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Influence on fetal blood pH when adding amniotic fluid: an in vitro model
Alexander Lösch1, Christian Kainz, Petra Kohlberger
1Department of Obstetrics and Gynaecology, St Pölten Hospital, Austria.
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.
