Fetal vascular pressure responses to nonlabor uterine contractions: dependence on amniotic fluid volume in the ovine fetus.

OBJECTIVE

This investigation tested the hypothesis that elevations in fetal vascular pressures during nonlabor uterine contractions would be eliminated during polyhydramnios and augmented during oligohydramnios.

STUDY DESIGN

Twenty-two chronically catheterized fetal sheep were divided into three groups: (1) control with normal amniotic fluid volume, (2) polyhydramnios produced by amnioinfusion, and (3) oligohydramnios produced by fluid drainage. Fetal arterial pressure, venous pressure, and amniotic fluid pressure were continuously recorded with on-line computer techniques. Amniotic fluid pressure was continuously subtracted from fetal arterial pressure and fetal venous pressure to correct to the fetal zero pressure reference. One minute averages were collected for 30 minutes centered around the nonlabor uterine contraction.

RESULTS

Amniotic fluid pressure increased significantly during nonlabor uterine contractions in the control group. During polyhydramnios the amplitude and duration of the amniotic fluid pressure elevation were similar to those of controls (not significant). During oligohydramnios the duration of the uterine contractions was similar to that of both controls and polyhydramnios groups; however, the amplitude was less (p < 0.001). Fetal vascular pressures increased significantly during nonlabor uterine contractions in the control group (p < 0.001) but did not change significantly during nonlabor contractions in the polyhydramnios group. The contraction-induced elevations in fetal arterial pressure and fetal venous pressure in the oligohydramnios group were significantly greater than those in the control group (p < 0.001).

CONCLUSION

This study shows that the fetal vascular pressure responses to nonlabor uterine contractions were modified significantly by increases or decreases in amniotic fluid volume. The fact that the fetal arterial pressure and fetal venous pressure responses were augmented during oligohydramnios and eliminated during polyhydramnios suggests that direct compression or a conformational change of the fetus may be the cause of the vascular pressure changes during nonlabor contractions.