Tachyarrhythmias are defined as fetal HRs of more than 180 beats per minute (bpm) [14
]. These are broadly classified as sinus tachycardia, SVT, atrial flutter and ventricular tachycardia. Advances in noninvasive evaluation outlined above have helped distinguish likely mechanisms in most cases of fetal tachycardia, though there are several limitations. The algorithm for the evaluation of tachycardia mechanisms based on echocardiographic findings is outlined in .
Algorithm for evaluation of mechanism of tachyarrhythmia based on Doppler and relationship of atrial and ventricular events
Sinus tachycardia may rarely present with fetal HRs of 180–200 bpm. This may be seen in the setting of maternal pyrexia, use of stimulants, maternal thyrotoxicosis or fetal systemic disease such as anemia, fetal distress and rare infections [14
]. A gradual increase or decrease in fetal HR, lack of abrupt initiation or breaks and preserved HR variability in the setting of 1: 1 atrioventricular conduction and normal atrioventricular conduction times would suggest sinus tachycardia rather than SVT.
SVT as defined by nonsinus mechanism, 1: 1 atrioventricular conduction and HRs above 180 bpm account for the majority (70%) of fetal tachyarrhythmia. They encompass tachycardias because of different mechanisms.
- Short ‘ventriculoatrial’ tachycardias, in which the ‘ventriculoatrial’ interval is less than half of RR interval, usually involve reentry mechanisms including atrioventricular reentry (AVRT) using a bypass tract or atrioventricular nodal reentry (AVNRT). As retrograde activation of the atria occurs shortly after ventricular activation, a short ‘ventriculoatrial’ interval is noted on Doppler evaluation. Using MCG, Strasburger et al. were able to show that complex mechanisms were involved in the initiation and maintenance of tachycardia in fetuses. Premature atrial contractions are also common . Preexcitation is noted in about 30% of cases [32,33]. Tachycardia typically shows a sudden onset and cessation and may be intermittent or sustained (present >50% of time in a 24-h period).
- Long ‘ventriculoatrial’ tachycardias include atrial ectopic tachycardia or paroxysmal junctional reciprocating tachycardia (PJRT). These can often be difficult to distinguish from each other with echocardiography, though use of tissue Doppler imaging (TDI) has shown promise. Atrial ectopic tachycardia is secondary to an automatic ectopic focus . Gradual warm up and cool down and variable atrioventricular conduction times may be noted. PJRT, on the contrary, is due to reentry using a slow conducting parahisian pathway that conducts retrograde with resultant delayed atrial activation and reentrant tachycardia. PJRT is usually incessant and difficult to treat and may be associated with tachycardia-mediated cardiomyopathy and cardiac failure due to the incessant nature of the tachycardia.
Atrial flutter accounts for about 30% of cases of fetal tachycardia [31
]. These are typically associated with high atrial rates of around 300–500 bpm and slower ventricular response in the setting of variable atrioventricular conduction. There is a high incidence of associated reentry in fetuses with atrial flutter, with reentry being noted in about 70% of cases [32
]. Atrial flutter is usually noted later in gestation.
There are very few multicenter organized studies with regard to the management and drug therapy for SVT; however, several excellent institutional reviews and general guidelines are available, with more recent publications outlining a more systematic approach based on arrhythmia mechanism [23••
]. The decision to treat or not to treat depends upon several factors including mechanism of tachycardia, persistence, fetal gestational age and well-being and the presence or absence of associated congenital heart disease (CHD). Sustained tachyarrhythmia may affect fetal well-being and result in hydrops due to impaired ventricular filling, altered flow patterns across the foramen ovale or cardiac dysfunction in the setting of incessant tachycardia. Left-sided path-ways and atrial flutter were noted to be associated with hydrops earlier, presumably secondary to altered flow across the foramen ovale [50
In general three options are available: no treatment with close monitoring, transplacental drug therapy and, finally, delivery of fetus [51
]. It is important to recognize that intermittent tachycardia in the fetus, especially later in gestation, is well tolerated and may not necessarily require transplacental therapy or emergent delivery in the absence of hydrops. In such cases, in-hospital monitoring of the fetus for 12–24 h to assess arrhythmia frequency and fetal well-being is recommended. Intermittent arrhythmias (<50% of time) in a healthy fetus can then be monitored on an outpatient basis. Close follow-up with repeat ultrasounds to assess rhythm and fetal well-being once or twice a week is recommended. On occasions, arrhythmia frequency may increase or concerns of fetal well-being may prompt hospital admission and institution of drug therapy or delivery if the fetus is near term. Rare cases of hydrops and neurologic sequelae have been reported in the setting of intermittent tachycardia [40
The comparison of some of the studies on the management of fetal tachyarrhythmia is shown in [38
]. Over the recent years, there has been an increasing recognition of the potential risks of proarrhythmia to both the mother and the fetus. The efficacy of transplacental therapy is highly dependent on the pharmacokinetics of a given drug, its ability to cross the placenta and fetal bioavailability [23••
]. For example, transplacental transfer of digoxin is significantly impaired in the presence of hydrops fetalis, whereas sotalol has been shown to have good transfer. On the contrary, higher levels of flecainide have been noted in the fetal amniotic fluid and conduction delay noted on ECG in infancy [53
]. In general, digoxin continues to be the first line of therapy in the absence of fetal hydrops. Second-line agents for refractory SVT or the presence of hydrops or both include sotalol, flecainide or amiodarone. Agents used include propranolol, propafenone and procainamide amongst others. Direct fetal therapy with intramuscular digoxin in the setting of fetal hydrops has been reported [41
]. Intravenous adenosine by cordocentesis has been used to interrupt incessant reentrant tachycardia or to exclude atrial flutter. Adenosine has a transient effect and is typically used in association with other antiarrhythmic agents to control the tachycardia. Finally, delivery and postnatal management of the arrhythmia, if persistent, is an option in the mature near-term fetus. The underlying arrhythmia makes fetal monitoring during pregnancy and labor challenging at times. However, adding the risks of prematurity is not doing the fetus a favor, and hence this should be reserved for patients with persistent, difficult-to-treat arrhythmias and associated fetal hydrops, in which transplacental or direct therapy or both have failed and the fetus is of a reasonable gestational age. Because of the real potential for proarrhythmia and adverse outcomes, a coordinated multidisciplinary approach including perinatologists, fetal cardiologist, adult cardiologist and neonatologist is beneficial.
This table depicts a few large series of fetal tachycardia or atrial flutter treatment
Ventricular tachycardia is rare and may present with ventricular rates of more than 180 bpm in the setting of atrioventricular dissociation. Atrial rates are normal, or rarely 1: 1 retrograde ventriculo-atrial rates secondary to retrograde atrial activation are noted. This makes diagnosis by echocardiography challenging, though TDI has been helpful in identifying the earliest activation in the ventricular myocardium. Ventricular tachycardia is usually seen in the setting of underlying conduction abnormalities such as CAVB, fetal myocarditis or LQTS [14
]. MCG has been very useful in the prenatal detection of LQTS and the quantification of arrhythmias [59
]. Regional referral should be considered in cases with a strong family history of LQTS or in the presence of atypical echocardiographic findings or both. Prenatal transplacental therapy with lidocaine and magnesium sulphate, mexiletine and beta-blockers has been reported [57