The current status of intrapartum continuous fetal tissue pH measurements

Continuous tissue pH monitoring during labor has now been possible for 15 years. Tissue pH is measured in the intercellular fluid, and the value differs in some cases from the blood pH value because of local capillary flow and local metabolism. The fetal scalp seems to be an area where tissue pH and capillary blood pH values are very close, but in acute acidosis there may be a time lag of 10-15 min before tissue pH is equilibrated. Normal values of scalp tissue pH are 7.38 in early labor declining by 0.016 per hour during the first stage of labor and by 0.12 per hour during the second stage of labor. Tissue pH values below 7.15 are defined as pathological. The correlation coefficient to umbilical artery blood pH (0.71), success rate (67%) and electrode drift (below 0.04) from the largest study of tissue pH monitoring using glass pH electrodes (n = 337) are described.