Low fetal oxygen saturation at birth and acidosis

Objective: To measure umbilical cord blood oxygen saturation, to calculate preductal oxygen saturation at birth, and to assess its predictive value fo r acidosis. Methods: Umbilical cord blood samples of 1537 live-born singleton neonates were analyzed. Oxygen saturation was measured by spectrophotometry; pH and base excess were measured by a pH and blood gas analyzer. Preductal oxygen saturation was calculated with an empirical equation. Acidosis was defined as 2 standard deviations (SDs) below the mean of umbilical artery (UA) pH o r base excess (7.09 and -10.50 mmol/L, respectively). The predictive value for acidosis of UA and umbilical vein (UV) oxygen saturation and calculated preductal oxygen saturation was determined with receiver operating charact eristic curves. Results: The mean values (+/-SD) of UV, UA, and calculated preductal oxygen saturation were 52 +/- 18%, 26 +/- 17%, and 31 +/- 16%, respectively. Fort y-seven neonates had UA pH less than 7.09 and 60 had UA base excess less th an -10.50 mmol/L. The UV, UA, and calculated preductal oxygen saturation sh owed considerably weaker relations to UA base excess (multiple r(2) = .056, .003, and .017, respectively; P < .001) than to UA pH (multiple r(2) = .11 2, .126, and .148, respectively; P < .001). Receiver operating characterist ic areas under the curve were higher when predicting low pH compared with l ow base excess (for UV, UA, and calculated preductal oxygen saturation: 0.7 16 versus 0.699, 0.747 versus 0.586, and 0.765 versus 0.628, respectively). The difference was significant for UA oxygen saturation (P < .05). All tes ts showed high sensitivity and negative predictive values, but low specific ity and positive predictive values. Conclusion: Low fetal oxygen saturation measured at birth seemed to be asso ciated with low fetal pH and base excess values, but its predictive value f or acidosis in an unselected population was limited, particularly if acidos is was metabolic. (C) 2000 by The American College of Obstetricians and Gyn ecologists.