Correlation between fetal scalp blood samples and intravascular blood pH, pO2 and oxygen saturation measurements

This study was designed to compare the values of blood gases in local scalp blood, obtained by scalp blood sampling, with direct measurements of fetal preductal arterial blood in fetal sheep.

Six fetal lambs were catheterized in the brachial artery and vein. Maternal oxygenation was reduced in steps from a fractional inspired oxygen concentration (FiO2) of 20 to 5% by addition of nitrogen to the inhaled gas mixture. Fetal scalp and arterial blood were sampled simultaneously at maternal FiO2 step intervals after maternal FiO2 and oxygen were stable for > 5 min. Blood pH, pO2 and oxygen saturation were measured and linear regression was performed to determine the correlation between these values.

Scalp pH correlated well with arterial pH, whereas scalp pO2, pCO2 and oxygen saturation did not. However, when a secondary analysis was performed taking into account the effects of aerobic contamination, scalp pCO2, pO2 and oxygen saturation became highly correlated with arterial values.

Local scalp blood oxygen saturation correlates highly with fetal preductal arterial values, when physiological artifacts are eliminated. The technique of scalp blood sampling introduces error into oxygenation saturation measurements, owing to difficulties in anaerobic sample collection. These results suggest that continuous measurement of fetal scalp oxygenation by noninvasive oximetry may be superior to direct sampling of scalp blood.