REFLECTANCE PULSE OXIMETRY IN FETAL LAMBS - SUBCUTANEOUS VESSELS AND VASOCONSTRICTION AFFECT ITS RELIABILITY

Objective. Reflectance pulse oximetry (RPOX) has been introduced for i ntrapartum fetal surveillance. The purpose of this study was to descri be two possible effects on the reliability of RPOX, namely the effect of the presence of a subcutaneous vein and the effect of vasoconstrict ion by adrenaline, both at fetal SaO(2) levels. Methods. In four anest hetized fetal lambs, a prototype 660/890 nm reflectance sensor (Nellco r Inc.) was placed on the fetal head, with the photodiode of the senso r precisely over a superficial subcutaneous vein. Measurements were ma de before and after coagulation of the vein. In five anesthetized feta l lambs, one or two reflectance sensors were placed on the fetal head and/or neck and adrenaline was administered in doses of 0.02 to 0.04 m g via a brachial artery. Pulse oximeter saturation readings (SpO(2)) w ere compared with continuous arterial oxygen saturation (SaO(2)) value s obtained using a fiberoptic catheter (Opticath, Abbott) in the carot id artery. Results. When the sensor was placed over the vein, the puls e oximeter read 18% to 24% too low at a SaO(2) level of 20% to 50%. Af ter coagulation of the vein, SpO(2) readings were in agreement with fi beroptic SaO(2) values. Administration of adrenaline resulted in a lar ge overestimation of the SaO(2) in 6 of the 7 measurements. Conclusion s. Subcutaneous veins and vasoconstriction can affect the reliability of reflectance pulse oximetry. As comparable situations may occur duri ng labor, SpO(2) readings should be interpreted with caution when this kind or comparable types of RPOX sensors are used at low SaO(2) level s.