Optic nerve head autoregulation during changes in arterial blood pressure

Purpose Previous studies in our laboratory have shown that an increase in blood pressure during isometric exercise leads to an increase in optic nerve head blood flow (ONHBF). An interesting phenomenon was found in some subjects when they were exhausted from isometric exercise and blood pressure decreased slightly. These periods were sometimes associated with a pronounced decrease in ONHBF even though ocular perfusion pressure (OPP) was still much higher than at baseline. The present study aimed to investigate this phenomenon in more detail and to gain insight in the regulatory mechanisms of ONHBF during short-term fluctuations in OPP. Methods 40 healthy subjects aged between 18 and 35 years were included in this study. Three periods of baseline measurements at rest and 3 periods of measurements during isometric exercise with a handgrip were alternated while ONHBF was measured continuously using laser Doppler flowmetry. Mean arterial blood pressure (MAP) was measured every minute and OPP was calculated as 2/3 MAP – intraocular pressure (IOP). Results OPP significantly increased during handgripping compared to the resting periods (p<0.001), whereas ONHBF was not significantly altered during these changes in OPP indicating for autoregulation. Three subjects showed a decrease of more than 10% in ONHBF during recovery at all 3 periods of isometric exercise. Unexpectedly, however, 3 other subjects consistently showed an ONHBF reduction of more than 10% during isometric exercise. Conclusion The present study indicates a complex regulation of ONHBF. Six subjects (15%) showed an abnormal ONHBF response during handgripping. This is most likely caused by an exaggerated vasoconstrictor response in order to counteract the increase in OPP.