Ocular hemodynamics during isometric exercise

The autoregulatory capacity of the human retina is well documented, but the pressure-flow relationship of the human choroid is still a matter of contr oversy. Recent data, using laser Doppler flowmetry to measure choroidal blo od flow, indicate that the choroid has some autoregulatory potential, where as most data using other techniques for the assessment of choroidal hemodyn amics indicate that the choroidal pressure-flow curve is linear. We used a new laser interferometric technique to characterize choroidal blood flow du ring isometric exercise. Twenty healthy subjects performed squatting for 6 min during normocapnia and during inhalation of 5% CO2 and 95% air. Ocular fundus pulsation amplitude, flow velocities in the ophthalmic artery, intra ocular pressure, and systemic hemodynamics were measured in 2-min intervals . To gain information on choroidal blood flow fundus pulsation amplitude wa s corrected for changes in flow pulsatility using data from the ophthalmic artery and for changes in pulse rate. Ocular perfusion pressure was calcula ted from mean arterial pressure and intraocular pressure. The ocular pressu re-flow relationship was calculated by sorting data according to ascending ocular perfusion pressure values. In a pilot study in 6 healthy subjects co mparable ocular pressure flow relationships were obtained when choroidal bl ood flow was assessed with the method described above and with laser Dopple r flowmetry. In the main study isometric exercise caused a significant incr ease in mean arterial pressure (56%, P < 0.001), pulse rate (84%, P < 0.001 ), and intraocular pressure (37%, P 0.004), but decreased fundus pulsation amplitude (-36%, P < 0,001). Significant deviations from baseline choroidal blood flow were observed only at ocular perfusion pressures >69% during no rmocapnia and 70% during hypercapnia. Our data indicate that during isometr ic exercise the choroid has a high capacity to keep blood flow constant des pite changes in perfusion pressure and that this pressure-flow relationship is not altered by moderate changes in arterial carbon dioxide levels. (C) 2000 Academic Press.