Fetal tachyarrhythmias detected in utero include irregular cardiac rhythm resulting from isolated extrasystoles, supraventricular tachycardia, and atrial flutter. The most common of the above arrhythmias is an irregular cardiac rhythm resulting from isolated extrasystoles (Silverman et al., 1985; Kleinman, 1986; Copel et al., 2000). Most of these extrasystoles originate in the atria and resolve spontaneously (Kleinman, 1986; Reed et al., 1987). An increased frequency of premature beats has been attributed to the maternal use of caffeine, tobacco, and alcohol (De Vore, 1984). Although premature atrial or ventricular contractions have not been considered a risk factor for anomalies in the past, more recent reports suggest that a fetal structural cardiac abnormality may be found in up to 2% of such cases (Beall and Paul, 1986; Reed, 1991).

Supraventricular tachycardia is the most common serious dysrhythmia detected prenatally (Bergmans et al., 1985; Kleinman et al., 1985a). The majority of supraventricular tachycardias are considered to be re-entrant dysrhythmias and are identified by a fetal heart rate of more than 200 beats per minute (bpm) with one-to-one atrial-to-ventricular activity (Figure 41-1) (Simpson and Marx, 1994). Paroxysmal supraventricular tachycardia may also occur as part of the Wolff–Parkinson–White syndrome, together with a short P–R interval, prolonged QRS, and presence of a delta wave on electrocardiogram. Typically, supraventricular tachycardia has not been associated with congenital heart disease; however, in 5% to 10% of cases, structural cardiac abnormalities can be found (Beall and Paul, 1986; Reed, 1989). The presence of structural heart disease in addition to supraventricular tachycardia is associated with a poor fetal outcome (Bergmans et al., 1985).

Fetal atrial flutter has been diagnosed and reported less commonly than supraventricular tachycardia (Kleinman, 1986). The atrial rate in flutter can be estimated between 300 and 500 bpm with a variable ventricular response dependent on the degree of atrioventricular block (Figure 41-2). Fetal atrial flutter has a poor prognosis, due in part to its association with structural heart disease (in up to 20% of cases) and with the development of hydrops fetalis (Kleinman, 1986; Reed, 1991; Simpson and Marx, 1994).

In contrast to benign arrhythmias such as isolated extrasystoles, fetal dysrhythmias have the potential for serious sequelae. It has been well described that fetuses with tachyarrhythmias have decreased cardiac output and are at risk for hydrops fetalis (Reed et al., 1987; Reed, 1989). Protracted supraventricular tachycardia can lead to cardiac failure in a matter of days (Allan, 1984). The exact mechanism for the development of cardiac failure and hydrops remains unclear. The progression to hydrops in utero seems to follow right heart failure with tricuspid regurgitation and right ventricular dilatation, signifying impending decompensation (Allan et al., 1983; Chao et al., 1992; Gembruch et al., 1993). It has been postulated that either passive liver congestion associated with cardiac failure or decreased hepatic perfusion from decreased cardiac output may result in hypoalbuminemia, leading to decreased oncotic pressure and transudation of fluid into interstitial spaces (Johnson et al., 1992). Pericardial and pleural effusions, fetal ascites, and subcutaneous edema are probably late manifestations of cardiac decompensation.

Arrhythmias are identified in approximately 1% of all fetuses; however, the actual incidence of fetal arrhythmias is expected to be higher because a significant proportion of them can be intermittent or resolve spontaneously (Reed, 1989; Simpson and Marx, 1994). The most common and clinically significant tachyarrhythmias identified in utero are supraventricular tachycardia and atrial flutter.

Ultrasound evaluation should include a complete fetal survey and a survey of the cardiac anatomy to confirm absence of structural malformations. The incidence of abnormal cardiac anatomy varies with the type of arrhythmia. The fetus should also be examined for evidence of hydrops as defined by fluid collections in the pericardium, the pleural spaces, ascites, or skin edema. Amniotic fluid may also be increased. An increase in fetal cardiac size or wall thickness may be an indication of abnormal cardiac hemodynamics (Reed, 1989).

Echocardiography, including M-mode assessment and Doppler analysis, is an important tool in the diagnosis and evaluation of fetal dysrhythmias (Kleinman et al., 1980; De Vore et al., 1983; Silverman et al., 1985). The atrial and corresponding ventricular activity can be viewed by two-dimensional echocardiography and the rates determined by placement of the M-mode cursor perpendicular to the atrial and ventricular walls. M-mode echocardiography can also be used to identify pericardial effusions and to measure wall thickness, chamber size, and fractional shortening (see Figures 41-1 and 41-2). Doppler evaluation of atrioventricular and semilunar valve flows can be used to time the ventricular contractions (Strasburger et al., 1986). The hemodynamic effects of a dysrhythmia are assessed by determining the size and function of the four chambers, the magnitude of the semilunar and atrioventricular flow-velocity integrals, and the presence of mitral or tricuspid regurgitation or nonimmune hydrops fetalis. Color flow mapping may also be used to identify flow disturbances. The approach to the ultrasound evaluation of arrhythmias is summarized in Table 41-1.

The differential diagnosis of fetal tachyarrhythmias includes benign isolated extrasystoles and more significant dysrhythmias such as atrial flutter and supraventricular tachycardia. M-mode echocardiography is useful in differentiating these conditions, as described above.

Premature atrial and ventricular contractions are considered to be benign and do not require treatment. However, it has been reported that a sustained tachyarrhythmia may subsequently develop in up to 1% of fetuses with premature beats (Kleinman, 1986).