Andersons pediatric cardiology 1825

before and after surgical interventions, are shown This multidistribution circulation is inherently inefficient and results in progressive congestive heart failure over time As can be seen, the variability in efficiency is dependent on anatomy, ventricular function, valvar function, pulmonary parenchymal function, electrophysiologic status, systemic and pulmonary vascular resistance, ductal patency (prior to surgery) and the status of surgical connections (after surgery) Perioperative management is tailored to the assessment and provision of adequate oxygen delivery When oxygen delivery is insufficient, targeted interventions toward the cause(s) are undertaken, specifically geared toward the individual anatomy and physiology present Given the complexity and individual variation of this multidistribution circulation, commonly described treatment strategies such as “balancing systemic and pulmonary blood flow” and “avoiding overcirculation” may be too simplistic Increased pulmonary blood flow in isolation does not generally result in inadequate systemic oxygen delivery over the short term and, if present, additional causes should be sought Pulmonary alveolar function must be adequate for effective gas exchange, and total ventricular output must be sufficient in volume to provide enough systemic venous return to mix with pulmonary venous return See text for details Both before and after surgical management in the neonate, the fUVH is volume-overloaded, as it supplies the pulmonary blood flow (Qp) plus the systemic blood flow (Qs) plus the regurgitant volume (Qr) if present Additionally, although the term cardiac output has frequently been used clinically to describe the systemic blood flow in children with a biventricular circulation, this is an incorrect term in a neonate with a fUVH The true “cardiac” output is distributed in two to three different compartments Clinically, it is important in the neonate with a fUVH to be clear regarding the difference between the ventricular output (Qp + Qs + Qr) and that fraction of the ventricular output that delivers oxygen to the tissues—the systemic blood flow In addition, a number of terms have come into common usage over the past decades but are neither sensitive nor specific in the assessment of physiologic stability and the management of adequate oxygen delivery in a neonate with a fUVH As our understanding of the fUVH circulation has improved, we suggest that the frequently used terms listed here are outdated37–40 and should be modified Terminology suggested to be modified/eliminated include: • Parallel circulation (discussed earlier) • Balancing the pulmonary and systemic blood flow • Pulmonary overcirculation • Single-ventricle physiology “Balancing the Pulmonary and Systemic Blood Flow” In vitro studies41 have suggested that the optimal ratio of Qp:Qs is approximately unity (balanced), and in some clinical scenarios this may be true.42,43 However, recent in vivo studies suggest a wide range in this ratio, which did not affect the systemic delivery of oxygen or hospital survival In this study by Li and associates,44 systemic delivery of oxygen was highly correlated with the absolute value of Qs, and much more so than the ratio of Qp:Qs In addition, there are situations in the neonate with a fUVH that, while the circulation is “balanced,” represent a precarious clinical scenario For example, a patient with a mixed venous oxygen saturation of 25%, a pulmonary venous oxygen saturation of 95%, and a systemic oxygen saturation of 60% indeed has a balanced circulation—the Qp to Qs ratio is 1.0—but nonetheless has severely impaired systemic oxygen delivery The term “balanced circulation” is thus neither a sensitive nor specific term to define the goals of management “Pulmonary Overcirculation” Technically, this term describes increased pulmonary blood flow to the lungs compared to the body, a clinical scenario seen in many children with septal defects, a patent arterial duct, and so on However, “pulmonary overcirculation” has unfortunately become synonymous colloquially with decreased systemic blood flow, with an implied imbalance of systemic and PVR as the cause of potential clinical deterioration In the preoperative neonate, as stated earlier and confirmed with the clinical experiences waiting for organ transplantation, slowly progressive congestive heart failure—over days to weeks—is the more common clinical scenario during the transitional circulation, rather than acute deterioration Should more acute clinical changes take place, prompt investigation of the patency of the arterial duct should be undertaken In the postoperative neonate, low systemic oxygen delivery may be due to preferential maldistribution of flow into the pulmonary vascular bed (“high Qp:Qs”) from a low PVR combined with an anatomically large shunt, but as stated above, a number of additional causes must be investigated before attributing the clinical scenario solely to “pulmonary overcirculation” (e.g., arch narrowing, low global univentricular output, low oxygen content or multiple combinations of these factors, see Fig 70.2) As in the preoperative state, the converse is true: not all babies with a “high” oxygen saturation are at risk of clinical deterioration In postoperative patients, both hyperventilation and significantly increased supplemental oxygen did not result in clinical deterioration.45 During hyperventilation, there were no changes in systemic or mixed venous saturation, arteriovenous saturation difference, oxygen excess factor (Ω), or blood pressure Importantly, high levels supplemental oxygen produced significant increases from baseline in systemic saturation (90% ± 1% vs 80% ± 1%; P < 01), mixed venous saturation (54% ± 3% vs 44% ± 2%; P < 01), and Ω (2.6% ± 0.2% vs 2.3 ± 0.2%; P < 01), with no change in arteriovenous saturation difference or blood pressure.45 This study, as well as our clinical experience, suggests that increased pulmonary blood flow per se in the face of normal systemic blood flow and normal pulmonary parenchyma may have no physiologic consequences For example, a patient with a mixed venous oxygen saturation of 65%, a pulmonary venous oxygen saturation of 95% and a systemic oxygen saturation 85% has a Qp:Qs ratio of 2 : 1; however, systemic oxygen delivery is maintained, as evidenced by a normal mixed venous oxygen saturation and a narrow difference between the arterial and venous oxygen saturations Fundamentally and more accurately, circulatory maldistribution is a more inclusive term, which can be used to describe some of the causes of a state of decreased oxygen delivery, in particular, redirection of ventricular outflow to the pulmonary vascular bed and/or significant atrioventricular valve regurgitation However, the clinical scenario of low oxygen delivery may also be due to low univentricular output, low oxygen content, or multiple combinations of these factors, as shown in Fig 70.2 Each of these causes of decreased systemic oxygen delivery has different management strategies; therefore an accurate and complete assessment of the multidistribution circulation must be undertaken In conclusion, although “pulmonary overcirculation” may cause clinical instability—particularly if it contributes to pulmonary congestion, inadequate alveolar gas exchange, tachypnea, or respiratory distress—undercirculation of either the pulmonary or systemic vascular beds is always an unstable clinical scenario Simplifying the complex physiology shown in Fig 70.2—with terminology such as high Qp:Qs, pulmonary overcirculation, and balancing flow