Aspirin: A Treatment for the Headache of Shunt-Dependent Pulmonary Blood Flow and Parallel Circulation?
D ependence on an expanded polytetrafluoroethylene graft for provision of pulmonary blood flow is a common yet precarious interval through which a population of patients with congenital heart disease must pass. The goals of the systemic-to-pulmonary artery shunt are to relieve cyanosis and to provide time before establishing in-series circulation by either complete repair of 2-ventricle lesions or, in the case of the single-ventricle patient, a bidirectional Glenn shunt. In the case of patients
... he case of patients with single-ventricle anatomy, this period of parallel circulation is necessary to permit the lung maturation and the reduction in pulmonary vascular resistance that are necessary for subsequent palliation. For patients who will ultimately achieve a 2-ventricle repair, the goals of a preliminary shunt may include increasing the size of the pulmonary artery size or having a larger, older patient at the time of repair. This period of parallel circulation is tenuous, and the patient remains at increased risk during this period of altered circulation. Article p 293 With parallel circulation, cardiac output from the heart is partitioned to the lungs and body based on the relative resistances of the pulmonary and systemic circulations. If the shunt is large, the patient will experience excessive pulmonary blood flow and congestive heart failure. Furthermore, with a large shunt, diastolic pressure is low and coronary circulation may be impaired. With stress, autonomic reflexes will result in an acute increase in sympathetic tone. The elevation of systemic vascular resistance leads to acute increase in the ratio of pulmonary to systemic flow. In the face of limited cardiac output, this acute increase in the ratio of pulmonary to systemic flow can result in a critical reduction in systemic oxygen delivery. Therefore, autonomic reflexes aimed at maintaining vital organ perfusion with in-series circulation have an adverse effect in patients with parallel circulation and can result in a critical reduction in oxygen delivery. This is likely to be one mechanism of death for patients with parallel circulation. 1,2 Alternatively, if the shunt is sufficiently limiting, then increasing pulmonary blood flow in response to increased metabolic demands such as may occur with fever or exercise will not be possible, increased cyanosis results, and again a critical reduction in oxygen delivery will occur. Additional limitations of parallel circulation include the potential for parenchymal lung disease, anemia, and decreased cardiac output to result in worsening cyanosis. Decreased cardiac output from the single ventricle may be the result of myocardial dysfunction, atrioventricular valve regurgitation, or arrhythmias. In addition, dehydration will result in decreased preload with decreased cardiac output and may occur in the course of an acute illness resulting from decreased fluid intake, gastrointestinal loss, and/or fever. Diuretic use is common in this patient population and will limit the ability of the patient to autoregulate fluid status in the face of an acute illness. Small expanded polytetrafluoroethylene grafts generally are used for these systemic-to-pulmonary artery shunts despite their thrombogenicity. The lumen of the grafts develops a neointima. Both the coagulation system and the inflammatory processes contribute to the development of this neointima, which is a proliferation of myofibroblasts with endothelial cell ingrowth. 3-6 The neointima is associated with organizing thrombus and tends to be most severe at the anastomotic sites. 7 Furthermore, these patients are uniformly cyanotic, and other growth factors such as vascular endothelial growth factor have increased expression in this patient group, potentially intensifying neointimal growth. 8, 9 As a result of this luminal in-growth, the caliber of the shunt is reduced over time even as the patient continues to grow. The patient with shunt-dependent pulmonary blood flow survives in an environment marked by both physiological and anatomic vulnerabilities. To identify patients with parallel circulation at risk of life-threatening complications, we developed a home monitoring program. 10 In addition to routine use of aspirin, parents were discharged with a pulse oximeter and a suitably sensitive infant scale. They were directed to obtain twice-daily arterial saturations and daily weights. Parents were instructed to call in case of an arterial saturation Ͻ75% or an acute weight loss of Ն30 g. An increase in cyanosis can be the result of a reduction in shunt caliber, a decrease in singleventricle cardiac output, pulmonary disease, or anemia. Acute weight loss is a sensitive indicator of dehydration, which can result in decreased total cardiac output, increased blood viscosity, and an increased risk of shunt occlusion. In our single-center experience, home monitoring proved successful in limiting the mortality of patients with shunt-dependent pulmonary blood flow.