Pathophysiology Circulatory Physiology The predominant physiologic abnormality in transposition is that the systemic venous return is recirculated to the body through the right ventricle and the aorta, while the pulmonary venous return is recirculated to the lungs through the left ventricle and pulmonary trunk Thus, although in the normal heart the systemic and pulmonary circulations are in series, in transposition they are set up as two separate and parallel circuits In the fetus, this arrangement results in more desaturation of the upper body than is usual but, although not associated with other significant circulatory instability, may be partially responsible for abnormal central nervous system findings later in life (see Chapter 76) Kept separated, the oxygenated pulmonary venous blood cannot reach the systemic circuit, and the systemic venous return cannot circulate to the lungs This results in severe systemic arterial desaturation following birth and is incompatible with life Although this is easy to understand, it is the lifesaving communications between these two circuits that add significant complexity to the physiology Determinants of Systemic Arterial Oxygenation and Mixing Systemic flow (QS) is all of the blood coursing through the systemic circulation Effective systemic flow (QES) is defined as the portion of QS that contains saturated pulmonary venous blood Likewise, pulmonary flow (QP) is the entirety of the blood in the pulmonary circulation, whereas effective pulmonary flow (QEP) is that desaturated portion, having completed its course through the systemic circulation, now directed toward the lungs In the normal heart, where the cardiac connections are concordant and no shunts exist, QP = QS = QEP = QES In transposition, if no shunts existed, QEP and QES would both equal 0 and the patient would die Fortunately, two areas of communication between the circuits typically exist in the newborn One is at the patent arterial duct The other is through the atrial septum In addition, a ventricular septal defect, present in approximately 40% of patients, may exist as well In the first few hours of life, when pulmonary vascular resistance is high, blood will shunt predominately from the pulmonary (saturated) circulation to the systemic (desaturated) circulation This may result in the interesting finding of “reversed differential cyanosis,” whereby the blood in the lower extremities is more highly saturated than the in upper extremities This phenomenon lasts for as long as there is pulmonary to aortic shunting at the ductal level It may be prolonged in the setting of transposition with preductal coarctation (or interruption) of the aorta, whereby the distal aortic pressure is reduced by the obstruction, driving saturated blood from pulmonary artery to aorta In the absence of such obstruction as the pulmonary vascular resistance drops, transductal shunting takes the opposite course, with desaturated aortic blood entering the pulmonary circulation, contributing to QEP In turn, pulmonary venous return to the left atrium is increased, and the resultant elevation in left atrial pressure serves as the impetus to drive the saturated left atrial blood across the second communication, the oval foramen or atrial septal defect, to join the systemic circulation, thereby contributing to QES Once the arterial duct has closed, it is the bidirectional flow across the atrial septum that maintains QEP and QES Interventions to augment flow at the ductal and atrial levels allow for the successful stabilization of the newborn with transposition Ventricular Septal Defect The presence of a ventricular septal defect can have a variable influence on the circulation This in part depends on the size of the defect, as well as on the presence of interatrial mixing, and the pulmonary vascular resistance A large interventricular communication may contribute to beneficial circulatory mixing, but the presence of unrestricted flow to the lungs may result in symptomatic heart failure If surgery is not performed during infancy, this can predispose to early pulmonary vascular disease, which is generally evident by 6 to 12 months of age.20–22 The presence of additional pulmonary or subpulmonary obstruction further modifies these relationships and may limit QP Patients who are well balanced due to pulmonary obstruction are generally asymptomatic, having a protected pulmonary circulation, adequate mixing, and a “well-trained” left ventricle Left Ventricular Pressure and Mass The essence of transposition is such that the left ventricle supports the pulmonary circulation In the neonatal period, when the pulmonary vascular resistance is high, left ventricular work and impedance are elevated In a patient with an intact or virtually intact ventricular septum, the normal postnatal reduction in pulmonary vascular resistance results in a progressive reduction in left ventricular pressure and work, so that as a result, left ventricular myocardial mass progressively falls This reduction in left ventricular pressure and myocardial mass will be attenuated in the presence of a large ventricular septal defect The effects of this postnatal transition are of crucial importance to the operative approach In a patient with transposition with an intact ventricular septum, who has traversed this postnatal transition, any procedure (including the arterial switch operation), in which the left ventricle is called on to support the high resistance systemic circulation will result in left ventricular failure in the early postoperative period Thus, as will be discussed, the arterial switch operation is performed in early postnatal life, before the postnatal transition is complete Older patients will require either a preliminary procedure to increase the load on the left ventricle, such as banding of the pulmonary artery to “train” the left ventricle in preparation for definitive surgery, or alternatively may require a period of left ventricular support with ventricular assistance, in the early postoperative period Finally, an atrial switch procedure may be desirable for some of these patients