Influence on fetal blood pH when adding amniotic fluid:: an in vitro model

Objective To assess the effect of contamination with amniotic fluid in different quantities on fetal capillary blood pH. Design In vitro model. Setting Department of Obstetrics, St Polten Hospital. Sample Venous umbilical blood and amniotic fluid from 35 women who underwent amniotomy during labour. Methods Venous umbilical blood was mixed in vitro with amniotic fluid in diluted series (venous umbilical blood/amniotic fluid, 10: 1 to 1:1). In every case two parallel runs of the dilution series in an inverted fashion were performed to rule out a possible time-dependent bias of the pH measurements. Main outcome measures pH change in dilutions of umbilical venous blood with amniotic fluid: 10: 1, +0.07 (0.02); 8:1, +0.08 (0.02); 6:1, +0.08 (0.03); 4:1, +0.07 (0.02); 2:1, +0.09 (0.03); 1: 1, +0.12 (0.05); all P < 0.001. Results Amniotic fluid pH increases, whereas umbilical blood pH decreases slightly over time since collection. Dilutions of umbilical venous blood with amniotic fluid resulted in a significant pH rise. There was no significant difference between the two inversely performed dilution series. Conclusion Amniotic fluid influences in vitro fetal venous blood pH immediately after contact. This observation indicates the possible masking of a fetus in distress by fetal scalp blood pH determination when amniotic fluid contaminates the sample.