superior cavopulmonary connection and increases again after the Fontan operation Pulmonary artery pressure and ventricular end-diastolic pressure fall throughout In addition to this serial hemodynamic data in a relatively small cohort, cross-sectional quantitative catheterization data in larger subsets of patients have confirmed nearly identical values for ventricular output as well as atrial and pulmonary artery pressures at each stage.51,66–69 Unanticipated Hypoxemia (Fig 70.5, Table 70.4) Unanticipated hypoxemia during and after staged reconstruction (beyond the typical expected values shown in Fig 70.5A) may be due to many causes related to various combinations of cardiac dysfunction, pulmonary dysfunction, and the status of the surgical procedures Despite the protean and multiple causes in the differential diagnosis of unanticipated hypoxemia, the etiology may be summarized in three broad categories: low mixed venous saturation (see Fig 70.5B), low pulmonary venous saturation (see Fig 70.5C), and decreased pulmonary blood flow (see Fig 70.5D) These factors may occur in isolation or combination and are summarized in Fig 70.6 FIG 70.5 Schematic representation of categorical etiologies of unexplained, atypical hypoxemia during and following staged reconstruction The normal arteriovenous difference in oxygen saturations (A-VD02) is assumed to be between 20% and 30% Values in the circles represent potential venous oxygen saturations; values in the boxes represent approximate arterial oxygen saturations (A) Typical values during staged reconstruction Lower than expected arterial oxygenation may be explained totally or in part by a low mixed venous oxygen saturation (B), low pulmonary venous oxygen saturation (C), and/or decreased pulmonary blood flow (D) Specific causes are addressed in Table 70.5 *Following the Fontan operation, there will be variable arterial hypoxemia depending on any residual right-to-left shunting (e.g., fenestration) Table 70.4 Typical Physiologic Changes During Staged Reconstruction (Fontan Pathway) Hemodynamic Variable Systemic blood flow (L/min per m2) Aortic oxygen saturation Mixed venous oxygen saturation Pulmonary blood flow (L/min per m2) Pulmonary-to-systemic flow ratio (Qp:Qs) Ventricular output (L/min per m2) Pulmonary artery pressure Ventricular end-diastolic pressure Prior to SCPC (n = 65) 2.8 ± 0.8 78 ± 6% 49 ± 7% 3.8 ± 1.3 1.4 ± 0.6 Prior to TCPC (n = 65) 3.3 ± 1.0% 85 ± 5% 64 ± 6% 2.6 ± 1.2 0.7 ± 0.3 After TCPC (n = 42) 3.0 ± 0.8% 91 ± 6% 64 ± 8% 2.7 ± 0.6 0.95 ± 0.2 6.6 ± 1.1 14 ± 5 9 ± 3 3.3 ± 1.0 11 ± 3 8 ± 3 3.0 ± 0.8 10 ± 2 6 ± 3 Qp, pulmonary blood flow; Qs, systemic blood flow; SCPC, superior cavopulmonary connection; TCPC, total cavopulmonary connection Serial changes in patients following staged reconstruction reported by Cohen et al.65 Additional studies have confirmed these general values in different patient populations.51,66–69 FIG 70.6 Differential diagnosis of hypoxemia across the surgical continuum aThese factors will only contribute to hypoxemia after a Fontan procedure if there is an anatomic right to left shunt, such as a fenestration, baffle leak, or decompressing venous collateral Venovenous Collaterals Following the SCPC, pressure in the upper venous compartment and pulmonary arteries is higher than that in the lower venous compartment, pulmonary veins, and atrium Because of this pressure gradient, previously small venous channels may dilate and progressively direct blood away from the higher-pressure circuit into the lower-pressure circuit (Fig 70.7) The physiologic effect of these connections is to reduce flow in the superior caval vein and therefore total pulmonary blood flow, resulting in unanticipated hypoxemia These venovenous collaterals are more prevalent and hemodynamically significant in the face of a larger venous pressure gradient from the upper body to the lower body, as in patients with elevated PVR, pulmonary artery hypoplasia, or an anatomically narrow superior cavopulmonary connection.70–72 The most common locations for these venous structures were previously described by McElhinney and colleagues.72a,73 The hypoxemia may be relieved by coil or device embolization during cardiac catheterization (see Fig 70.7; Videos 70.1 to 70.5) Although less common, these venovenous collaterals may be seen after the Fontan procedure as well.73–75