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 low arterial oxygen saturation (Sao2) values. Therefore, we evaluated transmission pulse oximetry in the unanesthetized fetal lamb at low Sao2 levels. In seven fetal lambs, fetal hypoxemia was induced by occlusion of the maternal common iliac artery, four days after the instrumentation of the animal. Two Nellcor prototype transmission Y-sensors (light emitting diodes: 660 and 890 nm) were applied, one around a forelimb 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 (Spo2) were compared with sample Sao2 values, over an Sao2 range of 13 to 63%. For the neck sensor the SD of the difference was 5.0% (n = 101). For the muscle sensor the mean difference was 19.5% and the SD of the difference was 5.9% (n = 206). Regression analysis showed a different calibration line for the muscle sensor with the equation: Spo2 = 0.92 x Sao2 + 21.90. Continuous recordings were obtained both from the forelimb muscle and from the neck, but the recordings from the neck showed a substantial loss of signal during the hypoxemia period. We conclude that transmission pulse oximetry is less accurate below an Sao2 of 70% in fetal lambs than above 70% Sao2. At these low levels of Sao2, pulse oximeters may need to be constructed with different calibration lines for various application positions of the body.