Background Acute circulatory failure is a clinical syndrome characterized by inadequate effective blood flow and reduced tissue perfusion with decreased delivery of oxygen to the capillaries.1 The reduction in oxygen delivery leads to impaired oxidative metabolism, lactic acidosis, and cell death Acute circulatory failure in children may be due to primary myocardial disease such as myocarditis, cardiomyopathy, cardiac allograft failure, or congenital heart disease; or secondary to systemic conditions such as sepsis and other inflammatory processes Myocardial performance in children with acute circulatory failure depends on the underlying condition and, in some cases, may change with time For instance, in children with congenital heart disease acute circulatory failure can present before cardiac repair, or it can complicate the early postoperative course Regardless of the etiology, the unifying feature for all children with acute circulatory failure is that the heart is unable to meet the circulatory demands of the tissues and the treatment should be directed at restoring this critical balance Advances in the approach to the management of acute circulatory failure have shifted away from a focus on myocardial contractility, toward favoring strategies that optimize systemic blood flow while protecting the myocardium through afterload manipulation, oxygen demand control, and myocardial rest The management of children with acute circulatory failure requires an individualized treatment strategy, characterized by initial stabilization, followed by subsequent tailored treatments This chapter will cover the pathophysiology, diagnosis, and treatment options for acute circulatory failure in children, focusing on ventilation, pharmacologic agents, and mechanical support Epidemiology of Acute Circulatory Failure Acute circulatory failure is one of the most common causes of death in children admitted to a cardiac intensive care unit (ICU) However, the exact incidence of acute circulatory failure is difficult to estimate as the definition is broad and circulatory failure may not be a primary reason for admission to the ICU The mortality from acute circulatory failure is higher in younger children, patients with congenital heart disease, after cardiopulmonary resuscitation, and those requiring mechanical support, estimated at 40% to 60% in the latter group.2,3 Pathophysiology of Acute Circulatory Failure Based on the intrinsic cardiac function, global cardiac output, and oxygen balance, the pathophysiology of acute circulatory failure in children can be divided into five categories Despite the common feature of systemic hypoperfusion, these categories differ significantly in their manifestations, underlying cause and the subsequent therapeutic strategies 1 Acute Myocardial Dysfunction With Reduced Cardiac Output and Increased Afterload Acute myocardial dysfunction with reduced systemic oxygen delivery characterizes the low cardiac output state that complicates the postoperative course of around one in four children early after cardiopulmonary bypass.4 The physiologic features of this state are an elevated ventricular afterload, abnormal ventricular-vascular interactions, and impaired systolic and/or diastolic performance Anecdotally this is the most commonly encountered manifestation of acute circulatory failure in children with cardiac disease 2 Preserved Myocardial Function With Normal Cardiac Output and Systemic Hypoperfusion Inadequate systemic oxygen delivery can affect infants with a functionally univentricular heart and normal ventricular contractility whose total cardiac output may be normal, but there is a maldistribution between flow to the pulmonary and the systemic circulations These infants are very dependent on the maintenance of stable pulmonary and systemic vascular resistances, and even small changes in these can precipitate rapid circulatory failure and systemic hypoperfusion 3 Preserved Systolic Function With Abnormalities of Diastolic Function A proportion of patients with normal systolic function after tetralogy of Fallot repair and Fontan-like operations can develop a low cardiac output state early after surgery, which is secondary to diastolic dysfunction and inadequate pulmonary blood flow In these patients, treatment is directed at optimizing diastolic function and cardiopulmonary interactions, while avoiding interventions that increase contractility 4 Residual Anatomic Lesions in Postoperative Patients In a minority of patients after cardiac surgery, a low cardiac output state may be secondary to residual or new anatomic problems In the absence of targeted investigations, these are often clinically indistinguishable from other causes of a low output, but are generally resistant to, or paradoxically may be worsened by conventional medical interventions In recent years, the incidence of residual lesions has decreased, in part due to the wide utilization of intraoperative echocardiography and thorough preoperative workup and surgical handoff.5 5 Preserved Myocardial Function With Normal or Increased Cardiac Output Inadequate systemic oxygen delivery in the presence of normal myocardial function, reduced afterload, and normal or increased cardiac output is an unusual cause of acute circulatory failure In this setting, despite a normal cardiac output, the total or regional demand for oxygen is excessively high This occurs in children with distributive shock The first four categories described above generally affect infants and children with congenital heart disease who are undergoing cardiac surgery Therefore acute circulatory failure in many infants and children with cardiac disease is to an extent predictable, and medical management should routinely include proactive strategies targeted at the prevention of this condition If acute circulatory failure does occur, this should prompt early therapeutic intervention with appropriate targets and subsequent investigations Clinical Manifestations The clinical signs of acute circulatory failure are primary perfusion failure, with a compensated or decompensated shock state ensuing Following the onset of hemodynamic dysfunction, several compensatory mechanisms are initiated in an attempt to maintain perfusion and function of essential organs As the acute circulatory failure progresses, the compensatory mechanisms can become