Physiology of ventricular septal defect shunt flow in the fetus examined by color Doppler M-mode.

The fetal circulation is unique in having the right ventricle perform as a systemic pump delivering oxygen-enriched placental blood to the distal fetal systemic circulation via the ductus arteriosus. Both ventricles then operate at similar systemic pressures. For this reason, the pressure gradient across the interventricular septum is known to be minimal. It has been noted, however, in fetuses with ventricular septal defects (VSDs) that shunt flow can be detected by color flow Doppler crossing the VSD, indicating that there are subtle differences in pressure between the ventricles at different times in the cardiac cycle. The isolated VSD thus provides a convenient physiological window for studying interventricular dynamics in the developing fetus. The rapid fetal heart rate and absence of a fetal ECG make accurate timing of shunt dynamics by standard 2D echocardiography with color flow Doppler difficult. We present a case in which careful hemodynamic evaluation could be performed by combination of information from several different cardiac ultrasound modalities that possess a higher temporal resolution— continuous-wave Doppler, pulsedwave Doppler, and color Doppler M-mode—to elucidate the temporal characteristics of VSD shunt flow. A systematic fetal echocardiogram was performed on a 36-year-old gravida 4 woman whose previous child had been diagnosed with a perimembranous VSD. At 22 weeks of gestation in this pregnancy, a small defect of the perimembranous ventricular septum, measuring 2.1 mm in diameter (Figure 1, left), could be clearly seen in the 2D fetal echocardiogram. Accessory tissue was burgeoning close to the tricuspid region but remained rudimentary. All other structures and flows were considered normal. A bidirectional flow across the VSD was seen by 2D color Doppler. Left-toright shunt was thought to occur during systole, but the temporal resolution of the cine-loop mode was not accurate enough to determine when reverse flow occurred (Figure 1, middle and right). On the color M-mode tracing (Figure 2, left), systole can be determined by the time between mitral valve closure and mitral valve opening. A red color signal is shown crossing the VSD from left to right during systole. However, reverse shunting from right to left (blue) also occurs during systole and can be discerned by the pulsed Doppler sampling of the VSD shunt flow shown in Figure 2, middle. Late systolic flow below the baseline indicates right-to-left shunt that ends just before the closure of the semilunar valves (denoted on the pulsed Doppler spectral trace by the arrows). Flow velocities remain low (0.55 m/s on the continuous-wave Doppler tracing, Figure 2, right), because the peak systolic pressure gradient between the ventricles is small, estimated at 3 mm Hg. No shunt flow was detectable during diastole.