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Acute Circulatory Failure: Pharmacologic and Mechanical Support
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. 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.
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. 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
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 harmful to other organs, leading to secondary organ dysfunction or failure, most typically the lungs, kidneys, and gastrointestinal tract. Hypotension is a late sign of acute circulatory failure, especially in neonates and infants, due to their higher systemic vascular resistance and vasoactive capacity compared with older children. Skin and muscles are affected early during acute circulatory failure as a result of blood being shunted away in order to perfuse other organs. This leads to ischemia of these vascular beds. Providers should recognize a prolonged capillary refill as a surrogate marker of decreased superior vena cava oxygen saturation, and if associated with hypotension, infers an increased mortality risk. In addition, the presence of neurologic dysfunction may suggest reduction in cerebral oxygen delivery beyond the point of cerebral autoregulation and is an ominous clinical sign in patients with acute circulatory failure ( Table 64.1 ).
Table 64.1
Signs and Laboratory Findings of Acute Circulatory Failure
| Organ System | Circulatory Instability Signs | Acute Circulatory Failure Signs | Laboratory Derangements |
|---|---|---|---|
| Respiratory | Tachypnea, increased WOB, grunting | Respiratory failure with hypoxia | PaO 2 /FiO 2 <300 in the absence of congenital heart disease or lung disease PaCO 2 >65 or 20 mm Hg over baseline PaCO 2 |
| Cardiovascular | Tachycardia, capillary refill <2 s, weak distal pulses | Tachycardia, bradycardia, capillary refill >2 s, arrhythmias, hypotension, weak central pulses | SvO 2 <60 O 2 ER >25 BNP >400 pg/mL |
| Renal | Oliguria | Anuria, tubular necrosis | Creatinine elevation >95th percentile for age or doubling of creatinine from admission |
| Neurologic | Agitation, anxiety, GCS >6 | Lethargy, somnolence, GCS <6 | NH 4 level >80 µg/dL, hypoglycemia |
| Gastrointestinal | Ileus, feeding intolerance | GI bleeding, distended abdomen with signs of peritonitis | |
| Hepatic | Right upper quadrant tenderness, hepatomegaly | Jaundice | AST >200 IU/L ALT >200 IU/L INR >1.5 in the absence of systemic anticoagulation |
| Hematologic | Endothelial and platelet activation | DIC | Platelets <50 × 10 3 or 400 × 10 3 PT 20 s aPTT 40 s Fibrinogen <100 mg/dL or > 400 mg/dL; D-dimers |
| Metabolic | Mild acidosis | Severe acidosis, hyperlactatemia | Lactate level >2 mmol/L |
ALT , Alanine aminotransferase; aPTT , activated partial thromboplastin; AST , aspartate aminotransferase; BNP , brain-type natriuretic peptide; DIC , disseminated intravascular coagulation; GCS , Glasgow Coma Scale; INR , international normalized ratio; NH 4 , ammonium; O 2 ER , oxygen extraction ratio; PaCO 2 , partial pressure of carbon dioxide; PaO 2 /FiO 2 , partial pressure of oxygen/inspired fraction of oxygen ratio; PT , prothrombin time; SvO 2 , mixed venous oxygen saturation; WOB , work of breathing.
Diagnosis of Acute Circulatory Failure
In extreme presentations, the clinical diagnosis of acute circulatory failure is usually straightforward. However, the majority of cases of acute circulatory failure present in a more subtle or insidious manner, sometimes presenting with respiratory (tachypnea, wheeze) or circulatory signs (tachycardia) that may lead frontline providers to initiate therapies that are inappropriate or even harmful. Frontline providers in the emergency department or the wards should be vigilant for clues during history taking or examination. Furthermore, the mode of presentation may vary by age; for instance, infants at risk of acute circulatory failure will develop poor feeding or irritability with feeds, whereas an older child may complain of excessive fatigue or sleep difficulties.
In the ICU, the diagnosis of acute circulatory failure is established by noninvasive and invasive methods. The noninvasive methods include assessment of vital signs, physical examination, pulse oximetry, near-infrared spectroscopy (NIRS) monitoring, and echocardiography. The invasive methods include central venous pressure monitoring, co-oximetry, and assessment of cardiac output via transpulmonary thermodilution and pulse contour analysis.
Laboratory Studies
The presence of anion gap metabolic acidosis is indicative of acute circulatory failure. Inadequate oxygen delivery will lead to lactate and lactic acid formation due to anaerobic metabolism via the Cori cycle. It is generally accepted that in the normal circulation, lactate levels less than 2 mmol/L correlate with superior vena cava O 2 saturation of 70% or greater. In addition, lactate levels greater than 6 mmol/L are associated with increased rate of adverse outcomes including mortality in neonates following cardiac surgery. Due to these associations, lactate levels are widely used in the ICU.
Monitoring
Pulmonary Artery Catheterization
The pulmonary artery catheter (PA catheter), also referred to as the Swan-Ganz catheter, is a balloon-tipped catheter that is used to assess mixed venous oxygen saturation, PA pressure, and pulmonary capillary wedge pressure, and to measure cardiac output by thermodilution. The PA catheter can be particularly helpful in conditions where pressure changes and assessment of response to interventions are immediately needed, such as treatment of severe pulmonary hypertension or the response to (or appropriateness of) fluid administration. However, the PA catheter, once considered a cornerstone in the management of critically ill patients, has been overshadowed by complications associated with its insertion and placement, as well as inaccuracies and difficulties with the interpretation of data. In patients with congenital heart disease with intracardiac shunts or valvar regurgitation, PA catheter data can be misleading or invalid.
Pulse Index Continuous Cardiac Output System
The pulse index continuous cardiac output (PiCCO, Maquet Cardiopulmonary) is an invasive, continuous cardiac output monitor. The PiCCO system utilizes transcardiopulmonary thermodilution and pulse contour analysis obtained from intraarterial and central venous catheterization. The PiCCO system may present fewer technical challenges than PA catheters, and studies have demonstrated close correlation in the data generated between systems. Some of the identified challenges with this PiCCO system are its invasive nature and the necessity of frequent calibration due to data drift from the pulse contour analysis techonology. Another downside of this system is its invasive nature, requiring arterial catheterization with a 3F or 4F catheter, often necessitating cannulation of the femoral artery for smaller patients, which presents an additional risk of arterial compromise.
Near-Infrared Spectroscopy
NIRS monitoring provides a noninvasive tool for continuous monitoring of regional tissue oxygen saturation or oximetry in critically ill children. The NIRS monitor analyzes the concentration and ratio of oxygenated to deoxygenated hemoglobin and assists at determining the balance between oxygen supply and demand, and is most commonly used to monitor brain and somatic oxygen saturation. The NIRS monitor employs single-use adhesive patches with an integrated near infrared light source and photodetector, which are applied close to the tissue of interest, for example the forehead or abdomen. In contrast to pulse oximetry, the NIRS monitor evaluates the nonpulsatile signal, reflecting the oxygen saturation of the microcirculation. The data derived from NIRS cerebral oximetry monitoring have demonstrated good correlation with jugular venous saturations. The cerebral NIRS also assesses regional cerebral oxygen saturation and can identify inadequate cerebral perfusion that is linked to neurologic injury and adverse outcomes.
Single-site cerebral and two-site NIRS (cerebral and somatic) are being increasingly used in patients with heart disease, and there is growing data to support that this may predict adverse outcomes including the need for extracorporeal membrane oxygenation (ECMO), neurodevelopmental impairment or death in selected patient groups.
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