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 one-week recovery, occluders were tightened to acutely increase and maintain systemic venous pressure at 15 mmHg (n=6), simulating one-stage Fontan conversion. Control animals (n=4) received identical surgery, but venous pressure was not manipulated.
Cardiac index decreased significantly (3.9±1.0 to 2.7±0.7 ml/min/m2, P<0.001), then normalized to Control at 2 weeks. Circulating blood volume increased (100±9.4 vs. 85.5±8.4 ml/kg, P=0.034) 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 (A-VO2 difference) and neurohormonal mediators increased transiently, 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 upregulation are temporary responses which 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.