How to differentiate second‐degree atrioventricular block from extrasystoles in fetuses with a bigeminal ventricular rhythm

First, we want to thank Dr Bijoy Balakrishnan, Department of Fetal Medicine, Cimar Fertility Center, Cochin, India, for recently making us aware of a typographical error in our article published in AOGS.1 When discussing ventricular rhythms with a shorter time interval between two consecutive ventricular contractions (VV-interval) that systematically alternated with a longer interval (bigeminal ventricular rhythms), we wrote that this pattern could be seen in 3:1 atrioventricular block (AVB), which will result in a slow regular rhythm, instead of writing 3:2 AVB. Secondly, we would like to share the experience we have accumulated in recent years, as we believe this may improve the diagnosis and management of fetuses with bigeminal ventricular rhythms. As discussed in our article, we recommend starting by establishing whether the longer VV-interval corresponds to a heart rate >100 bpm, suggesting that every second beat is a premature extrasystole (in bigeminy), or <90 bpm, suggesting that every third atrial beat is blocked. Ventricular extrasystoles will not affect the atrial rhythm, which will remain regular (Figure 1A). Conducted atrial extrasystoles in bigeminy will also affect the atrial rhythm, but are rarely diagnosed in the fetus. In our recent experience, not only blocked atrial extrasystoles in trigeminy (BAT) but also 3:2 second-degree AVB results in a ventricular rhythm with a long VV-interval that is only 70%-80% of twice the short VV-interval (Figure 1B-F), and will be difficult to differentiate by analyzing the ventricular rhythm. That a premature blocked atrial extrasystole will result in an incomplete compensatory pause is easy to understand, as it resets the sinus node.1 In addition, a delayed ventricular beat will lead to an increased diastolic filling period that will improve systolic performance and shorten the time lag from atrial to ventricular ejection.2 This is illustrated in Figure 1B, where the time interval between events related to atrial and ventricular contractions is shorter during a longer than a shorter VV-interval (100 vs 125 ms), even though these two beats have the same electrical delay in the atrioventricular node. This effect will make the longer VV-interval shorter. In our cases with 3:2 AVB there is also a progressive lengthening of atrioventricular conduction between the two beats preceding the non-conducted beat (Wenckebach or Mobitz type I block, Figure 1E,F), which makes the shorter VV-interval significantly longer and the compensatory pause incomplete. The atrial rhythm consequently needs to be clarified in cases where every third atrial beat is blocked. It is trigeminal in BAT and regular in 3:2 AVB with or without (Mobitz II) Wenckebach conduction. This can be demonstrated by using Doppler recordings where both atrial and ventricular depolarizations can be identified by their hemodynamic consequences. Besides showing a closer “resemblance” to an electrocardiogram, these recordings also allow us to estimate mechanical time intervals, used as an indirect measure of atrioventricular conduction.3 As illustrated, these intervals increase from approximately 145 ms (+4 SD) to 225-250 ms between the two conducted beats, and confirm Wenckebach conduction in our fetuses with 2:3 AVB (Figure 1E,F). The clinical management of these arrhythmias differs markedly. Ventricular extrasystoles usually disappear spontaneously within a few weeks. Fetuses with blocked atrial beats in bigeminy and trigeminy have a 6%-14% risk of developing supraventricular tachycardia.4 The risk may be even higher when extrasystoles occur as couplets (Figure 1D). Fetuses with incomplete AVB might benefit from transplacental treatment with fluorinated steroids.5 In conclusion, we recommend that fetuses with bigeminal rhythms should be diagnosed by someone who can record and interpret Doppler recordings, where the timing of both atrial and ventricular depolarizations can be recognized.