TRANSMISSION PULSE OXIMETRY IN THE FETAL LAMB - IS THERE A UNIVERSAL CALIBRATION

Transmission pulse oximetry is widely used for oxygen monitoring. The use of pulse oximeters is steadily expanding toward situations with lo w arterial oxygen saturation (Sao(2)) values. Therefore, we evaluated transmission pulse oximetry in the unanesthetized fetal lamb at low Sa o(2) levels. In seven fetal lambs, fetal hypoxemia was induced by occl usion of the maternal common iliac artery, four days after the instrum entation of the animal. Two Nellcor prototype transmission Y-sensors ( light emitting diodes: 660 and 890 nm) were applied, one around a fore limb muscle and one around a skinfold in the neck, and were connected to Nellcor pulse oximeters. The pulse oximeter was calibrated for the skin measurements. Pulse oximeter saturation readings (Spo(2)) were co mpared with sample Sao(2) values, over an Sao(2) range of 13 to 63%. F or the neck sensor the SD of the difference was 5.0% (n = 101). For th e muscle sensor the mean difference was 19.5% and the SD of the differ ence was 5.9% (n = 206). Regression analysis showed a different calibr ation Line for the muscle sensor with the equation: Spo(2) = 0.92 . Sa o(2) + 21.90. Continuous recordings were obtained both from the foreli mb muscle and from the neck, but the recordings from the neck showed a substantial loss of signal during the hypoxemia period. We conclude t hat transmission pulse oximetry is less accurate below an Sao(2) of 70 % in fetal lambs than above 70% Sao(2). At these low levels of Sao(2), pulse oximeters may need to be constructed with different calibration lines for various application positions of the body.