Innovative treatments developed during the past three decades have dramatically altered the course of selected fetal anomalies and conditions. Over time, fetal interventions have become less invasive, and the number of fetal centers has expanded. The North American Fetal Therapy Network now includes 36 medical centers in the United States and Canada. Some of the fetal abnormalities and conditions amenable to either medical or surgical treatment are presented here. The management of fetal anemia and thrombocytopenia is reviewed in Chapter 18, and treatment of some fetal infections is discussed in Chapters 67 and 68.
Selected medications administered to the pregnant woman are transported across the placenta in concentrations high enough to treat fetal conditions. Pregnancy physiology affects drug concentration, and transfer depends on maternal and placental metabolism (Chap. 8, p. 145).
Abnormal fetal cardiac rhythms are grouped into three categories: tachyarrhythmias, heart rates >180 beats per minute (bpm); bradyarrhythmias, heart rates <110 bpm; and ectopy, typically premature atrial contractions. If a fetal arrhythmia is suspected, M-mode ultrasound, described in Chapter 15 (p. 294), is used to measure the atrial and ventricular rates and to clarify the relationship between atrial and ventricular beats, thereby diagnosing the type of rhythm disturbance.
Premature Atrial Contractions
If the fetal heart rate is normal but the rhythm is irregular, the most common etiology is premature atrial contractions (PACs). These atrial ectopic beats are found in 1 to 2 percent of uncomplicated pregnancies (Hahurij, 2011; Strasburger, 2010). PACs represent immaturity of the cardiac conduction system. They typically resolve later in gestation or in the neonatal period. PACs are usually an isolated finding but may be associated with redundancy of the foramen ovale flap—formerly termed a foramen ovale aneurysm.
When a PAC is conducted, an extra beat is heard with handheld Doppler. However, the premature contraction more commonly arrives at the atrioventricular node during the refractory period. This results in a compensatory pause, which sounds like a dropped beat. M-mode evaluation confirms the diagnosis (Fig. 15-46, p. 295).
PACs may occur as frequently as every other beat, which means that the auscultated fetal heart rate may be as low as 60 to 80 bpm. Known as blocked atrial bigeminy, this condition is benign and does not require treatment (Strasburger, 2010). However, monitoring the fetus in labor may be challenging and necessitate cesarean delivery. M-mode ultrasound will differentiate atrial bigeminy from other causes of bradycardia, such as third-degree atrioventricular block.
Up to 2 percent of fetuses with PACs are later found to have supraventricular tachycardia (Copel, 2000; Srinivasan, 2008). Given the importance of prompt identification and treatment of supraventricular tachyarrhythmias, pregnancies with PACs are often monitored with fetal heart rate assessment every ...