CHANGES OF VENOUS-BLOOD FLOW VELOCITY WAVE-FORMS IN FETUSES WITH SUPRAVENTRICULAR TACHYCARDIA

Studies in the fetal lamb have shown that atrial pacing beyond a rate of 300-320 beats/min may be associated with dramatic changes of venous blood velocity waveforms, an increase of venous pressure by up to 75% hydrops, polyhydramnios and placental edema. The aim of our study was to determine the 'critical' heart rate frequency in the human fetus. In 11 fetuses (five with and six without hydrops) with supraventricular tachycardia, pulsed wave Doppler analysis of flow velocity waveforms of the inferior vena cava, the ductus venosus and the left hepatic vein were performed before and after drug treatment. In ten cases cardioversion was achieved by in utero antiarrhythmic drug therapy; in one case treated with digoxin and flecainide the supraventricular tachycardia was decreased to 160-190 beats/min with disappearance of hydrops. Before intrauterine treatment of supraventricular tachycardia, pulsatile reversal of blood flow in the inferior vena cava, ductus venosus and left hepatic vein was visible, with monophasic forward flow during systole and reversed flow during diastole in ten of 11 fetuses. One fetus with supraventricular tachycardia of 195 beats/min showed a normal biphasic forward flow pattern. During drug-induced sinus rhythm, a normal biphasic forward venous bloodflow pattern was shown in all ten cases. In five cases pulsatile reversal was demonstrated during a drug-induced reduction of the heart rate from 280 to 210 beats/min and a nor mal biphasic forward flow velocity waveform appeared during supraventricular tachycardia below 210 beats/min. From our results it appears that pulsatile venous flow velocity waveform changes to a biphasic forward pattern below a 'critical' heart rate of approximately 210 beats/min in the human fetus.