Uncomplicated Fetal Tachycardia in Labour: Dilemmas and Uncertainties

Fetal Tachycardia (FT) is described as increase in baseline fetal heart rate (FHR) above 160bpm. Mild fetal tachycardia is described as 161-180bpm and severe tachycardia is defined as greater than 180bpm for at least three minutes. The fetal tachycardia causes include maternal fever, dehydration or anxiety, maternal ketosis, medications like anticholinergic medications, sympathomimetic medications like terbutaline, fetal movement, preterm fetus, maternal thyrotoxicosis and maternal anaemia¹. Fetal tachycardia is considered significant (any range >160-180bpm) in the presence of maternal pyrexia as Chorioamnionitis is suspected. Fetal arrhythmia or congenital defect is associated with FHR more than 200 bpm. Baseline FHR tachycardia represents an increase in sympathetic and or a decrease in parasympathetic autonomic nervous system tone¹.

Complicated fetal tachycardia in the presence of decelerations or maternal fever qualify the decision for delivering the baby in view of fetal distress and suspected chorioamnionitis respectively. However in clinical day to day practice, it’s not easy to deal with uncomplicated tachycardia because clinicians don’t have clear guidance for intervention in cases of uncomplicated fetal tachycardia in the absence of maternal pyrexia where tachycardia is not settling even when conservative measures like left lateral tilt, hydration and pain control have been explored. So the dilemma which clinicians face in labour is that “what duration of uncomplicated fetal tachycardia is significant and how long conservative management is safe in the absence of maternal pyrexia and deceleration’’? Should conservative management with watchful wait be limited to 30-45 min, 45-90 min or more than 90 min? One option which may be available is to perform fetal blood sampling (FBS) if feasible to do so in labour. But again another dilemma appears: when to undertake FBS in uncomplicated fetal tachycardia? Should it be within 30-45 min, 45-90min or more than 90 min from the start of conservative management like left lateral tilt, hydration and pain control? Another big question that surfaces is whether doing FBS is really justified in uncomplicated fetal tachycardia in the absence of deceleration?

We know that fetal hypoxia, congenital heart anomalies and fetal tachycardia itself can cause decreased variability so one can argue that fetal tachycardia with reduced variability is not a reassuring sign and may warrant delivery. But evidence shows that fetal tachycardia with reduced variability in cases of intrapartum hypoxia will always be preceded by decelerations². Hence clinicians may face another dilemma in cases where fetal tachycardia is not preceded by deceleration in the absence of maternal fever. To err on the side of caution, possibly the majority of clinicians will still suspect infection in fetal tachycardia with reduced variability in the absence of either maternal pyrexia or preceded decelerations; however, the issue of role and duration of expectant/conservative management and absence of clear guidance from professional bodies regarding urgency of delivery bears complex fetal safety, risk management and litigation issues.

Dr Junaid Rafi MBBS, MRCPI, EFOG-EBCOG, DFSRH
Ipswich Hospital NHS Trust, Heath Road, Ipswich, IP4 5PD, UK

References:
1: Smith R.P. Netter’s Obstetrics and Gynecology E-Book. Philadelphia: Elsevier Health Sciences; 2017
2: Ugwumadu A. Are we (mis)guided by current guidelines on intrapartum fetal heart rate monitoring? Case for a more physiological approach to interpretation. BJOG 2014; 121:1063–1070

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