The goal of this study is to evaluate if vasopressin can elevate systemic arterial blood pressures without having a significant effect on pulmonary arterial pressures. Because patients who have undergone Fontan procedures rely on low pressures across the pulmonary vascular bed to maintain cardiac output, vasoactive agents that concomitantly increase systemic and pulmonary pressures may have a deleterious effect in this specific patient population.

Hypothesis: In patients with Fontan physiology, vasopressin will increase systemic BP by 20% above baseline, without increasing the transpulmonary gradient.

Patients with Fontan physiology are at increased risk of developing hypotension during periods of illness or with anesthesia. These patients have changes in venous capacitance, compliance, and venomotor tone. When hypotension occurs, the approach to management requires a careful understanding of these changes and the selection of appropriate, efficacious vasoactive agents. Vasoconstrictive medications are frequently used to treat hypotension when patients are refractory to fluid resuscitation. However, most of these medications have an effect on both the systemic and pulmonary circulations, raising the vascular pressures of both circuits. However, these patients often do not tolerate abrupt increases in pulmonary arterial pressures or vascular resistance, and a low transpulmonary gradient is imperative to maintain cardiac output. The ideal agent would be one that raises systemic vascular pressure while having a minimal effect on the pulmonary circulation.

One promising medication for treating hypotension in Fontan patients is vasopressin. One prior study evaluated the hemodynamic effect of vasopressin in pediatric patients with pulmonary hypertension. Compared to other vasopressor agents (phenylephrine, epinephrine), vasopressin increased systemic blood pressure with the least effect on pulmonary artery pressure. In a second study evaluating the use of vasopressin specifically for patients undergoing cardiac surgery for the creation of a Fontan, those who were treated with vasopressin as part of their postoperative management demonstrated lower transpulmonary gradients than those treated with placebo. Other studies have evaluated the use of vasopressin in Fontan patients for other clinical outcomes, such as decreasing postoperative chest tube output. On the basis of these studies, as well as our clinical experience managing these patients in the operating room, procedural suites, and in the intensive care unit, the investigator believes that vasopressin is not only safe but may be the preferred agent for this patient population.

Although vasopressin has been studied extensively in the context of sepsis, post cardiopulmonary bypass, and other vasoplegic disease states, the use of vasopressin has not yet been studied in the general population of patients who have Fontan physiology. The investigator hopes to demonstrate the efficacy of vasopressin as a medication specifically for these patients, as one that can elevate systemic blood pressure without compromising cardiac output or having a deleterious effect on the pulmonary vasculature.

Inclusion Criteria:

1. All patients scheduled for cardiac catheterization at Lucile Packard Children's Hospital with Fontan circulation

Exclusion criteria:

1. Critical illness severe enough to preclude extended cardiac catheterization time
2. Patients already on vasopressin
3. Patients with pulmonary hypertension.
4. Patient not scheduled for cardiac catheterization at Lucile Packard Children's Hospital (LPCH).