Risk stratification for irregular fetal heart rhythm: practical approach to management

Auscultation of the fetal heart using Doppler devices is an integral part of routine antenatal care. It is reassuring for mothers and healthcare professionals to hear the fetal heart beating regularly and at a fast pace compared to that in postnatal life. In practice, fetal heart rates (FHRs) between 120 and 160 bpm are considered normal and accepted as a gestational age-independent reference range. However, it has long been known that FHR decreases with gestational age1. More recently, a gestational-age reference centile chart has become available and used as a predictor of long QT syndrome2. However, despite FHR being an important and easily obtainable parameter to assess fetal wellbeing, the actual value (in bpm) is often not recorded during the 20-week anomaly scan, subsequent growth scans or routine antenatal visits. Rhythm irregularity is relatively common, occurring in 1.7% of normal pregnancies towards term3, and is mostly due to atrial ectopic beats, also known as premature atrial contractions (PAC). Their audial or visual perception by Doppler auscultation or visualization of the fetal heart by ultrasound is of the heart missing or skipping a beat, which is often worrisome for the pregnant woman and health professionals alike, despite PAC being benign, self-limited and of no clinical significance in the majority of cases. However, in a small proportion of cases, irregular rhythms may be associated with clinically significant arrhythmia (∼ 2–3%)4,5 or may evolve into tachyarrhythmia (∼ 5%)6. In most instances, PAC occur in structurally normal hearts and are not associated with increased risk of congenital heart disease (CHD), although the two can certainly coexist4,6,7. These observations give rise to an important clinical question: how should an irregular rhythm be managed when it is detected incidentally during a routine antenatal appointment or obstetric scan? The systematic review by Bet and colleagues8 in this issue of the Journal aimed to evaluate two important clinical aspects of rhythm irregularities: the incidence of CHD and the risk of complications. The meta-analysis suggested that the overall risk of CHD in fetuses with PAC is 4–5 times that encountered in the general population. It also concluded that the risk of tachyarrhythmia, heart failure and fetal demise was low (< 2%). Based on these results, the authors advised performing an advanced ultrasound assessment in all cases of fetal PAC to exclude CHD and better characterize the arrhythmia. Furthermore, they recommended weekly FHR monitoring for early detection of tachyarrhythmia in order to avoid development of heart failure. However, these recommendations also raise several practical questions. Should they apply to all fetuses with PAC without considering other variables? Are there sufficient medical and allied-health-professional resources to implement this strategy of advanced scanning and weekly FHR surveillance for all cases of PAC? Are weekly antenatal visits to monitor FHR necessary and/or practical for all pregnant women, if most cases of PAC resolve with no further consequences? A closer look at the meta-analysis, which included 19 studies, indicates that only six were conducted in a low-risk population, that is, pregnancies referred solely because of a rhythm irregularity. In nine, there were additional reasons for referral and the pregnancies were considered high-risk. In the remaining four studies, the indications for referral were not available and these populations were analyzed together with those referred only for rhythm abnormalities, i.e. low-risk pregnancies. From the pooled analysis, the overall incidence of CHD in all 19 studies was 2.8% (95% CI, 1.5–4.1%), but it decreased to 1.7% (95% CI, 0.8–2.7%) when two outliers were removed. Considering only the 10 low-risk population studies, the pooled incidence was much lower at 0.9% (95% CI, 0.0–2.0%), which was similar to that encountered in the general population, quoted from a historical reference registry (0.6%)9. These data suggest that when patients are referred for an irregular rhythm without other known risk factors, the probability of associated CHD is similar to the background risk. With regards to risk factors for the development of tachyarrhythmia, such as their frequency or presence of bigeminal or trigeminal patterns, the meta-analysis did not allow better characterization of PAC, which is relevant to their management. To return to the question of how to manage rhythm abnormalities, the simple answer is that they should not be ignored, but pregnancies should be stratified by risk. An appropriate clinical algorithm should identify those fetuses at greater risk, so that their care can be escalated appropriately and in a timely fashion, and at the same time, pregnancies at lower risk can be similarly reassured without causing unnecessary anxiety. The aim of such a strategy is two-fold: (1) to exclude CHD or to identify the small number of cases in which cardiac malformations may trigger the arrhythmia and (2) to plan further investigations and FHR surveillance appropriate