[Acid base balance in the fetus during labor: pathophysiology and exploration methods]

Although the majority (about 90%) of cerebral palsies are related to antenatal or postnatal causes, search for acute fetal distress during labor remains one of the primary objectives of obstetrical surveillance. Acute fetal distress leads to hypoxemia which induced hypoxia and then asphyxia of most of the organs, including the noble organs such as the brain. Most of the parameters used for fetal monitoring have good sensitivity but low specificity. Furthermore, there is a weak correlation between the different parameters measured and the neonatal status which remains difficult to predict with certainty. The general "philosophy" behind fetal monitoring consists in detecting warning signs and, when warning signs are observed, to measure one or more other parameters to assess the state of hypoxia and its consequences. Fetal heart rate is a warning sign used by all teams. Irrespective of the method used to analyze fetal heart rate when anomalies are detected, metabolic acidosis induced by anoxia is an indispensable element for assessing severity. The development of fetal hypoxia is related to two essential phenomena: anaerobic glycolysis with lactate production and glycogenolysis to meet energy needs. Metabolic acidosis can be measured directly by scalp pH or lactate measurements, or indirectly by measuring its effect on heart conduction as expressed in the ST segment. The underlying physiological mechanisms and the methods used to evaluate acid base balance are described here.