PURPOSE. The Valsalva maneuver (VM) is known to increase the IOP during a forced expiration against a closed glottis. Our objective was to elaborate the mechanism by which the IOP is increased. METHOD. Five healthy subjects between 20 and 52 years of age forced expiration into a pressure gauge for some 10 seconds in 20mm Hg increments. During each VM, the sub-foveal choroidal blood flow (ChBF) was measured continuously by dynamic Laser Doppler Flowmetry (LDF). Simultaneously, the systolic and diastolic pressures were sampled at 20Hz by a non-invasive blood pressure system (Colin 7000). The IOPs were measured for each level of expiration in a second session with a Tonopen tonometer.
RESULTS. Maximum expiration pressures ranged between 40 and 110 mmHg. A low expiration force did not change the ChBF. However, at the end of moderate expiration, the ChBF showed a rapid undershoot and recovery of baseline within 5 sec. The expiration pressure at which the ChBF exceeded resting values varied across subjects (45, 80, and 110mmHg). While both the systolic and diastolic BPs exceeded baseline during the VM, the diastolic pressure rapidly approached the systolic level with progressively higher expiration pressures. In one subject the diastolic pressure increased at a rate of 0.65 mmHg/mmHg expiration, while the ChBF increased at 1.05 Arbitrary Units/mmHg expiration. LDF indicated that the increase in ChBF was due to an increase in the ChBF volume but decrease in velocity. The average maximal increase in IOP during the VM was about 5 mmHg.
CONCLUSIONS. A decrease in intraocular venous outflow signaled by a rise in diastolic BP congested the choroid and thus increased the intraocular volume. Since the choroid represents 90-95% of total blood flow in the eye, an increase in its volume is large enough to increase the IOP.