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 ).
| 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 |
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.
Management of Acute Circulatory Failure
The management of acute circulatory failure varies according to its etiology. In children with congenital heart disease the presentation is more predictable, giving the opportunity for timely preventative interventions. Conversely, in children with cardiomyopathy presenting with acute circulatory failure, such prevention is not possible. In this case, a more reactive approach with early investigations, establishment of therapeutic targets, and appropriate intervention is necessary.
Early Recognition
Early recognition, appropriate treatment, and rapid reversal of acute circulatory failure or any shock state have been shown to influence outcomes in critically ill patients. A recent study in pediatrics revealed that the predominant factor that reduces mortality and neurologic morbidity in children transported to tertiary care pediatric hospitals is the reversal of shock through early recognition and resuscitation in the referring emergency department.
Detailed Investigation
In children with circulatory instability early after surgery for congenital heart disease or in those where the cause of this is unclear, electrocardiography (seeking out arrhythmias) echocardiography, cardiac catheterization, or computed tomographic angiography (investigating cardiac function and residual lesions) may be necessary for the identification of the underlying cause.
Therapeutic Targets
One of the fundamental tenets of the management of acute circulatory failure is to restore the systemic oxygen balance by manipulating one or more of the following:
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Systolic function
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Diastolic function
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Preload
- ■
Afterload
- ■
Oxygen demand
- ■
Cardiopulmonary interactions
Therapeutic Tools
A variety of tools are available for the treatment of acute circulatory failure in children. The remainder of this chapter addresses how these essential hemodynamic tools can be applied to infants and children with acute circulatory failure in the ICU.
Ventilation
In addition to its primary function, which is to maintain gas exchange, ventilation is an important hemodynamic tool in children with cardiac disease, and can be used to optimize the systemic perfusion. Cardiopulmonary interactions describe the interplay between spontaneous or mechanical ventilation and the cardiovascular system. These interactions differ greatly in health and disease. In addition, unique interactions are present in children with cardiac disease. The application of mechanical ventilation in children with acute circulatory failure requires an understanding of the underlying diagnosis, physiology, and how cardiopulmonary interactions may be tailored for an individual.
Cardiopulmonary Interactions in the Healthy Circulation
In the healthy circulation, the fall in intrathoracic pressure during spontaneous inspiration is associated with an increase in cardiac output secondary to increased right ventricular (RV) preload. Conversely, positive pressure ventilation produced a reduction in venous return and right heart filling, resulting in a small reduction in cardiac output that was proportional to the mean airway pressure ( Fig. 64.1 ).
The effects of ventilation are not confined to the preload of the right heart. Positive pressure ventilation can also impede the emptying of the right heart through its effects on pulmonary vascular resistance and may also reduce left ventricular (LV) afterload through a reduction in transmural LV pressure. Although these hemodynamic effects are of minimal importance in the healthy individual, in the presence of circulatory instability, cardiopulmonary interactions become much more relevant in both the development of the problem and in its treatment.
Cardiopulmonary Interactions in Children With Systolic Ventricular Dysfunction
Positive pressure ventilation decreases work of breathing and RV filling and reduces LV afterload. These cardiopulmonary interactions can be beneficial in patients with impaired systolic ventricular function. Similar hemodynamic effects and improved exercise tolerance were demonstrated in patients with decompensated heart failure treated with noninvasive positive pressure ventilation.
Due to these benefits, positive pressure ventilation should be considered as a form of hemodynamic support for children with LV systolic dysfunction, particularly early after cardiac surgery. Common postoperative examples where infants may benefit from the reduction in RV preload, LV afterload, and work of breathing obtained by positive pressure ventilation include the arterial switch operation, reimplantation of an anomalous left coronary artery from the PA, or relief of LV obstruction. These infants, and others with significant systolic dysfunction, may also benefit from a period of continuous positive airway pressure as ongoing hemodynamic support following extubation. In addition, children with acute myocardial dysfunction secondary to sepsis or myocarditis may also benefit from invasive or noninvasive positive pressure ventilation.
Cardiopulmonary Interactions in Children With Abnormalities of Diastolic Function
A low cardiac output state in the presence of normal systolic ventricular function can complicate the early postoperative period of infants and children after right heart surgery, where the pulmonary blood flow is critically related to the intrathoracic pressure. For instance, positive pressure in Fontan patients can impede pulmonary flow, and extubation has been associated with clinical improvement. Similarly, a subgroup of patients early after repair of tetralogy of Fallot have a reduced cardiac output secondary to restrictive RV physiology, where their cardiac output is dependent on diastolic forward pulmonary flow. The cardiac output of these patients and those after the Fontan operation is augmented by negative pressure ventilation, as this mimics spontaneous breathing. In practice, preemptive or early proactive circulatory management should include the use of low ventilatory pressures and early extubation when possible.
Cardiopulmonary Interactions in the Functionally Univentricular Circulation After Stage 1 Palliation
The maintenance of a stable pulmonary vascular resistance is important for the early optimization of these infants and can be greatly influenced by ventilation. Seemingly, minor increases in pulmonary blood flow secondary to alkalosis, or excess inspired oxygen can compromise systemic blood flow. A time of particularly high risk is immediately after birth, when there may be a temptation to resuscitate these infants with high levels of inspired oxygen, or hyperventilation. Hyperoxygenation and respiratory alkalosis can be detrimental to these patients and can precipitate metabolic acidosis and acute circulatory failure secondary to impaired systemic oxygen delivery. This is an important factor that differentiates the resuscitation of infants with a prenatal diagnosis, from those without, in whom high levels of supplemental oxygen are more likely to be administered.
In infants with a functionally univentricular circulation who are hemodynamically unstable in the preoperative period, conservative levels of positive pressure ventilation can be used to control pulmonary flow. Ventilation using high airway pressures or slow rates is no longer used deliberately to induce respiratory acidosis and pulmonary vasoconstriction, as acidosis is not advantageous to these infants. Instead, mechanical ventilation stabilizes the pulmonary resistance, and in turn this helps optimize the systemic perfusion.
The use of supplemental inspired carbon dioxide to create a mild respiratory acidosis, or nitrogen to create hypoxic inspired gas, delivered directly with the ventilator gases, has been investigated as a tool to control pulmonary blood flow and enhance systemic oxygen delivery in infants undergoing stage 1 palliation for hypoplastic left heart syndrome. While the addition of 3% CO 2 (but not nitrogen) has been shown to improve systemic oxygen delivery, it is fair to say that the use of supplemental gases is almost if not completely obsolete in the current era.
Summary
Children with cardiac disease have complex and diverse cardiopulmonary interactions. Ventilation should be tailored individually to manipulate hemodynamic performance depending on the patient’s underlying diagnosis, type of surgery, and associated myocardial function ( Table 64.2 ).
| Nature of Cardiac Failure | Key Considerations | SPONTANEOUS RESPIRATION | POSITIVE PRESSURE VENTILATION/CPAP | ||
|---|---|---|---|---|---|
| Cardiopulmonary Features | Hemodynamic Effect | Cardiopulmonary Features | Hemodynamic Effect | ||
| Systolic cardiac failure (postoperatively, myocarditis) | Increased LV afterload Systolic LV Dysfunction | Increased work of breathing Exaggerated negative intrapleural pressure | Increased LV afterload Jeopardizes the systemic delivery of oxygen | Reduced work of breathing Obliterates negative swings in pleural pressure | Reduced venous return Reduced LV afterload Improved LV function |
| Postoperative tetralogy of Fallot | Good systolic function Diastolic RV dysfunction Preload dependent | Increased RV preload Improved diastolic pulmonary artery flow | Improved cardiac output | Reduced RV preload Reduced diastolic pulmonary artery flow | Reduced cardiac output |
| Postoperative Fontan | Good systolic function Preload dependent Cardiac output depends on pulmonary blood flow | Increased preload | Improved pulmonary flow and cardiac output | Reduced preload Reduced pulmonary blood flow | Reduced cardiac output |
| Duct-dependent systemic flow | Excessive pulmonary flow leading to reduced systemic flow Control difficult if infant is spontaneously breathing | Respiratory alkalosis and oversaturation often associated with low pulmonary vascular resistance | May result in excessive pulmonary flow, reduced systemic delivery of oxygen | Better control of pulmonary flow, pH, and pulmonary resistance | Improved systemic cardiac output |
Cardiovascular Drugs
A better understanding of the pathophysiology and hemodynamic manifestations of circulatory failure in children has resulted in a shift away from therapy using pure inotropes aimed at improving contractility, to measures that also focus on the peripheral vasculature and the interactions between the periphery and the myocardium. Current approaches are aimed at optimizing afterload and manipulating contractility with careful, not excessive, inotropic therapy, while avoiding any unwanted increases in vascular resistance or myocardial oxygen consumption.
Drug therapies for acute circulatory failure are generally categorized according to their pharmacologic actions and also by their physiologic effects. The classes of drugs most commonly used to treat acute circulatory failure in children are catecholamines and phosphodiesterase-3 inhibitors. In addition, a number of other drugs that influence cardiovascular function through very different mechanisms including sensitization to intracellular calcium, and neurohormonal effects, have also become available for clinical use in children.
It is important to consider some unique pharmacodynamics and pharmacokinetic factors when approaching treatment of acute circulatory failure in children. In addition, a comprehensive list of frequently used vasoactive medications in the management of acute circulatory failure is presented ( Table 64.3 ).
| Drug | RECEPTOR ACTIVITY | IV Infusion Dose | Receptor Effect | Organ Effect | Potential Side Effects | |||
|---|---|---|---|---|---|---|---|---|
| α-1 | β-1 | β-2 | Dopamine | |||||
| Dopamine | −+ | − | − | ++ | 1–5 µg/kg/min | D 1 -like (D 1 -D 5 ): ↓ sensitivity to postsynaptic intracellular Ca 2+ D 2 -like (D 2-4 ): ↓ norepinephrine release from nerve terminal | ↑ vascular tone ↑ contractility ↑ HR | Arrhythmias Tachycardia |
| ++ | ++ | + | ++ | 5–10 µg/kg/min | ||||
| ++ | + | ++ | 10–20 µg/kg/min | |||||
| Epinephrine | −+ | ++ | + | 0 | 0.01–0.03 µg/kg/min 0.03–0.1 µg/kg/min > 0.1 µg/kg/min | α-1: ↑ Ca 2+ influx to postsynaptic cell receptor β-1, β-2: ↑ intracellular cAMP | Systemic vasoconstriction ↑ contractility ↑ LV afterload | Arrhythmias Hypertension Hyperglycemia |
| ++ | ++ | + | 0 | |||||
| ++ | + | 0 | ||||||
| Levosimendan | Calcium sensitizer | Loading 6–12 µg/kg for 10 min Continuous 0.05–0.1 µg/kg/min for 24 to 48 h | Opening of ATP-dependent mitochondrial K + channels in vascular smooth muscle | Systemic vasodilation Coronary vasodilation ↑ inotropy ↑ contractility ↓ LV afterload | Arrhythmias Hypotension Hypokalemia | |||
| Milrinone | Type III phosphodiesterase inhibitor | 0.25–1 µg/kg/min | Inhibition of intracellular hydrolysis of 3′5′ cAMP | Systemic vasodilation ↑ inotropy ↑ lusotropy | Hypotension Renal accumulation | |||
| Nesiritide | Recombinant B-type natriuretic peptide | 0.01–0.02 µg/kg/min | ↑ cGMP in endothelial and vascular smooth muscle cells | Systemic vasodilation Coronary vasodilation ↑ GFR ↓ Na + reabsorption ↑ diuresis | Bradycardia Hypotension | |||
| Norepinephrine | ++ | ++ | − | 0 | 0.01–1 µg/kg/min | α-1: ↑ Ca 2+ influx to postsynaptic cell receptor | Systemic vasoconstriction ↑ LV afterload | Arrhythmias Hypertension |
| Phenylephrine | ++ | 0 | 0 | 0 | 0.15–0.75 µg/kg/min | α-1: ↑ Ca 2+ influx to postsynaptic cell receptor | ↑ vascular tone ↑ LV afterload | Bradycardia Hypertension |
| Vasopressin | V1 receptor agonist | 0.01–0.1 µg/kg/min | IP 3 signal transduction in the vascular smooth muscle | Systemic vasoconstriction ↑ LV afterload | Hypertension | |||
Maturational Influences
The neonatal myocardium differs significantly from the more mature heart in its innervation and contractile reserve. The neonatal heart is less densely supplied with sympathetic nerve terminals than older infants and adults, resulting in reduced myocardial effects and less reuptake of catecholamines. This latter factor may also predispose to the neonatal cardiotoxicity of catecholamines as previously described. The newborn myocardium is more sensitive to changes in intracellular calcium compared to the more mature heart.
Pulmonary Vasculature
Pulmonary hypertension, or lability of the pulmonary vascular resistance, is commonly encountered in newborns and infants with heart disease. Changes in pulmonary vascular tone can play a role in the development of acute circulatory failure in some patients. Patients at increased risk of pulmonary hypertension include those with structural heart disease, resulting in excessive pulmonary blood flow, pulmonary venous hypertension, or a functionally univentricular circulation. Pulmonary vascular instability can further deteriorate in the newborn transitional circulation and by cardiac surgery and cardiopulmonary bypass, which disturbs the balance between endogenous pulmonary vasodilators and constrictors.
Complex Circulations
Careful control of vascular tone is a prerequisite for the circulatory management of patients with more complex congenital heart lesions, in particular those with a functionally univentricular heart. In these patients, sudden changes in pulmonary or systemic vascular resistance can immediately impact on the systemic oxygen delivery and can rapidly precipitate into acute circulatory failure. Moreover, a stable pulmonary vascular resistance and an appropriately dilated systemic vasculature are highly desirable.
The presence of complex congenital cardiac disease can also impact the responsiveness of the myocardium to exogenous agents. Sympathetic dysregulation is most marked in newborns and young infants with cyanotic or critical acyanotic heart disease. In these patients, reduction of the density of β-adrenoreceptors is associated with elevated endogenous levels of noradrenaline and a partial uncoupling of the receptor to adenylate cyclase. As a result, the myocardium may be less responsive to β-adrenergic stimulation.
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