Comparison of four pulse oximeters: effects of venous occlusion and cold-induced peripheral vasoconstriction. The ability of four pulse oximeters (the Ohmeda 3700, Nellcor N100 and N200 and the Datex Oscar) to detect hypoxaemia was determined in the presence of venous obstruction and cold-induced peripheral vasoconstriction. Significant increases in detection time for hypoxaemia were found in both cases. There were no significant differences in detection time between the instruments, although the Ohmeda 3700 displayed smaller values of SaO2 under certain conditions. Peripheral vasoconstriction was induced using three differing methods which gave differing results, thus emphasizing the importance of methodology in assessments of pulse oximetry. 