Purpose

Inferior vena cava flow (IVCF) and abdominal aortic flow (AAF) are essential factors of the systemic circulation.¹ Although postural changes might alter IVCF and AAF by the gravity effect, the exact details are unknown. This study aimed to evaluate the differences of IVCF and AAF in the supine and upright positions of healthy volunteers, using an original magnetic resonance imaging (MRI) system that can obtain images in any posture (multi-posture MRI).²

Materials and Methods

We obtained caval velocity-mapped images with ECG-triggered cine phase-contrast technique using multi-posture MRI (N = 12) in the supine and upright positions, and determined mean IVCF and AAF velocities in the region of interest in each cardiac phase. In addition, we obtained flow curves in the cardiac cycle from the IVCF and AAF velocity multiplied by each cross-sectional area. We assessed the mean IVC velocity (IVC-V_mean), the maximum IVC velocity (IVC-V_max), the mean IVCF (IVCF_mean), the maximum IVCF (IVCF_max), and the cross-sectional area of the inferior vena cava (IVC-CA) in the supine and upright positions. We also assessed the mean AA velocity (AA-V_mean), the maximum AA velocity (AA-V_max), the mean AAF (AAF_mean), the maximum AAF (AAF_max), and the cross-sectional area of the abdominal aorta (AA-CA) in the supine and upright positions. Further, we assessed heart rates in the supine and upright positions.

Results and Discussion

The IVC-V_mean in the upright position (1.62 ± 1.03 cm/s) was significantly lower than that in the supine position (9.55 ± 5.03 cm/s; P = 0.002)(Fig. 2a), the IVC-V_max in the upright position (2.1 ± 0.93 cm/s) was significantly lower than that in the supine position (14.6 ± 5.66 cm/s; P = 0.002)(Fig. 2b), the IVCF_mean in the upright position (165 ± 109 mL/min) was significantly lower than that in the supine position (1403 ± 534 mL/min; P = 0.002)(Fig. 2c), the IVCF_max in the upright position (23.4 ± 10.2 mL/min) was significantly lower than that in the supine position (249 ± 94.1 mL/min; P = 0.002)(Fig. 2d), and the mean IVC-CA in the upright position (211 ± 115 mm²) was significantly lower than that in the supine position (342 ± 203 mm²; P = 0.008)(Fig. 2e). The AA-V_mean in the upright position (2.10 ± 0.79cm/s) was significantly lower than that in the supine position (6.71 ± 2.30 cm/s; P = 0.002)(Fig. 3a), the AA-V_max in the upright position (5.22 ± 2.39 cm/s) was significantly lower than that in the supine position (17.9 ± 6.52 cm/s; P = 0.002)(Fig.3b), the AAF_mean in the upright position (241 ± 113 mL/min) was significantly lower than that in the supine position (699 ± 306 mL/min; P = 0.002)(Fig. 3c), and the AAF_max in the upright position (572 ± 303 mL/min) was significantly lower than that in the supine position (1823 ± 630 mL/min; P = 0.003)(Fig. 3d). However, no significant difference was observed between the upright position and the supine position in terms of the mean AA-CA (184 ± 50.1 mm² and 175 ± 43.6 mm², respectively; P = 0.583)(Fig. 3e). The heart rate in the upright position (85 ± 7.4 times/s) was significantly higher than that in the supine position (73 ± 6.9 times/sec; P = 0.002) (Fig. 4). The findings of this study indicate that gravity reduces IVCF and AAF, despite autoregulation.

Conclusion

Both IVCF and AAF decrease in the upright position. Multi-posture MRI makes it possible to evaluate the effect of gravity on systemic circulation.

Acknowledgements

No acknowledgement found.