Case
Hi, I’m a new obstetrician in town and I am seeing a mother who is 20 weeks along in her pregnancy. She has been handling the pregnancy well without any issue and this is the second pregnancy for the mother with the first resulting in live birth so she is a G2P1. Anyway, the reason I am calling is because when I was performing her ultrasound I noted that the fetal heart rate was very irregular and occasionally would go very fast. To me it sounds like an arrhythmia and I am concerned that it will affect the pregnancy. Is there someone I should be coordinating with to help manage the fetus? I know in my fellowship we coordinated with adult cardiology but when I called the local adult cardiologist they put me in touch with you. Any advice?
What am I thinking?
This sounds like a clear-cut case of a fetal arrhythmia. The issue, however, is figuring out: what type of arrhythmia, how serious it is for the fetus’s health and development, and how to manage it. Management of fetal arrhythmias should be considered a team effort that involves a combination of the treating obstetrician, a fetal cardiologist, a pediatric electrophysiologist, and/or adult electrophysiologist. Decisions on who should be involved also depend on the suspected treatment plan that may additionally involve a cardiac surgeon.
Fetal arrhythmias can be simplified to bradyarrhythmias and tachyarrhythmias. Identification of the rhythm most commonly involves the use of a fetal echocardiogram and mechanical contraction of the atria and ventricles on an M-mode tracing that provides a surrogate electrocardiogram. A Doppler pattern of inflow and outflow from the left ventricle may act as a surrogate electrocardiogram as well. Fetal magnetocardiography can also, rarely, be utilized but is offered in limited centers.
Fetal bradycardia can be secondary to the development of fetal heart block. A fetus can present with varying degrees of heart block including first-degree, second-degree Mobitz I, second-degree Mobitz II, and third-degree or complete heart block. Infants born with heart block are at increased risk of having mothers who carry anti-SSA and anti-SSB antibodies. However, the converse is not true (i.e., mothers with anti-SSA and anti-SSB antibodies are not at higher risk for infants with heart block). Infants with early first-degree heart block noted on fetal ultrasound should be carefully monitored for progression with collaboration between the obstetrician and pediatric cardiology team. A fetal echocardiogram should be performed for any evidence for congenital heart disease as heart block may be the first indication. Studies have been performed at the utilization of various medications and treatments (e.g., maternal steroids, immunoglobin) to the mother to help minimize the risk of progression of heart block with mostly inconclusive results. Once the fetus progresses to third-degree or complete heart block, the fetal heart rate may be too low for hemodynamic stability and may result in the development of hydrops fetalis. Early work has been performed in the creation of fetal pacing that may provide promise to prevent the development of hydrops fetalis and provide a bridge to a term delivery; however, this technology is not clinically available at this time. If possible, it is recommended to keep the fetus in utero for as long as possible with close monitoring for any worsening developments or hydropic features. During this time, planning should occur among the team to determine delivery options and the need for immediate pacing via a temporary wire. Eventually, the infant will need a permanent pacing system.
For fetal tachyarrhythmias, similar methodologies of diagnosis are needed using M-mode echocardiography, Doppler, or rarely, fetal magnetocardiography. In most instances, M-mode provides a very clear picture of atrial and ventricular contractions and can even provide indication of premature beats. Premature atrial contractions are commonly seen in the fetus and can sometimes lead to tachyarrhythmias such as an atrial tachycardia or atrial flutter with a rhythmic 2:1 or 3:1 conduction to the ventricle (see Fig. 20.1 ). This results in varying ectopic beats followed by a rapid, sustained rhythm much like the scenario presented. The fetus may also have a reentrant form of supraventricular tachycardia. Frequently recurrent or persistent episodes of tachycardia may begin to have detrimental effects on the fetus and could lead to hydrops fetalis. In such situations, the treatment team must initiate medical management via the mother.
