Fetal atrioventricular and outflow tract flow velocity waveforms during normal second half of pregnancy.

Maximum flow velocity waveforms at atrioventricular and outflow tract level were studied longitudinally in 40 fetuses from 18 to 38 weeks' gestation at 3- to 4-week intervals. Flow velocity parameters studied were peak systolic and time-averaged velocity and acceleration time at outflow tract level and time-averaged velocity, peak E-wave and peak A-wave velocities, and E/A ratio at atrioventricular level. An increase of all flow parameters with advancing gestational age was found. Aortic parameters were higher than pulmonary ones. Time-averaged, peak E-wave, and peak A-wave velocities were higher at tricuspid level than at mitral level, whereas E/A ratios were higher at mitral level. Outflow tract parameters showed no correlation with period time. A positive correlation for peak E-wave velocity and E/A ratio, and a negative correlation for time-averaged and peak A-wave velocity was found with period time. These results suggest that the normal second half of pregnancy is characterized by rising ventricular stroke volume and output and reduction in ventricular afterload. The left ventricular afterload seems to be lower than the right ventricular afterload. Atrioventricular flow velocities are heart rate dependent.