Andersons pediatric cardiology 1855

decrease in myocardial VO2 (shifts the ventricular pressure/volume loop down and to the left, illustrating a reduction in myocardial O2 demand) Ideal agents for this strategy include nitroprusside, and the combined inodilator milrinone Optimizing the hematocrit (40% to 45%) and ensuring adequate ionized calcium are important; the latter is of particular importance for the neonatal cardiomyocyte The patient with respiratory distress and marginal cardiac output should undergo intubation and mechanical ventilation as described earlier In addition to addressing impaired gas exchange, replacing exaggerated negativepressure breathing with positive ITP reduces systemic ventricular afterload while unloading the respiratory pump, allowing for a redistribution of the limited CO to other vital organs, while improving the respiratory muscle, myocardial and global VO2/DO2 relationship.47a A second cause for clinical deterioration is systemic-to-pulmonary artery shunt malfunction This is clinically manifested by a decrease in PaO2/SaO2, resulting in systemic hypoxemia Because impedance to Qp is elevated, Qs is preserved and compensates for a decrease in CaO2 until the fall in CaO2 reaches a critical point The SaO2 decreases; however, the O2ER remains normal until the fall in CaO2 becomes severe enough that the compensatory increase in Qs no longer suffices and the O2ER begins to increase Because Qp is limited, the spontaneously breathing neonate compensates by increasing minute ventilation In the neonate receiving PPV and a fixed minute ventilation, the arterial- to endCO2 gradient becomes elevated (>5 to 10 mm Hg), consistent with “wasted” ventilation Temporizing measures include acute anticoagulation and increases in the systemic arterial pressure with the administration of vasopressors such as norepinephrine (provides significant inotropic support as well), phenylephrine and vasopressin Increasing airway pressure and minute ventilation may be of some benefit by increasing CO2 excretion, or it may increase the extent of wasted ventilation, limiting or negating the benefit of the increase in minute ventilation As ITP rises, the pressure gradient for systemic venous return may diminish If alveoli become overdistended (creating zone 1 and 2 conditions), impedance to Qp increases further An echocardiogram may demonstrate partial shunt occlusion by a thrombosis or a narrowing of the shunt otherwise; however, the sensitivity of an echocardiogram demonstrating such a lesion is low and echo should not be used to rule out shunt malfunction The cardiac catheterization and surgical teams should be notified immediately, as an urgent intervention may be indicated Acute Deterioration, Cardiac Arrest, and Mechanical Circulatory Support In both of these clinical scenarios—low systemic cardiac output and progressive hypoxemia from shunt thrombosis—progression to cardiac arrest is common For both Norwood-type palliation as well as isolated aortopulmonary shunt placement, ECMO is typically used in between 8% and 12% of patients.163,165,168–170 Standard resuscitative maneuvers including medications and chest compressions are typically unsuccessful, as the mechanisms to provide oxygenation and circulation that have been well studied in a patient with a structurally normal heart are vastly different in the baby with a fUVH and multidistribution circulation Chest compressions are unlikely to provide pulmonary blood flow through a shunt in the same manner as pulmonary blood flow is provided during chest compressions in a biventricular circulation Additionally, forward ventricular output during chest compressions is likely to be compromised by AV valve regurgitation, which is so common in these babies Furthermore, the effectiveness of cardiopulmonary resuscitation (CPR) may be compromised by delayed sternal closure, bleeding with reduced oxygen-carrying capacity, and significantly reduced intravascular volume If possible, avoiding the cardiac arrest with the preemptive use of ECMO may be of value In many cases this involves a paradigm shift from “ECMO as rescue” to “ECMO as treatment” for a state of low DO2—typically from undercirculation of the systemic or pulmonary vascular beds To this end, many pediatric cardiovascular centers are beginning to study and implement cardiac arrest reduction strategies for patients with cardiovascular disease, similar to those implemented in general inpatient pediatrics The most common causes of cardiac arrest in this patient population are residual lesions resulting in low cardiac output (e.g., systemic outflow obstruction, valvar regurgitation), excessive pulmonary blood flow at the expense of systemic blood flow, airway compromise, bleeding, tamponade, arrhythmias, and shunt thrombosis Prompt recognition and the initiation of standard resuscitative measures are essential; but in the patient with a fUVH and multidistribution circulation, inability to achieve a return to spontaneous circulation with conventional measures is common If a prompt return to stable circulation is not achieved within minutes, the use of rescue ECMO should be considered The use of rescue ECMO in circumstances of cardiac arrest has come to be termed extracorporeal life support during cardiopulmonary resuscitation, or eCPR, and is a potentially lifesaving intervention to reverse refractory cardiopulmonary arrest Rapid initiation of circulatory support is essential to a successful outcome For the patient in the early postoperative period following a sternotomy incision, central cannulation may be the most expeditious method of instituting ECMO However, during CPR, this results in critical interruptions of flow For the patient who is more remote from surgery or is receiving CPR, neck cannulation may be used Although the various institutions will use different approaches, there has been little research on critical ECMO settings such as appropriate ECMO flow rates, degree of PPV (“rest” settings vs active ventilation), wholebody cooling, antibiotics, nutrition, and many more In general, for the neonate with a cardiac source of pulmonary blood flow such as native pulmonary stenosis, a banded pulmonary artery or a Norwood with a right ventricle-topulmonary artery conduit, initial ECMO flow rates of 120 to 140 mL/kg per minute provide a systemic blood flow of only ~1.5 to 2.0 L/min per square meter (normal being 2.5 to 5.0 L/min per m2); however, there will be additional systemic blood flow if there is any ejection from the ventricle Importantly, providing adequate systemic blood flow for the patient with shunt-dependent pulmonary blood flow on ECMO can be challenging Maintaining an open shunt has been associated with improved lung function.52 However, flow rates of 120 to 140 mL/kg per minute will provide significantly less systemic blood flow as a percentage (different in every patient) of the typical 1.5 to 2.0 L/min per m2 of the ECMO output will be directed to the lungs (through the shunt) instead of the body, reducing systemic blood flow and DO2 even further Ventilatory maneuvers to increase PVR, such as low inspired fraction of oxygen and hypoventilation, may be helpful However, in the presence of a significant “runoff” through the shunt, physical reduction of shunt flow with clips or banding53 may be necessary to provide adequate systemic blood flow Rather than using a “standard” of 100 to 150 mL/kg per minute, a general approach should be taken to provide the maximal flow rates possible from the ECMO circuit that does not require significant additional volume to maintain, does not cause high outflow pressures, and does not cause hemolysis Unfortunately, total ECMO output is generally limited by intravascular volume status and/or the size of the cannulas Early after initiation of ECMO, the patient should undergo echocardiography ... treatment” for a state of low DO2—typically from undercirculation of the systemic or pulmonary vascular beds To this end, many pediatric cardiovascular centers are beginning to study and implement cardiac arrest reduction strategies... centers are beginning to study and implement cardiac arrest reduction strategies for patients with cardiovascular disease, similar to those implemented in general inpatient pediatrics The most common causes of cardiac arrest in this patient population are residual lesions resulting in low cardiac output (e.g., systemic outflow