Episcleral venous pressure: a comparison of invasive and noninvasive measurements.

Noninvasive (pressure chamber) measurements of episcleral venous pressure were followed by invasive (direct cannulation) measurements at the same venous site in seven eyes of four anesthetized rhesus monkeys. There were three definite effects on the vein caused by the pressure chamber that could be used as endpoints: (1) slight indentation; (2) intermittent collapse; and (3) sustained collapse of the vein lumen. The mean pressure in the chamber corresponding to these endpoints was 9.9 +/- 0.9, 23.5 +/- 2.9, and 31.4 +/- 4.0 mmHg (+/- SE), respectively. After the chamber was removed, the pressure in the veins determined by cannulation measurements at the same sites was 11.3 +/- 0.5 mmHg (+/- SE). Therefore, the first endpoint with the pressure chamber (slight indentation) correlates most closely and slightly underestimates the cannulated pressure. Endpoints defined by partial or complete venous collapse overestimates the venous pressure. Simultaneous measurements with the chamber and a cannula show a rise of local venous pressure caused by the chamber.