BACKGROUND: Despite the existing dogma that women undergoing cesarean delivery under spinal anesthesia should be positioned with a 15° left-lateral tilt, the patients were actually positioned in a right-lateral tilt position in several of the original studies. The superiority of right versus left positioning for optimal inferior vena cava volume is unknown. We used magnetic resonance imaging to compare the effects of right-lateral and left-lateral tilt positions on abdominal aortic and inferior vena cava volumes in pregnant women. METHODS: Thirteen women with singleton pregnancies and gestational age 31–39 weeks underwent magnetic resonance imaging while in the supine position, and in the left-lateral (15° and 30°) and right-lateral tilt (15° and 30°) positions, which were maintained by placing a 1.5-m–long piece of polyethylene foam under either side of the body. Abdominal aorta and inferior vena cava volume were measured between the L1–L2 disk and L3–L4 disk levels using magnetic resonance images. RESULTS: Aortic volume did not differ significantly among any of the positions examined. Mean inferior vena cava volume was significantly greater in the 30° left-lateral tilt position than in the 15° right-lateral tilt (10.7 ± 7.5 vs 5.9 ± 5.1 mL; mean difference, 4.8; 95% CI, 1.2–8.5; P = .002) and 30° right-lateral tilt (10.7 ± 7.5 vs 5.9 ± 2.5 mL; mean difference, 4.8; 95% CI, 1.2–8.4; P = .002) positions. Mean inferior vena cava volume in the 15° left-lateral tilt position did not differ significantly from that in the 15° right-lateral tilt (mean difference, 0.4; 95% CI, −3.2 to 4.0; P = 1.000) or 30° right-lateral tilt (mean difference, 0.4; 95% CI, −3.3 to 4.0; P = 1.000) positions. Mean inferior vena cava volume in the supine position only differed significantly from that in the 30° left-lateral tilt position (5.2 ± 3.8 vs 10.7 ± 7.5 mL; mean difference, 5.5; 95% CI, 1.8–9.1; P < .001). The greatest inferior vena cava volume was observed in the 30° left-lateral tilt position in 9 of 13 subjects (70%), and in the 30° right-lateral tilt in 3 subjects (23%). CONCLUSIONS: The 30° left-lateral tilt position most consistently reduced inferior vena cava compression by the gravid uterus compared with the supine position. Mean inferior vena cava volume in pregnant women was not increased at either angle of the right-lateral tilt position compared with the 30° left-lateral tilt position. However, in a subset of patients, the 30° right-lateral tilt position achieved the optimal inferior vena cava volume. Further investigation to understand this variability is warranted.

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The present retrospective cohort study was designed to determine body mass index (BMI) at the delivery in women undergoing cesarean section in a Japanese urban area, and whether the nerve block height after spinal anesthesia upon the cesarean delivery relates to the lower maternal BMI, less gestational age, or underweight fetus at birth in the population. A total of 401 pregnant women undergoing cesarean delivery with spinal anesthesia were evaluated retrospectively. We examined background differences, including BMI at the delivery, gestational age, and fetal birth weight between the cases with and without the adequate initial nerve block height less than the sixth thoracic vertebral level (Th6) after the spinal dose administration. The data demonstrated advanced maternal age pregnancy (median 35.5 years) and normal BMI (median 24.7) at the delivery in the population. The patients with the inadequate initial nerve block height immediately after the spinal dose administration documented significantly lower block height compared with those with adequate block height (Th8 [n = 55] vs Th4 [n = 346], P < 0.001). There was a risk of the low initial block height caused by either preoperative BMI <23, gestational age <37 weeks, or fetal birth weight <2500 g in the population. In a Japanese urban area, parturient median BMI undergoing cesarean delivery is in the normal range. Such lower BMI, in addition to less gestational age or underweight fetus, seems one of the factors causing the low initial block height upon spinal anesthesia.

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