Determination of fetal acidemia at birth from a remote umbilical arterial blood gas analysis

OBJECTIVE: Sampling of the umbilical artery for determination of acid-base status is performed within 60 minutes of birth, but this is not feasible in all hospitals on a 24-hour basis. A desirable alternative would be to perform the arterial cord blood gas analysis hours after the sample was obtained at delivery with reliable identification of whether the newborn was acidotic. STUDY DESIGN: After 19 deliveries multiple umbilical arterial blood samples were withdrawn into five preheparinized syringes, and these were analyzed at 0.5, 15, 30, 45, and 60 hours after delivery. On the basis of observed changes in pH and base deficit over 60 hours, two separate regression equations were generated. These two equations permit calculation of the original pH and base deficit if the following are known: (1) the time interval from delivery to blood gas analysis and (2) the values of the remote pH and remote base deficit. The regression models were validated among 23 subsequent deliveries to assess how accurately they identified newborn acid-base status at birth. RESULTS: The original pH and base deficit can be calculated with two separate equations from linear regression models if two variables are known: the results of the remote umbilical arterial blood gas analysis and time interval from delivery to analysis. Of 23 newborns during the validation phase of the study, 16 were not acidotic, one had respiratory acidosis, and six had metabolic acidosis. The second umbilical arterial blood gas analysis was performed at a mean (± SD) interval of 53.8 ± 41.5 hours (range 7.5 to 138 hours). The remote gas analysis inaccurately identified 68.7% (11/16) of nonacidotic newborns as being acidotic at birth. However, insertion of these data into these equations produced results with none of the newborns predicted to have normal acid-base when they were acidotic at birth or predicted to be acidotic when the cord pH was normal. CONCLUSIONS: Use of these mathematic models allows the clinician to perform an umbilical arterial blood pH analysis ≤60 hours after delivery yet with accurate estimation of true acid-base status at birth. (AM J Obstet Gynecol 1994;170:1705-12.)