Accuracy and utility of a new reflectance pulse oximeter for fetal monitoring during labor.

Objective: To evaluate the accuracy of a new pulse oximeter in estimating the oxyhemoglobin (O2Hb) concentration in fetal arterial blood (SaO2) with either a high concentration of hemoglobin F (HbF) or a low concentration of O2Hb during labor.

Patients and methods: Fetal forehead arterial oxygenation was determined in 44 fetuses by reflectance pulse oximeter (SpO2) during labor and was compared with SaO2 determined immediately after birth. Because HbF has little or no known effect on pulse oximetry, but does affect the laboratory multiwavelength "CO-oximeter" type reading, the SaO2 was corrected by HbF concentration. SpO2 and SaO2 were simultaneously measured in five hypoxic adult volunteers achieved by inhaling 11% oxygen.

Results: A gradual decline in HbF concentration was seen during weeks 37 to 40 of gestation. HbF concentration varied near term, ranging between 53 and 88% of the fetal hemoglobin concentration (mean +/- SD = 74.6 +/- 6.3%). This alteration produced a lower %O2Hb percent by 4% at the most. The corrected SaO2 in cord blood correlated with fetal SpO2 (y = 0.974, x -7.279, r = 0.90). In five adults, SpO2 reflected well SaO2 with a mean +/- SD of bias of -1.1 +/- 2.9%.

Conclusions: SpO2 determined by a new reflectance pulse oximeter at the end of labor correlated with an immediate post-natal cord arterial blood sample before the first breath, with a mean and SD of bias of 8.5 +/- 6.2%. Reflectance pulse oximetry is a useful tool for continuous noninvasive monitoring of the fetal oxygen status during labor.