Author: Melissa Colizza, MD - CHU Sainte-Justine, Montreal, Quebec
A 4-year-old male with a history of hypoplastic left heart syndrome (HLHS) and previous bidirectional Glenn with extensive bilateral pulmonary artery reconstruction undergoes a fenestrated Fontan procedure. On post-operative day one, the blood pressure decreases gradually to 64/32 from 92/45 despite increased vasoactive support with epinephrine and vasopressin, and the lactate increases from 1.4 to 4.2 The Fontan pressure is 20 mmHg with a common atrial pressure of 7 mm Hg and an oxygen saturation of 94% in room air. What is the MOST likely cause of the hypotension?
Early Fontan failure consists of low cardiac output with high Fontan pressure that remains refractory to medical therapy. In recent years, the risk of early Fontan failure in experienced centers varies from 2-6%. It is usually caused by residual defects along the Fontan pathway such as atrioventricular valve (AVV) regurgitation, inadequate pulmonary artery (PA) size or dysrhythmias. Other factors that are known to be associated with a high risk of Fontan failure include heterotaxy, dominant-right ventricle (RV) morphology, common AV valve, increased pre-operative pulmonary artery pressure (PAP), increased post-operative Fontan pressure, elevated left ventricular end-diastolic pressure (LVEDP), and prolonged cardiopulmonary bypass (CPB) and cross-clamp times.
In 1989, the concept of fenestration was introduced as a technique to preserve cardiac output, albeit at the expense of systemic oxygen desaturation, and to minimize the complications associated with high systemic venous pressures such as, pleural effusions, ascites, and lymphatic dysfunction. The fenestration has allowed for expanded Fontan candidacy, including patients previously deemed high-risk. Early reports demonstrated decreased length of hospital stay, leading to the widespread use of a Fontan fenestration in the 1990s. Its popularity waned in the face of undesirable long-term complications, including lower resting systemic oxygen saturation resulting in diminished exercise tolerance and higher risk for systemic thromboembolism. Despite being widely used, there is still much debate about the risks and benefits of fenestration. Two meta-analyses by Bouhout et al. and Li et al. have come to different conclusions. Bouhout et al. reported that the main benefits of a fenestration were lower PAP and decreased chest tube drainage. Li et al. described a lower burden of dysrhythmias. Both studies concluded that there was no significant difference in early mortality, need for Fontan takedown, length of hospital stay, or incidence of stroke or thrombosis. A retrospective study by Daley et al. demonstrated that patients with a fenestration had significantly lower survival, lower freedom from failing Fontan physiology and lower thromboembolic disease. However, the groups were overall unmatched; fenestrated patients had a higher incidence of HLHS, RV dominance, lateral tunnel Fontan, significant AVV regurgitation or AVV surgery, need for PA plasty, and higher mean PAP. These findings illustrate the challenge to objectively determine the benefits of Fontan fenestration since it is typically reserved for patients at higher risk for Fontan failure, which creates selection bias across studies.
Fontan failure can be categorized as circulatory failure or mechanical pump failure. Circulatory failure refers to the inability to maintain adequate pulmonary blood flow through the pulmonary vascular bed with an acceptable Fontan pressure. The pulmonary vascular bed is the most critical “bottleneck”, and successful Fontan circulation requires low pulmonary vascular resistance (PVR) and adequately sized pulmonary arteries. Mechanical pump failure relates to inability of the heart to meet systemic oxygen demand and may be secondary to systolic or diastolic dysfunction, dysrrhythmia, obstruction along the systemic vascular pathway or AVV regurgitation. Isolated systolic dysfunction remains fairly uncommon in the immediate post-operative period and should prompt investigation into an acute cause. It rarely causes profound cardiogenic shock unless the cardiac function is so poor that high atrial pressures prevent blood flow through the fenestration or through the pulmonary circulation. The child in the question has low cardiac output, low common atrial pressure and high Fontan pressures which would be consistent with circulatory failure. In the case of mechanical pump failure due to severe AVV regurgitation or systolic dysfunction, the common atrial pressure would likely be higher than 7 mm Hg. However, if the common atrial pressure remains lower than the Fontan pressure, shunting of desaturated blood through a patent fenestration is expected resulting in a systemic oxygen saturation ranging from of 70-80%. Shunting of desaturated blood is also expected in patients with a fenestrated Fontan and high pulmonary artery pressures secondary to hypoplastic pulmonary arteries or high pulmonary vascular resistance.
Post-cardiopulmonary bypass vasoplegia is a type of distributive shock typically occurring within the first 24 hours post-operatively. It is characterized by hypotension, high cardiac output, and low systemic vascular resistance that is relatively resistant to vasopressors and fluids. Vasoplegia tends to occur in cases of long CPB/cross-clamp times, and the pre-operative use of systemic vasodilators. It classically presents with hypotension requiring treatment with vasoactive medications in the absence of low mixed venous saturation or high lactate. Therefore, it is less likely in this patient.
In the above case, our patient is a high-risk Fontan due to the presence of a high fixed resistance to pulmonary blood flow secondary to hypoplastic pulmonary arteries (note the history of bilateral pulmonary artery reconstruction with likely residual pulmonary hypoplasia). The new finding of high Fontan pressures, low common atrial pressure, and high systemic saturation with evidence of a low cardiac output state points to an occluded fenestration. In this scenario, there is inadequate pulmonary blood flow through the lungs resulting in low cardiac output, and an absence of blood flow through the fenestration resulting in high systemic oxygen saturation given absent mixing of pulmonary venous return and systemic venous return in the common atrium. Although re-opening a fenestration is feasible in the cardiac catheterization lab, this may be associated with a high risk of systemic thromboembolism as the most likely cause of occlusion in the early post-operative period is a thrombus. Since most fenestrations are only 4 mm in diameter, reopening it may only have marginal benefits on such tenuous hemodynamics and further diagnostic evaluation, such as computed tomography with angiography or cardiac catheterization, may be needed. In the worst-case scenario, the Fontan may have to be taken down.
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