Does the hemoglobin concentration in fetal blood interfere with the accuracy of fetal reflection pulse oximetry?

OBJECTIVE

Our goal was to investigate the influence of the fetal hemoglobin (Hb) concentration on the accuracy of reflection pulse oximetry.

METHODS

179 fetuses were monitored by a reflection pulse oximetry system (RPOX MK2). Pulse oximetry measurements (SO2POX) were compared with the oxygen saturation and pH of umbilical artery and vein (UA-, UV-SaO2) immediately after delivery. Correlation and linear regression analyses were performed. Because it is unknown whether low or high Hb concentrations might interfere with the SO2POX measurements, the influence of the Hb level on the correlations was evaluated by weighting the linear regression in both ways; by Hb concentrations as weighting factor and as an inverse weighting factor (1/Hb).

RESULTS

There was a significant correlation between SO2POX and UV-SaO2 (for the last 10 min of delivery, r = 0.45, p < 0.0005) but not with pH or UA-SaO2. Linear regression analysis between SO2POX and UV-SaO2 showed a multiple R of 0.45. Hb concentration in fetal blood ranged from 9.2 to 20.5 mg/dl. The weighted regression indicated a multiple RHb of 0.49, which did not differ by the inverse weighting (multiple Rinverse Hb: 0.49).

CONCLUSIONS

Pulse oximetry measurements seem to reflect oxygen saturation in fetal blood, however further improvement is necessary. The Hb concentration in fetal blood does not interfere with the accuracy of fetal pulse oximetry and need not be taken into consideration in calibration curves.