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 Pölten 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.