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.

BACKGROUND Left-lateral tilt position is used to reduce assumed aortocaval compression by the pregnant uterus. METHODS Magnetic resonance images of 10 singleton parturients at full term and 10 healthy nonpregnant women were obtained for measurement of the abdominal aorta and inferior vena cava volume between the L1-L2 disk and L3-L4 disk levels in both the supine and left-lateral tilt positions (15°, 30°, and 45°) maintained by insertion of a 1.5-m-long polyethylene foam placed under the right side of the parturient's body. RESULTS Aortic volume did not differ significantly between parturients and nonpregnant women in the supine position (12.7 ± 2.0 vs.12.6 ± 2.1 ml, mean ± SD; mean difference, -0.1; 95% confidence interval [CI], -2.0 to 1.9; P = 0.95). Inferior vena cava volume in the supine position was significantly lower in parturients than in nonpregnant women (3.2 ± 3.4 vs.17.5 ± 7.8 ml; mean difference, 14.3; 95% CI, 8.3-20.2; P < 0.001). Aortic volume in parturients did not differ among left-lateral tilt positions. Inferior vena cava volume in the parturients was not increased at 15° (3.0 ± 2.1 ml; mean difference, -0.2; 95% CI, -1.5 to 1.2; P > 0.99), but was significantly increased at 30° (11.5 ± 8.6 ml; mean difference, 8.3; 95% CI, 2.3-14.2; P = 0.009) and 45° (10.9 ± 6.8 ml; mean difference, 7.7; 95% CI, 2.2-13.1; P = 0.015). CONCLUSIONS In parturients, the aorta was not compressed, and a 15° left-lateral tilt position did not effectively reduce inferior vena cava compression.