The oscillating 'vena cava syndrome' during quiet standing--an unexpected observation in late pregnancy.

While studying the lung function of pregnant women at term in four different postures, we were surprised to note marked cyclic accelerations in the heart rate in two-thirds of the women when in a standing position. The mean cycle length was 105 s (range: 1-4 min) and the amplitude had a mean of 27 beats/min (range: 9-51). Blood flow velocity measurements with ultrasound Doppler over the femoral vein showed that there was an intermittent reduction of flow during quiet standing. When the venous return ceased, maternal heart rate increased, cardiac output decreased and blood pressure fell. After the venous blood flow was restored, maternal heart rate, cardiac output and blood pressure returned to normal until the cycle started again. Concomitant with these maternal heart rate changes, different patterns of fetal heart rate were observed. About 70% of the fetuses showed reduction in the long-term variability, increase in fetal heart rate or periodic accelerations. Although no woman fainted during quiet standing, the maternal circulatory changes were consistent with those seen in the classical vena cava syndrome.