Purpose: The objective of this study was to establish the ability of laser Doppler flowmetry to detect relative changes in human optic nerve head hemodynamics caused by physiologic blood gas perturbations.
Methods: Laser Doppler flowmetry permits the noninvasive assessment of relative blood velocity, volume, and flow (flux) in a sample volume of the nerve head. Such measurements were performed in two groups of healthy subjects. The first group (n = 11) was tested during normal room air breathing and then while breathing 100% oxygen (isocapnic hyperoxia). The second group (n = 10) was also tested under normal conditions as well as during isoxic hypercapnia (+ 15% end-tidal carbon dioxide). Results were analyzed by paired t tests.
Results: Hyperoxia created a significant 25% (p = 0.002) decrease in optic nerve head blood flow, with blood volume decreased by 9% (p = 0.095) and blood velocity reduced by 13% (p = 0.154) compared to the room air condition. During hypercapnia, optic nerve head blood flow was increased by 28% (p = 0.012), with blood volume increased by 22% (p = 0.017) and blood velocity increased by 9% (p = 0.218) as compared to the normal room air condition.
Conclusion: Blood flow in the optic nerve head capillaries changes in response to hyperoxia and hypercapnia as demonstrated in the brain and retina. Laser Doppler flowmetry permits the noninvasive assessment of these responses in humans under conditions within the physiologic range.