Subdiaphragmatic venous hemodynamics in the Fontan circulation

We investigated the subdiaphragmatic venous physiology in patients subjected to the Fontan operation to understand some of the early and late problems of this circulation.

Flows were evaluated by Doppler ultrasonography in the subhepatic inferior vena cava, hepatic vein, and portal vein during respiratory monitoring and with a tilt table. Twenty control subjects (group A) and 56 patients who had the Fontan operation, 27 in functional class I (group B) and 29 in class III or IV (group C), were studied. Inspiratory/expiratory flow ratio was calculated to reflect respiratory effects, and upright/supine flow ratio was calculated to assess gravity effects. Inferior vena caval, hepatic venous, and wedged hepatic venous pressures were measured during catheterization in 21 control subjects and 25 Fontan patients. The difference between wedged and hepatic venous pressures represents the transhepatic venous pressure gradient.

Fontan hepatic venous flow depended more on inspiration than control, but without difference between groups B and C (inspiratory/expiratory flow ratios: 1.7, 2.9, and 2.9, respectively; P <.02). Normal portal venous flow was higher in expiration; this effect was lost in group B and reversed in group C (inspiratory/expiratory flow ratios: 0.8, 1.0, and 1.3; P <.0005). Gravity reduced portal venous flow in groups A and B, but progression to functional class III or IV (group C) exacerbated this effect (upright/supine flow ratios: 0.8, 0.7, and 0.5; P <.01). Inferior vena caval, hepatic venous, and wedged hepatic venous pressures (in millimeters of mercury) in the Fontan groups were all elevated compared with the control group (inferior vena cava, 14.4 +/- 4.4 vs 5.9 +/- 2.3; hepatic vein, 14.7 +/- 4.5 vs 5.9 +/- 1.9; wedged hepatic vein, 14.7 +/- 4.0 vs 8.3 +/- 2.6; P <.0001). However, transhepatic venous pressure gradient in the Fontan group was lower than in the control group (0.5 +/- 0.5 vs 2.4 +/- 2.0; P <.001). Univariate analysis of inferior vena caval pressure and transhepatic venous pressure gradient showed significant inverse correlation (r = 0.6, P <.002).

In patients who are in functionally poorer condition after the Fontan operation, portal venous flow loses normal expiratory augmentation and adverse gravity influence is enhanced. These suboptimal flow dynamics, coupled with higher splanchnic venous pressures and lower transhepatic venous pressure gradients, suggest that hepatic sinusoids are congested, acting as "open tubes." Transhepatic gradient loss is incrementally worse with higher caval pressures. These observations may be responsible for late gastrointestinal problems in patients who have had the Fontan operation.