Intrathoracic pressure effects on hepatic flow and inferior vena cava diameter: an ultrasonographic study

Objective

To compare the effect of an increased intrathoracic pressure on the inferior vena cava (ICV) diameter and hepatic flow (HF) in healthy subjects.

Patients and methods

Ten healthy subjects (seven females, three males; age 27.3 ± 4.5 years) were investigated in a supine position before and after application of continuous positive airways pressure (CPAP) of 10 cmH₂O by nasal mask. The study was performed using sonographic equipment with a multiprobe (convex 3.5–5 MHz; sector 2.5–3.5 MHz) and color-Doppler capability (Hitachi H 21). IVC was visualized by a two-dimensional echographic sector probe and M-mode was used to measure the inspiratory and expiratory diameters at the origin of the suprahepatic veins. HF is composed of portal flow (PF) and hepatic artery flow (HAF). Portal velocity, assessed near the liver hilum, was used as a measure of PF, and the left intrahepatic branch resistivity index (RI) was used as a measure of HAF. Measures were repeated twice for each value of intrathoracic pressure by two different examiners and the mean value was given for the statistical analysis. Results are given as the mean ± SD. Data were evaluated by paired t test and P < 0.05 was taken as statistically significant.

Results

CPAP determined a reduction of portal vein velocity: 30.0 ± 9.1 cm/s vs 19.7 ± 5.0 cm/s (P = 0.01). IVC diameters are increased by CPAP: inspiratory diameter 9.49 ± 2.5 cm vs 12.05 ± 3.9 cm (P = 0.002), expiratory diameter 16.46 ± 2.9 cm vs 18.08 ± 3.65 cm (P = 0.05).

Conclusion

The results of this study demonstrate that, in healthy subjects, variation of intrathoracic pressure by CPAP influences venous return. HF reduction could be due to an increased IVC pressure, as displayed by the bigger diameters measured during CPAP, other than a diaphragmatic descent. Ultrasonography is able to detect this effect and could be useful in a more complete evaluation of patient haemodynamic status in various clinical settings.