Mechanisms of systemic adaptation to univentricular Fontan conversion

After univentricular Fontan conversion, systemic venous pressure serves as the sole driving force for transpulmonary blood flow. Consequently, systemic venous return is markedly altered and ventricular filling is subnormal. The mechanisms and time course of systemic adaptation to Fontan conversion are incompletely understood. We hypothesized that acute elevation in systemic venous pressure induces an adaptive response similar to conversion to a univentricular Fontan circulation.

Adjustable vessel occluders were placed around the superior and inferior vena cavae in juvenile sheep. After 1-week recovery, occluders were tightened to acutely increase and maintain systemic venous pressure at 15 mm Hg (n = 6), simulating 1-stage Fontan conversion. Control animals (n = 4) received identical surgery, but venous pressure was not manipulated.

Cardiac index decreased significantly (3.9 ± 1.0 mL/min/m(2) to 2.7 ± 0.7 mL/min/m(2), P < .001) and then normalized to control at 2 weeks. Circulating blood volume increased (100 ± 9.4 mL/kg vs 85.5 ± 8.4 mL/kg, P = .034) as a persistent response. Cardiac reserve improved and was not different from control by week 3. Resting heart rate decreased in both groups. Oxygen extraction (arteriovenous oxygen difference) and neurohormonal mediators increased transiently and then normalized by week 2.

Adaptation to global elevation in systemic venous pressure to Fontan levels is complete within 2 weeks. Increased blood volume and reduced heart rate are persistent responses. Increased oxygen extraction and neurohormonal up-regulation are temporary responses that normalize with recovery of cardiac output. With improved physiologic understanding of systemic adaptation to Fontan conversion, approaches to single-ventricle palliation can be more objectively assessed and optimized.