3%, 10.0±3.3%, and 3.4±1.3% (mean ± S.E.) respectively. The dependent one-tailed student''s t-test was used for statistical comparisons.
CONCLUSIONS. When challenged with the FDT visual field test, subjects responded with an increase in cerebral blood flow. This increase was observed under both control (21% O2) and experimental (15% O2) conditions of hypoxia.