During staged reconstruction, perhaps the most abrupt physiologic change to the circulation occurs immediately after the superior cavopulmonary connection (SCPC) or bidirectional Glenn Prior to the procedure, the fUVH distributes pulmonary blood flow (Qp), systemic blood flow (Qs), and, if present, the regurgitant volume (Qr) Immediately after the procedure, most studies report that ventricular volume decreases significantly (only Qs + Qr) whereas mass does not (Fig 70.3), although these results have been variable depending on ventricular morphology, age, and other factors This results in an acute mass-tovolume mismatch and transiently mildly decreased ventricular function.49–53 In addition, the upper body has an acute elevation in central venous pressure, and it has been speculated that this acute change in venous pressure results in increased intracranial pressure (similar to what is seen in traumatic brain injury) to explain the transient irritability and Cushing response—systemic hypertension and relative bradycardia—so frequently seen following this procedure FIG 70.3 Schematic representation of the changes in the mass-tovolume ratio immediately following the superior cavopulmonary connection Immediately after surgery, ventricular mass is unchanged while ventricular volume is decreased In general, arterial flow to the upper body accounts for 40% to 60% of the ventricular output Following the bidirectional Glenn, the pulmonary blood flow is essentially obligate; that is, in the absence of decompressing venovenous collaterals (discussed further on), the upper body's arterial flow to the brain, neck, and arms must return to the pulmonary arteries through the superior caval vein (Fig 70.4) The superior cavopulmonary connection coupled with the elimination of other sources of pulmonary blood flow results in an immediate decrease in ventricular work and ventricular stroke volume by 30% to 60% The physiologic benefits of this step in surgical management are apparent within weeks of surgery, as most babies experience a considerable clinical improvement in growth and development An alternative strategy is to leave an additional source of pulmonary blood flow (shunt, antegrade flow, etc.) in addition to the superior cavopulmonary connection The advantages of this strategy are improved pulmonary artery growth, higher oxygen saturations, and a continued supply of “hepatic factor” to the pulmonary vascular bed Disadvantages include persistent volume loading of the fUVH and, in most studies, a longer hospital length of stay and a higher incidence of pleural effusions and chylothorax.54–57 FIG 70.4 Schematic representation of upper body flow following the superior cavopulmonary connection The arrows represent direction of blood flow In the absence of venous collaterals from the upper body to the lower body (decompressing venovenous collaterals), the pulmonary blood flow is identical to the brachiocephalic outflow from the heart, or approximately 50% to 60% of the cardiac output in an infant Importantly, it is at this stage of staged reconstruction, as well as between the Glenn and Fontan, that additional procedures, if necessary, are most safely performed These include cardiac procedures (e.g., pulmonary arterioplasty, atrioventricular valvuloplasty) and noncardiac surgery (e.g., gastrointestinal, facial, orthopedic, and other procedures—see also Chapter 90).51,58–60 Total Cavopulmonary Connection (Fontan Procedure) Following the bidirectional Glenn procedure, lower-body venous return enters the fUVH in an unobstructed manner, thus maintaining adequate ventricular preload with low venous pressure in the lower half of the body However, following the Fontan procedure, which separates the systemic and pulmonary circulations, both the lower- and upper-body venous return must travel through multiple potential resistors to flow, as shown in Fig 70.1 Although systemic oxygenation improves, the multiple resistors to systemic venous return result in increased pressure in both the superior and inferior caval veins as well as in the pulmonary arteries Studies have also shown that, both acutely and chronically, the fUVH is chronically preload-deficient, and cardiac output falls slightly compared with the preoperative state (see further on) The chronically elevated central venous pressure is thought to be primarily responsible for the majority of long-term consequences of the Fontan circulation, particularly for the liver, lung, intestine, and lymphatic system.22–24,26,61–64 These consequences are discussed in detail in Chapter 73 Serial Physiologic Data During Staged Reconstruction The hemodynamic consequences of the anatomy, physiology, and surgical interventions have been qualitatively described by echocardiography and quantitatively by cardiac catheterization and magnetic resonance imaging Cohen and colleagues65 reported serial data on 65 patients undergoing cardiac catheterization before the superior and total cavopulmonary connections as well as during late follow-up (see Table 70.4) In summary, there is a fall in both ventricular output and systemic blood flow that is most pronounced with the Glenn procedure but continues after the Fontan procedure The arterial and venous oxygen saturation is narrowest (highest oxygen delivery) following the Glenn and before the Fontan Pulmonary blood flow is reduced following the