Scalp heat flux and its relationship to scalp blood pH of the fetus.

A method for directly assessing the metabolic rate of the fetus for optimal management of labor and delivery is still being studied. Since heat, an end product of metabolism, is dissipated from the fetal surface, we measured the heat flux from the fetal scalp and related it to the pH of fetal scalp blood, a measure of metabolism. In 25 human fetuses at risk of intrapartum hypoxia, after the membranes had ruptured and the cervix dilated to greater than 3 cm, we attached a heat flux transducer, a platelet of 2.5 cm diameter and 1 mm thickness, to the fetal scalp and recorded continuously the heat flux from then until delivery. We also obtained one sample of fetal scalp blood in all patients and two such samples in 13 patients for the analysis of pH. We found a significant correlation between heat flux measured immediately before the scalp blood sampling (presampling values) and the pH of fetal scalp blood (r = 0.736; n = 18 presampling values; p less than 0.001). Out of 13 fetuses with two scalp blood samples, the changes in the scalp heat flux paralleled changes in scalp blood pH in 11. We conclude that fetal scalp heat flux is related to the metabolic condition of the fetus. Measuring scalp heat flux during labor could be developed into a noninvasive method for a continuous and more direct assessment of the fetal metabolic rate.