Fetal blood gas analysis at delivery.

Correspondence Fetal blood gas analysis at delivery Sir, In a recent article D'Souza et al.1 note that it is simpler and more convenient to obtain umbilical venous than arterial blood to assess the acid base status of a newborn infant at delivery. They add, however, that sampling arterial blood would be preferable to venous blood as the former reflects more closely the metabolic condition of the fetus. This clinical dilemma can be resolved very easily by sampling blood from the fetal arteries that cross the chorionic plate of the placenta. As in the retina of the eye the arteries can be accurately identified as they cross over the veins. For a number of years we have obtained fetal arterial blood in this manner to determine the degree of metabolic acidosis in infants born depressed or after an obstetric diagnosis of fetal distress. The method is easy, especially if the fetal blood is not permitted to drain before delivery of the placenta. The measurement of base excess or buffer base is preferred to pH as the latter is rapidly altered by changes in maternal Pco2. For exemple, the presence of hypo-carbia provides the explanation of the finding of an alka-losis in many of the infants studied by D'Souza et al. Lastly it remains debatable whether a maternal metabolic acidosis influences the acid base status of the infant at birth.2 Drs D'Souza and Richards comment: Obtaining a sample of blood from fetal arteries crossing the chorionic surface of the placenta may be feasible but we are concerned that this blood is less likely to reflect the pH, Po2, and Pco2 in the infant at birth, since there is a delay in the delivery of the placenta. Several minutes generally lapse between birth of the infant and delivery of the placenta during which time fetal blood will not be circulating in the placenta after the cord is clamped. This raises some uncertainty about the relevance of pH, P02, and Pco2 measurements in fetal blood from the placenta. In our study base excess was calculated from blood pH, Pco2, and haemoglobin. There is no way that base excess can be measured, it can only be calculated from pH, Pco2, and haemoglobin. One can either use the Siggard-Andersen alignment nomogram" or one of the several equations, for example Siggard-Andersen4; both these are incorporated in the Radiometer slide rule5 and microcomputer chips incorporated into many modern …