Increasingly, more patients with congenital heart disease (CHD), including those with more complex disease, survive into adulthood. They present to critical care physicians by virtue of their underlying cardiac disease, following surgical or cardiologic intervention to replace failing valves and conduits or with unrelated reasons such as pregnancy or surgery for noncardiac conditions. Moreover, CHD is associated with a range of other noncardiac pathologies ( Table 16.1 ).
| System | Associations |
|---|---|
| Respiratory | Congenital pulmonary lesions (eg, hypoplastic lung) Musculoskeletal abnormalities Phrenic nerve palsies following cardiac surgery Hemoptysis secondary to aortopulmonary collaterals, pulmonary embolisms etc. |
| Renal | Glomerulosclerosis and renal dysfunction Proteinuria Hyperuricemia Congenital abnormalities of the renal tract |
| Gastrointestinal | Asplenia Congenital abnormalities of the gastrointestinal tract Liver dysfunction Protein-losing enteropathy (in Fontan circulation) |
| Endocrine | Thyroid dysfunction |
| Neurologic | Cerebral abscesses |
The classification adapted from the Canadian Consensus Document provides a useful guideline concerning the degree of support critical care teams will require from cardiologists specializing in adult congenital heart disease (ACHD), imaging specialists, electrophysiologists, and cardiac surgeons. Thus, patients with mild or surgically repaired lesions such as a bicuspid aortic valve or ligated patent ductus arteriosus often pose few additional problems on the critical care unit aside from considerations of prophylaxis for infective endocarditis or complications following previous surgery. By contrast, patients with complex (eg, cyanotic disease, univentricular circulation) and moderate disease (eg, tetralogy of Fallot, Ebstein anomaly) may require considerable resources in terms of specialist cardiac opinion and intervention, but also clinical specialties such as gastroenterology, rehabilitation, and nephrology. The critical care physician is often key in coordinating these opinions and balancing the risks and benefits of proposed interventions.
In this chapter, general considerations are presented for critical care physicians caring for patients with moderate or severe CHD. It also considers the relevance of some topical concepts in the general critical care arena such as rehabilitation after critical illness, the role of extracorporeal membrane oxygenation (ECMO) support, and delirium. The consequences of specific anatomic arrangements and pregnancy are considered elsewhere in this book.
Acute Cardiovascular Management of Oxygen Delivery
Much consideration is given to maintaining sufficient oxygen delivery to the organs of the body to prevent ischemic organ dysfunction. There is a complex balance between cardiac output, oxygen saturations, hemoglobin levels, the affinity of hemoglobin for oxygen, systemic arterial pressure, and systemic venous pressure. The latter is often overlooked, although systemic venous hypertension in combination with a low cardiac output can be particularly damaging to end organs. All of these parameters are easy to measure apart from cardiac output. In CHD there may be anatomic considerations that limit certain techniques ( Table 16.2 ). Cardiac output may be manipulated through fluid therapy, vasoactive drugs, management of pulmonary vascular resistance (PVR), pacing, or mechanical support.
| Technique | Comments |
|---|---|
| Fick principle | Oxygen consumption is difficult to measure in the clinical setting. The Fick technique measures transpulmonary flow assuming that there is no intrapulmonary shunting. |
| Pulmonary artery thermodilution | Pulmonary artery catheter is not possible to place if there is no subpulmonary ventricle. This technique measures flow through the subpulmonary ventricle but it is less accurate if there is severe tricuspid regurgitation. |
| Transpulmonary dilution | Indicators that can be used are lithium (LIDCO) or thermal (PICCO). They measure flow through the entire heart presuming minimal intracardiac shunt. |
| Esophageal Doppler interrogation | Measures flow in descending aorta and estimates cardiac output based on nomograms of aortic size according to the patient’s height and weight. It may not be possible to obtain a Doppler signal if the aorta is right sided. The nomograms may be inaccurate in congenital heart disease. |
| Fick partial rebreathing | Carbon dioxide production is difficult to measure in the clinical setting. Transpulmonary flow is measured assuming there is no intrapulmonary shunting. |
| Pulse contour analysis (calibrated) | Variable reports about the accuracy of the pulse contour algorithms in patients with congenital heart disease |
| Pulse contour analysis (uncalibrated) | The accuracy of these devices is low even in normal circulations, particularly in low cardiac output settings. |
| Echocardiography | Provides excellent physiologic and anatomic data at a specific time point but is less useful for real-time titration of therapy. The accuracy is very dependent on the skill of the operator. |
Cardiac Anatomy
An appreciation of the patient’s cardiac anatomy is paramount. When patients present following cardiac surgery, their cardiac anatomy will have been well defined preoperatively with a combination of echocardiography, cardiac catheterization, and magnetic resonance imaging. However, the fallibility of these investigations is reflected by the occasional conflicts with observations made during surgery. In the setting of an emergency admission or a patient who has been lost to follow-up, the anatomy may be less well defined. It is important to gather data from the patient, next of kin, and other institutions that have cared for the patient previously. Echocardiography in complex CHD may be very difficult and requires clinicians with extensive experience in this setting. Key questions to attempt to answer are the presence of abnormal shunts, the nature of the systemic and subpulmonary ventricles, and previous palliative or corrective procedures that have been undertaken.
Detailed anatomic knowledge helps physicians predict the effects of interventions such as increasing systemic vascular tone and increasing heart rate. It is often necessary to compromise certain physiologic targets (eg, the systemic saturations) to achieve other targets (eg, sufficient cardiac output, systemic pressure). The goal of hemodynamic manipulations is to achieve just enough oxygen delivery to end organs such as the kidney or the brain to prevent organ damage. Frequently, this must be accomplished with parameters that are different from those in patients without CHD. Because patients with CHD tend to be younger and have less atherosclerotic disease, they often tolerate moderate hypotension better.
Fluid Therapy
Fluids can be administered to increase systemic preload to the right side of the heart. Initially, fluid expansion will improve right-sided heart function, particularly if it has restrictive physiology, although this will be at the expense of systemic venous hypertension. However, excessive fluid administration may result in ventricular dilation and a reduction in cardiac output. This is particularly true in patients who have had cardiac surgery and whose hearts are not acutely constrained within a pericardial sac. Fluids should be titrated to markers of cardiac output (direct or indirect, such as clinical examination, metabolic status, urine output). Patients who have a Fontan-type circulation are particularly dependent on adequate venous filling to ensure transpulmonary blood flow in the absence of a subpulmonary ventricle. There are few data to support the use of a specific colloid or crystalloid solution. Synthetic colloids are increasingly less favored. A large, multicenter, high-quality randomized trial of hydroxyethyl starch versus saline in critically ill patients demonstrated a higher incidence of renal replacement in patients receiving starch for fluid resuscitation. In another study, the use of synthetic gelatins was temporally associated with increased renal replacement following cardiac surgery. In both studies, the use of colloids was only associated with a small reduction in the total volume of fluids administered to patients. Human albumin solution did not improve mortality in comparison to saline in a large, high-quality, randomized trial performed in a heterogeneous population of critically ill patients. Therefore, the use of albumin is hard to justify in the context of its higher cost. It is possible that selected populations, such as patients with severe sepsis, may benefit from the use of albumin solutions in fluid resuscitation, but this has not been proven. The role of albumin’s wide range of noncolloid effects in these patient groups is actively investigated.
Blood Transfusion
Anemia is common in critically ill patients. Blood is often administered in an attempt to increase oxygen delivery. Moreover, cyanotic patients have increased red cell mass at baseline. This is one part of their adaptation to chronic hypoxemia and is triggered by increased erythropoietin production in the kidney. The increase in red cell numbers is termed correctly as a secondary erythrocytosis, in contrast to a polycythemia, which relates to increases in more than one hematologic lineage. Patients are frequently iron deficient because of consumption by erythropoiesis, inappropriate phlebotomy, gastrointestinal losses, or poor dietary intake.
The hemoglobin threshold that should trigger transfusion is unclear. A large study in critically ill patients demonstrated that targeting hemoglobin concentrations to 70 to 90 g/L was as effective and perhaps superior to higher targets. This study excluded patients who had undergone cardiac surgery and most likely patients with cyanotic heart disease because the hemoglobin level had to fall below 90 g/L within 48 hours of admission to the intensive care unit. Nevertheless, more restrictive targets limit the exposure of the detrimental effects of transfusion that may increase morbidity and even mortality. Transfused red cells are immunosuppressive; have poorer rheologic properties, which reduce microvascular flow; and are depleted in 2,3-diphosphoglycerate, which impairs their oxygen-carrying capacity for some days. Targets should be customized to the patient. Typically, a threshold of 70 to 90 g/L is used in currently noncyanotic patients who do not have acute coronary ischemia. Cyanotic patients need higher hemoglobin levels, but the exact levels are hard to estimate. Transfusion is better titrated to markers that suggest oxygen delivery is insufficient (eg, low venous saturations or poor end organ function) despite optimization of arterial oxygen saturations and cardiac output.
Inotropes and Vasoconstrictors
Inotropes are used to increase cardiac contractility in low-output states. Catecholamines such as epinephrine, dobutamine, and dopamine are agonists at β-adrenergic and dopaminergic receptors. Although they may increase the force of contraction by increasing intramyocyte calcium levels, this is often at the expense of an increased heart rate, increased myocardial oxygen consumption, and impaired relaxation of the heart during diastole. Epinephrine, per se, can induce hyperlactatemia, which may complicate the interpretation of systemic acid-base balance. There are potential advantages to phosphodiesterase inhibitors, such as milrinone and enoximone, and the newer calcium sensitizer levosimendan, in patients with significant right ventricular dysfunction.
Milrinone, when compared with dobutamine, causes less tachycardia, more pulmonary and systemic vasodilation, more lusitropy, and causes a lesser increase in myocardial oxygen consumption. Because the morphologic right ventricle is so susceptible to afterload changes, the vasodilating properties are advantageous, even in the setting of needing some vasoconstrictor to maintain systemic pressures.
Levosimendan acts by sensitizing cardiac troponin C for calcium during systole. Because intracellular calcium levels are not elevated, there is a lesser increase in myocardial oxygen consumption and better lusitropy. Experimental data suggest it may also be a pulmonary vasodilator. This appears to be borne out clinically and makes it an attractive agent in patients with right ventricular failure and those with pulmonary hypertension. It has been used successfully in pediatric patients with CHD.
Norepinephrine is an α-adrenergic agonist that is a systemic and pulmonary vasoconstrictor. It is administered in vasodilated states to restore systemic vascular resistance (SVR) and mean arterial pressure to ensure adequate organ perfusion. Although autoregulation maintains perfusion of organs during hypotension, there is a threshold at which this fails, and administration of vasoconstrictors will restore organ perfusion and function such as urine output. Vasodilation is common in critically ill patients because of sepsis, systemic inflammation postoperatively, and the administration of vasodilating drugs such as milrinone and levosimendan.
Arginine vasopressin (up to 0.04 IU/hour) is an alternative systemic vasoconstrictor to norepinephrine. It acts at vasopressinergic (V1) receptors and may be vasodilating at low doses in the pulmonary circulation through a nitric oxide–dependent mechanism. This may manifest clinically as a reduction in the PVR/SVR ratio. It has been used successfully in severe sepsis and safely in patients with pulmonary hypertension. These data provide a rationale for selecting arginine vasopressin above norepinephrine in settings of pulmonary hypertension and systemic vasodilation.
Management of the Pulmonary Vascular Resistance
Management of the PVR is often the cornerstone to the care of patients with complex CHD. In patients with a failing subpulmonary ventricle, reduction in the afterload presented by the pulmonary circulation may improve cardiac output; morphologic right ventricles (the usual scenario) are particularly susceptible to failure with acute increases in the PVR. The balance between pulmonary and systemic blood flow in patients with unrestricted shunting is influenced by the balance between PVR and SVR. Thus, in high PVR–low SVR settings, systemic cardiac output will increase at the expense of decreased pulmonary flow, greater venous admixture, and systemic desaturation. The converse will occur in low PVR–high SVR settings.
Inhaled Nitric Oxide
Inhaled nitric oxide forms a mainstay of acute therapy in many institutions. It increases smooth muscle cyclic guanosine monophosphate, thereby causing arteriolar vasodilation. Because nitric oxide is inhaled and has a short half-life, its effects are generally limited to the pulmonary circulation. Administration of inhaled nitric oxide may profoundly drop the PVR. It is important to administer it properly. In general, it is delivered using an injector system that is attached to the mechanical ventilator. Doses used in clinical practice range from 0 to 40 ppm. It is clear that ever-increasing doses of nitric oxide do not increase pulmonary vasodilation further and may exacerbate the situation. Moreover, data from 20 patients with elevated pulmonary artery pressures as a result of acute respiratory failure suggest that the dose response to inhaled nitric oxide changes over time and may result in a situation in which a previously efficacious dose is ineffectual. Thus, inhaled nitric oxide therapy should be titrated at least every 48 hours, targeting a clear physiologic goal such as cardiac output. Despite concerns about the generation of nitrogen dioxide and methemoglobin, in practice, this seems to be unusual.
Prostacyclin Analogues
Epoprostenol, iloprost, and treprostinil are prostacyclin analogues that differ primarily in their stability and half-lives. All can be administered by continuous infusions, although only epoprostenol is licensed for this in pulmonary hypertension. Although they are pulmonary vasodilators and inhibit platelet aggregation, their use in this form in critically ill patients can be limited by the concomitant systemic vasodilation. By contrast, epoprostenol and iloprost can be nebulized with fewer systemic effects. The longer half-life of iloprost means that it does not need to be nebulized continuously. All have been associated with rebound pulmonary hypertension on withdrawal.
Phosphodiesterase Inhibitors
Sildenafil is a phosphodiesterase-5 inhibitor that causes vasodilation by increasing intracellular cyclic guanosine monophosphate levels. Because phosphodiesterase-5 is particularly abundant in pulmonary vascular tissues and the corpus cavernosum, the vasodilation is relatively selective to these tissues beds. It has been used with great success in patients with chronic pulmonary hypertension, increasing exercise capacity and improving hemodynamics, symptoms, and longevity. Sildenafil may be additive with inhaled nitric oxide, prostacyclin analogues, and bosentan.
In general, sildenafil is only available as an oral preparation. It has been used acutely in critically ill patients. An intravenous form is available on a compassionate basis from the manufacturer and is used at rates of 2 to 16 mg/hour. Intravenous sildenafil can be associated with profound systemic vasodilation, particularly when there is concomitant use of inhaled nitric oxide.
Other Factors Influencing Pulmonary Vascular Resistance in Critically III Patients
Pulmonary hypertension may be exacerbated by hypercapnia, acidemia, hypoxemia, and pain. With respect to analgesia, fentanyl and thoracic and lumbar epidural analgesia do not affect pulmonary vascular tone.
Mechanical ventilation also increases PVR because elevated airway pressures compress perialveolar capillaries. The relationship between positive end-expiratory pressure (PEEP)/lung inflation and PVR is U-shaped, such that low levels of PEEP reduce PVR by recruiting collapsed areas of lung, but as PEEP increases further, PVR increases. PEEP must therefore be titrated so that PVR is not unduly increased while maintaining lung volumes and preventing hypoxemia. Spontaneous ventilation minimizes airway pressures and is the favored mode of ventilation if possible.
Sedation and neuromuscular blockade are used to facilitate mechanical ventilation on intensive care units. Propofol and midazolam do not increase PVR and have been used safely in patients with pulmonary hypertension. Another study in patients undergoing coronary artery surgery demonstrated that PVR was not changed by atracurium, vecuronium, or pancuronium.
Endothelin Receptor Antagonists
Bosentan is a competitive antagonist of endothelin A and B receptors. It has become an important drug in the treatment of pulmonary hypertension, in which endothelin-1 has been implicated in pulmonary vasoconstriction per se and the proliferation of vascular smooth muscle cells, which results in the remodeling of pulmonary arterioles. It increases exercise capacity and hemodynamics and slows disease progression in chronic pulmonary hypertension. It improves exercise capacity and hemodynamics in patients with Eisenmenger syndrome. Longevity is improved when bosentan is used as part of a package of pulmonary hypertension management in these patients.
There are few data about the acute use of bosentan or other more selective endothelin antagonists in critically ill patients. Bosentan’s usefulness is limited by its availability only as an oral preparation and that it causes an idiosyncratic hepatic transaminitis. It is therefore not recommended in patients who also have Child-Pugh class C cirrhosis. It is a known teratogen.
Atrial Fenestration
Occasionally, when pulmonary hypertension is refractory to medical intervention resulting in a low cardiac output state that threatens organ perfusion, an atrial septostomy will allow shunting of desaturated blood to the left side of the heart and an increase in cardiac output, albeit at the expense of systemic desaturation. This intervention can be lifesaving but may fail in the setting of significant left ventricular dysfunction when elevated left atrial pressures will reduce the degree of shunt. Sometimes fenestration will be undertaken at the time of cardiac surgery to allow shunting if pulmonary hypertension occurs. Fenestrations may be closed at a later date, if appropriate, usually by a percutaneous approach.
Mechanical Support
Temporary mechanical support can bridge a patient through the temporary cardiac dysfunction that may occur after cardiac surgery. This temporary dysfunction may be a result of relative ischemia during the application of the surgical cross clamp, to myocardial dysfunction associated with postoperative systemic inflammation, or to perioperative elevation of PVR. Mechanical support may be indicated to bridge a patient through a period of malignant arrhythmia when an ablation procedure is undertaken.
Intraaortic balloon counterpulsation lowers the afterload of the systemic ventricle and improves coronary blood flow. These effects are less significant in younger patients with more elastic aortas. Balloon counterpulsation does not improve mortality in adults with cardiogenic shock following acute myocardial infarction ; there are no trial data in patients with adult congenital heart disease. A pragmatic approach or assessing whether a key physiological goal is achieved by balloon counterpulsation is probably best.
Venoarterial ECMO can provide a full systemic cardiac output to patients with no cardiac function. It is associated with significant complications such as bleeding (including cerebral hemorrhage) and vascular damage. It is not a light undertaking in patients with ACHD, and cannulation may be complex. The pattern of venous drainage is important. For example, patients with left isomerism have 25% of their cardiac output returning from the hepatic and portal veins via a route other than the vena cavae. Peripheral arterial cannulation may be difficult in patients with small femoral vessels that have been used for surgical and interventional procedures previously. Central cannulation may be challenging in patients with multiple previous sternotomies. Persistent Blalock-Taussig shunts and aortopulmonary collaterals may steal blood from the systemic circulation and prevent full cardiac unloading.
Short-term ventricular support can be achieved by transvascular devices. The right heart can be supported by the Impella (Abiomed) or PROTEK Duo cannula with a Tandem Heart (Cardiac Assist). The left heart can be supported by the Impella (Abiomed), Tandem Heart (Cardiac Assist), or HeartMate PHP (Thoratec). All have specific anatomic requirements, and experience is very limited. They should only be used in centers with special clinical governance arrangements in place and extensive experience in extracorporeal support.
Medium-term and durable ventricular assist devices can be equally challenging to place anatomically. However, the suitability of patients for these devices is often limited because of pulmonary hypertension and/or the unsuitability of the patients for transplantation in the long term.
Management of Cardiac Rhythm
Arrhythmias are common postoperatively and in patients with complex lesions. Loss of atrial transport can be associated with a dramatic fall in cardiac output. Atrial arrhythmias tend to be more common because of the substantial substrate of atrial tissue and often multiple operations and scars. Atrial tachycardias are common, particularly in patients with Ebstein anomaly or Fontan circulations. They can be hard to differentiate from sinus tachycardias, but previous electrocardiograms and interrogation of any implanted pacing system may help in this respect.
Management of cardiac arrhythmias follows standard algorithms of replacing electrolytes such as potassium and magnesium, antiarrhythmics, and electrical cardioversion. Amiodarone is often the first drug of choice but can reveal other problems, such as poor sinus node function, atrioventricular/intraventricular conduction delays, and aberrant pathways. A more detailed consideration of arrhythmias is presented elsewhere in the book. Early input from specialist electrophysiologists is highly advisable for rhythm disturbances that do not respond to standard measures.
Pacing
Patients who have undergone cardiac surgery generally have an external temporary pacing system attached to epicardial pacing wires on the right atrium and one or both ventricles. Transvenous access to the heart for pacing is not possible in some cases, such as following a total cavopulmonary connection. In emergencies, pacing wires can be passed retrogradely over the aortic valve into the systemic ventricle, but this may be associated with aortic valve damage and systemic thromboembolism. Further difficulties with pacing may occur because previous and extensive surgery often makes electrical capture difficult. In some cases, patients will have permanent implanted pacing systems.
Optimization of heart rate and atrioventricular delay pacing may lead to important increases in cardiac output. Simply increasing heart rate can dramatically influence cardiac output in the acute setting. Echocardiography is often used to guide optimization. For example, patients who have restrictive physiology demonstrated on echocardiography (with ventricular filling ending in early diastole) may benefit from an increased heart rate and short atrioventricular interval. Interventricular dyssynchrony may be improved by left ventricular pacing or multisite pacing. Furthermore, biventricular pacing, or pacing with a longer atrioventricular delay, may allow the heart’s intrinsic conduction pathways to work, albeit with a degree of heart block. It is unknown and probably unlikely that conventional criteria for cardiac resynchronization therapy would apply to patients with CHD.
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