The effect of augmentation of labour with syntocinon on the fetal CTG using objective computerised analysis: a nested case-control study.

OBJECTIVE

To investigate the effect of syntocinon augmentation on the fetal cardiotocogram (CTG) using computerised analysis. We hypothesised that syntocinon will have no direct effects on the fetal heart rate if used correctly.

STUDY

DESIGN

A retrospective, nested case-control study.

SETTING

Intrapartum CTG records from the digital archive at the John Radcliffe Hospital, Oxford, UK.

SUBJECTS: 110 women with singleton pregnancies of >36 weeks gestation, no known congenital abnormality, spontaneous onset of labour and syntocinon augmentation for failure to progress, with start time of syntocinon recorded, from between August 1998 and December 1999, extensively matched to 110 controls who had normally progressing labours.

METHODS

Eight different CTG features were measured during four time points with OxSys, a computerised numerical analysis system.

STATISTICAL ANALYSIS: Differences in the CTG features over time in cases and controls using ANOVA and Friedman's ANOVA and at each time point between case-control pairs using Student's t-test and the Wilcoxon signed rank test.

RESULTS

After administration, syntocinon increased the frequency, decreased the duration and decreased the resting time between contractions (p<0.001), resulting in no significant difference between normally progressing labours and those requiring augmentation. The case group had a significantly higher signal stability index (SSI) and fewer decelerations compared to the control group - differences which disappeared after augmentation was commenced (p=0.025 and 0.033 respectively). Syntocinon did not affect the baseline heart rate, short term variability (STV) or phase rectified signal averaging (PRSA) (p=0.518, 0.215 and 0.138) in comparison with controls. There was a significant increase in the PRSA in babies born with acidaemia (arterial pH≤7.05) 60-120min after syntocinon was commenced that was not seen with in babies with a normal pH (p=0.002).

CONCLUSION

Syntocinon "normalises" ineffective uterine activity without any direct effect on the fetal heart rate. Therefore its administration does not confound objective computerised analysis. There may be a specific response in PRSA shortly after commencing syntocinon augmentation in the fetus which is subsequently born acidaemic which requires further investigation.