Effect of Lateral Tilt Angle on the Volume of the Abdominal Aorta and Inferior Vena Cava in Pregnant and Nonpregnant Women Determined by Magnetic Resonance Imaging

Abstract

Assumed aortocaval compression (inferior vena cava [IVC] compression and compression of the abdominal aorta) by the gravid uterus in parturients in the supine position can cause hemodynamic disturbances and uteroplacental hypoperfusion. The left-lateral tilt position is used to reduce aortocaval compression, although this assumption has not been morphologically validated. Reported recommended angles, although controversial, are 15 degrees after spinal anesthesia for cesarean delivery, and 30 degrees is recommended during resuscitation in pregnant women. This study, using magnetic resonance imaging (MRI), aims to investigate whether the left-lateral tilt angle reduces aortocaval compression. Two groups of women, 10 healthy women at full term with cephalic singleton pregnancies and 10 nonpregnant healthy women, volunteered and gave written consent for the study. Magnetic resonance imaging scans were taken to measure the volume of the abdominal aorta and IVC from the L1-L2 disk level to the L3-L4 disk level at supine and 15-, 30-, and 45-degree positions. These angles were achieved by using a 1.5-long hard V block, running the length of the woman’s body. Mean arterial pressure, heart rate, and cardiac output were recorded in each position for each subject. Statistical significance was achieved when P < 0.05. The fetus was in the right occiput position in 7 of the group of 10 and in the left occiput position in 3. While the shape of the aorta in pregnant women was different in some MRI slices, there was no significance in the difference in aortic volume between groups in any position, nor was there significant difference in either group in any of the left-lateral tilt positions. In the supine position, IVC volume in pregnant women was significantly smaller than that in nonpregnant women (3.2 ± 3.4 vs 17.5 ± 7.8 mL; mean difference, 14.3 mL; P < 0.001). In pregnant women, IVC volume between the supine position and 15-degree left-tilt position was not significantly different (3.0 ± 2.1 mL; mean difference, −0.2 mL; P > 0.99), but IVC volume was significantly different between the supine position and 30-degree left-tilt position (11.5 ± 8.6 mL; mean difference, 8.3 mL; P = 0.009) and was also significantly between the supine position and 45 degrees (10.9 ± 6.8 mL; mean difference, 7.7 mL; P = 0.015). While tilt positions of 30 and 45 degrees partially relieved IVC compression compared with the supine position, a tilt position of 15 degrees did not. However, there are limitations of this study; for example, the pregnant and nonpregnant women did not receive anesthesia, and the parturients were not in labor. Future research should attempt to examine physiologic measures as well.