Mechanisms in fetal bradyarrhythmia: 65 cases in a single center analyzed by Doppler flow echocardiographic...

Fetal bradyarrhythmias have various underlying mechanisms. As blocked atrial bigeminy (BB) generally resolves spontaneously, but incomplete atrioventricular block (AVB) might respond to steroid treatment, correct diagnosis is of major importance. Our objectives were to assess the underlying mechanisms in fetal bradyarrhythmia and the accuracy of Doppler techniques in differentiating between them.

Seventy-eight patients referred to our tertiary center between 1990 and 2007 for evaluation of fetal bradycardia were analyzed retrospectively. Besides Doppler recordings from the mitral valve/aorta, superior vena cava/aorta and pulmonary vein/peripheral pulmonary artery, we used recordings from the pulmonary trunk and ductus venosus. We calculated the ratio of the time interval between conducted and consecutive blocked atrial contractions divided by the interval between two conducted atrial beats (a(cb) /a(cc) ), to analyze more meticulously the atrial rhythm in BB and second-degree AVB.

Fetal bradycardia ( ≤ 110 bpm) was confirmed in 65 of the 78 referred cases. Twenty-five had AVB (of which 20 were complete AVB), 29 had BB (of which 23 were intermittent) and 11 had sinus bradycardia. The bradyarrhythmic mechanism was identified correctly in all but one fetus with an atrial ectopic rhythm. Heart rates < 65 bpm were not seen in fetuses diagnosed with BB and rates < 60 bpm were seen only in cases with complete AVB, but heart rate did not distinguish between BB and AVB in the 60-75 bpm range. The a(cb) /a(cc) ratio clearly differentiated between fetsues with BB and those with second-degree AVB, including during midgestation, when it was difficult to distinguish these fetuses.

Using Doppler flow recordings, the mechanism causing fetal bradycardia can be clarified. In most cases this can be accomplished by visual validation only, and meticulous measurements are needed mainly to distinguish midterm fetuses with BB from those with second-degree AVB.