Postoperative Management

General Approach to the Postoperative Patient
Mechanical Support of the Circulation

General Approach to the Postoperative Patient

Use and Care of Monitoring Lines

Left atrial catheters provide indirect data on left ventricular function, and analysis of the pressure waves allows assessment of the function of the left-sided atrioventricular valve, information that is particularly important after repair of atrioventricular septal defect. They are also useful as a site for saline injection during echocardiography to determine the presence and location of a residual left-to-right shunt. Great care must be taken to prevent air or particulate embolization through this catheter.

Pulmonary artery catheters have multiple uses, and are usually employed when ventricular septal defects are closed, when postoperative pulmonary hypertension is likely to occur, (truncus arteriosus, atrioventricular septal defect, and obstructed total anomalous pulmonary venous return), or when cardiac output determinations are to be made. Samples for determinations of pulmonary artery oxygen saturation are drawn on the day of operation and on the first postoperative day. A falsely elevated pulmonary artery oxygen saturation occurs if the catheter is in the wedge position, therefore the location of the catheter needs to confirmed by both a chest x-ray and the pressure tracing. Simultaneous right atrial, systemic arterial, and pulmonary arterial determinations of oxygen saturation provide data for the calculation of pulmonary-to-systemic flow ratios, and hence residual ventricular septal defects can be diagnosed with reasonable accuracy in the intensive care unit.

For example, if the SaO₂ was 100%, the right atrium saturation 65%, and the pulmonary artery saturation 85%, then,

Qp:Qs = (100 - 65) / (100 - 85) : 1

Qp:Qs = 35/15 : 1= 2.3 : 1

 

In patients following repair of tetralogy of Fallot, the pulmonary artery catheter may be used to identify residual right ventricular outflow tract obstruction, by withdrawing the catheter through the right ventricular outflow tract while the pressure tracing is being recorded.

The uses of right atrial catheters are similar to those for left atrial catheters. Right ventricular and right-sided AV valve function can be assessed and right atrial oxygen saturation measured. Right atrial oxygen saturation must be interpreted with knowledge of the position of the tip of the catheter, since an elevated oxygen saturation may be obtained when the tip is positioned in the inferior vena cava below the liver (renal vein oxygen saturation may exceed 85%) and a low oxygen saturation is obtained if the catheter tip is located near the ostium of the coronary sinus. An echocardiogram performed during the injection of contrast material (or fine bubbles) through the right atrial catheter can identify right-to-left shunts, especially at the atrial level. Right atrial catheters can also be used to administer parenteral nutrition beyond the immediate postoperative period if care is taken to prevent contamination of the catheter. In neonates and small infants, right atrial catheters are preferred over other types of central venous access catheters, such as those placed through the subclavian or axillary vein, in that the latter are associated with venous thrombosis and complicated by the superior vena cava syndrome, chylothorax, and pulmonary emboli.

Transcutaneous oxygen saturation monitors are particularly useful in patients with a functional single ventricle and parallel pulmonary and systemic circulation since variations in systemic or pulmonary vascular resistance may occur rapidly and result in changes in arterial saturation.

Umbilical artery catheters are frequently employed in neonates, and are useful for preoperative management, as a site for administration of mediations and fluid, and as access for cardiac catheterization. These catheters pose a risk to the mesenteric circulation when descending aortic flow is compromised, as occurs with coarctation, IAA, or hypoplastic left heart syndrome. Umbilical artery catheters are useful as a source of central aortic blood pressure data, especially early postoperatively when peripheral vasoconstriction may be present. These catheters should no be used for more than two or three days, and should be removed earlier if the status of descending aortic blood flow remains questionable.

Postoperative Echocardiography and Angiocardiography

The completeness of the anatomic repair remains the most critical factor in determining the outcome of operation for congenital cardiac anomalies. When a significant residual anatomic abnormality is suspected, the patient must be investigated completely to assess the severity of the problem, and reoperated on to correct the residual problems if significant. Echocardiography should be freely used during the postoperative period when questions exist about the adequacy to the anatomic repair. Contrast echocardiography can aid in the diagnosis of residual shunts, and color-flow Doppler echocardiography has become increasing useful in localizing the site. Pericardial effusions that may cause tamponade several days following an operation are easily identified by echocardiography. The presence of absence of AV valve regurgitation can also be identified, although quantification may be difficult. Measurement of residual pressure gradient can be made at a number of sites in the cardiac chambers or great vessels, and obstruction in venous pathways, such as occurs after a Senning or Mustard procedure, can be reliably identified as well.

Cardiac catheterization can be used as both a diagnostic and therapeutic modality in the postoperative period. Examples of the latter include coil embolization of aortopulmonary collateral, dilation of branch pulmonary artery stenosis, and possibly closure of residual ventricular septal defects.

Management of the Cardiovascular and Pulmonary Systems

Cardiovascular and pulmonary management involves manipulation of contractility, pulmonary vascular resistance (pulmonary vascular resistance), systemic vascular resistance, preload, and cardiac rate and rhythm. The immature myocardium differs in several important ways from adult myocardium, which has implications in postoperative management. These are summarized in the following table:

Table 1: Characteristics of the immature myocardium compared to the adult

Contractility

The contractile state of the myocardium can be depressed in the early postoperative period, and especially after major repairs, is usually supported by catecholamine inotropic agents or phosphodiesterase inhibitors. The relative effects of the various medications used are summarized in the "medications" section. Notably, the immature myocardium is more resistant to the effects of the catecholamine inotropic agents. This is perhaps due to the incomplete development of the sympathetic nervous system at birth. While there is a somewhat attenuated responsiveness to these inotropic agents, the immature myocardium nevertheless responds to these agents, albeit sometimes at a slightly higher dose.

Pulmonary Vascular Resistance

Patients at increased risk of developing problems with pulmonary vascular resistance are typically those with a single functional ventricle and pulmonary and systemic circulation in series, and neonates undergoing repair of truncus arteriosus, atrioventricular septal defect, ventricular septal defect, obstructed total anomalous pulmonary venous connection, or hypoplastic left heart syndrome.

Physiologic manipulations

Several physiologic and pharmacologic variables experimentally affect pulmonary vascular resistance (pulmonary vascular resistance). The most clinically useful physiologic variables are inspired oxygen concentration, arterial carbon dioxide concentration, hydrogen ion concentration, the level of sympathetic nerve activity, and mean airway pressure. There is extensive muscularity of the pulmonary resistance vessels in fetal life, which shows greater responsiveness to stimuli such as hypoxia than do adults. The degree of muscularization of intraacinar pulmonary arteries correlates strongly with the level of preoperative pulmonary arterial pressure, and thus the potential for elevated postoperative pulmonary vascular resistance can be anticipated to a certain extent.

Inspired Oxygen Concentration. There is a vasoconstricting response of the pulmonary vasculature to hypoxia (not necessarily hypoxemia) which has been known about for many years. This hypoxic vasoconstricting response of the pulmonary vasculature is widely believed to play an integral role in matching regional perfusion to ventilation. Conversely, although no clinical studies have documented that elevated inspired oxygen concentration consistently reduces pulmonary vascular resistance in the intensive care unit setting, the clinical impression is that increasing FiO₂ can reduce pulmonary vascular resistance in patients with elevated resistance due to increased muscularization of the pulmonary vasculature, as in patients after repair of truncus arteriosus, atrioventricular septal defect, ventricular septal defect, obstructed total anomalous pulmonary venous connection, or hypoplastic left heart syndrome.

Carbon Dioxide and H+. The effect of CO₂ seems primarily to result from changes in H+ concentration, since CO₂ seems to have direct smooth muscle relaxing effect. Acidosis causes pulmonary vasoconstriction under normoxic condition, and has a potentiating effect on hypoxic pulmonary vasoconstriction. The effects of alkalosis are the opposite, and attenuate hypoxic vasoconstriction and produce vasodilatation under certain conditions. Therefore, patients with increased pulmonary vascular resistance are hyperventilated to a pH of ~7.50 or PaCO₂ of 25 - 30 torr, while the PaCO₂ is allowed to rise above 40 torr if pulmonary vascular resistance needs to be increased.

Sympathetic Nervous System. Sympathetic nerve stimulation increases pulmonary vascular resistance, and is responsible for the reflex sympathetic pulmonary vasoconstriction that follows distention of the main pulmonary artery. This latter reflex is particularly strong in the muscularized resistance vessels of the pulmonary circulation of an infant when compared to adults, and may have significant pathophysiological implications in pulmonary hypertensive crises. Studies in patients following cardiac operations show that fentanyl (Sublimaze) blunts the pulmonary vasoconstrictor response to the stress of endotracheal suctioning, probably by blocking the sympathetic response at the central nervous system. Hence patients at risk of developing pulmonary hypertensive crises may be placed on a continuous intravenous fentanyl infusion. Muscle paralysis must be used simultaneously due to the thoracic wall rigidity due to fentanyl.

Mechanical Factors. Pulmonary vascular resistance is generally lowest at functional residual capacity, and the hypoxic pulmonary vasoconstrictive response is reduced with physiologic degrees of lung inflation. Below the functional residual capacity, pulmonary vascular resistance increases as less alveoli are recruited and there is a subsequent ventilation-perfusion mismatch, while above the functional residual capacity pulmonary vascular resistance increases as alveolar pressure exceeds pulmonary capillary pressure. Therefore, low levels of end-expiratory pressure may reduce pulmonary vascular resistance if local areas of atelectasis are expanded and the associated portions of the pulmonary vascular bed are recruited, while sufficiently raising end-expiratory pressure may lead to an increase in pulmonary vascular resistance in patients with increased pulmonary blood flow.

Pharmacologic Interventions

No selective pulmonary vasodilating or vasoconstricting agent has yet been identified.

Nitric oxide

Adrenergic Agonists. Isoproterenol has a mild vasodilating action in the normal pulmonary circulation. Dopamine in low doses (< 10 m g/kg/min) does not appear to induce significant changes in pulmonary vascular resistance, but occasionally is associated with a deleterious effect at higher doses. Dobutamine is thought to have effects similar to isoproterenol, and is a mild pulmonary vasodilator in adult patients. Reports on epinephrine effects on the pulmonary vasculature are varied, but some studies in man have demonstrated a vasoconstrictor activity. Amrinone

Smooth Muscle Relaxants. Nitroglycerin, nitroprusside, prostaglandins, tolazoline, calcium channel blockers, and amrinone have relatively nonspecific vasodilatory properties. Nitric oxide, administered intratracheal. holds some promise in reversible pulmonary arterial hypertension.

Table 2: Treatment of Pulmonary Hypertension

Systemic Vascular Resistance

Shunt dependent pulmonary circulation

The immature myocardium is clearly vulnerable to ischemia, and depressed ventricular function resulting in reduced cardiac output is a significant and real factor in postoperative morbidity and mortality. In addition to improving myocardial performance primarily, however, it is also useful to manipulate the systemic vascular resistance after operative palliation of functional single ventricle where there is shunt dependent pulmonary circulation and the pulmonary and systemic blood flows are in parallel. Ideally, the patient should have a Qp:Qs of approximately 2:1, yielding an arterial saturation of 80 - 85%. Two cases of unbalanced Qp:Qs exist:

When Qp:Qs is high, pulmonary blood flow is excessive and systemic perfusion tends to be inadequate. The former may result in pulmonary edema and eventual hypoxemia, while the latter results in hypotension, acidosis, and reduced mixed venous O₂. Maneuvers to increase pulmonary vascular resistance include reducing FiO₂, decreasing minute ventilation to increase PaCO₂ and lower pH, and increasing end-expiratory pressure by increasing Vt or PEEP. Maneuvers to lower systemic vascular resistance include addition of vasodilator agents and warming surface lights. The ongoing presence or addition of vasopressors may be counterproductive, since elevation of systemic vascular resistance will further increase Qp:Qs.

When Qp:Qs is low, pulmonary blood flow is inadequate and the SaO₂ drops below 70%. Measures to decrease pulmonary vascular resistance (increase pH by hyperventilation and sodium bicarbonate infusion, increase FiO₂, lower mean airway pressure) and raise systemic vascular resistance (calcium gluconate, and maintaining or starting inotropic vasopressors) are undertaken.

Cardiac Rate

Since the beneficial effects of augmenting preload and increasing ejection fraction with inotropic agents are more limited in immature hearts, augmentation of cardiac rate becomes more important as a means of increasing the cardiac output. By increasing intracellular cAMP, catecholamines enhance ventricular relaxation in addition to promoting contractility, and may be the preferred method of increasing cardiac rate if successful.

Arrhythmias

Immature conduction tissues differs from that of the adult in the following important ways:

There is depressed sodium channel and enhanced calcium channel activity,

higher resting membrane potentials,

faster repolarization with shorter action potential duration, and

increased sensitivity to positive chronotropic effects of catecholamines.

These properties make the immature heart more prone to automatic tachyarrhythmias, and may account for the relatively high incidence of postoperative atrial and junctional rhythms that are refractory to cardioversion, overdrive pacing, or conventional antiarrhythmic medications. Conversely, primary ventricular arrhythmia is rarely a major problem following cardiac operations in infants, and are usually due to secondary effects of hypokalemia, hypoxia, elevated digoxin levels, and severe myocardial dysfunction.

Diagnosis of Arrhythmias

Surface Electrocardiogram

Although much information can be gained from a well-scrutinized rhythm strip, a formal 12-lead EKG should always be obtained to evaluate sustained and/or symptomatic arrhythmias. Due to the intrinsically rapid cardiac rates in infants, the P wave on the surface EKG may be obscured by the preceding T wave, and accurate rhythm diagnosis may be difficult.

Atrial Pacing-Wire Electrocardiogram

Temporary atrial pacing wires are important both diagnostically and therapeutically in the postoperative period. They allow for a method of atrial pacing and overdrive interruption of some atrial arrhythmias, in addition to providing accurate recordings of atrial electrical activity when the P wave is uncertain or obscured on the surface EKG. The simplest way to record the atrial EKG is to attach each of the two atrial wires to the right and left arm electrodes of the standard EKG recorder, and attach the leg electrode in the usual location. Lead I then records a bipolar atrial EKG, while leads II and III record unipolar tracings. If only one atrial lead is present, unipolar signals can still be obtained by attaching the wire to the left arm electrode and recording either lead I or II.

Transesophageal Atrial Electrocardiogram

When atrial wires are unavailable, an esophageal catheter can be placed for atrial recordings. The catheter is inserted either transnasally or transorally far down the esophagus, and then slowly withdrawn wile constantly monitoring the EKG. When the recording electrodes pass behind the left atrial, large bipolar atrial signals are displayed.

Atrial Electrocardiograms via Fluid-filled Catheters

An alternative to atrial wires or an esophageal lead is a fluid column used to transmit atrial electrical activity. Any catheter with its distal tip in the atrium can be used for this technique. The catheter is filled with NS or sodium bicarbonate, and the proximal end fitted with a metal Luer-lock needle. A unipolar recording can be obtained if care is taken to remove all air and blood from the catheter.

Specific Arrhythmias

Sinus Node Dysfunction

Sinus node dysfunction can follow any cardiac procedure, although the procedures associated with the highest risk are those following the Mustard or Senning operations, baffling of PAPVR, and the modified Fontan procedure.

The early postoperative manifestation of sinus node dysfunction is usually inappropriate sinus bradycardia or marked sinus arrhythmia with an ectopic atrial junctional escape rhythm. The junctional escape rate is usually sufficient to maintain stable hemodynamics in the presence of satisfactory ventricular function and no hemodynamic residua.

Atrial pacing reliably increases the cardiac rate while assuring AV synchrony. Drugs which suppress escape beats, such as b -blockers, procainamide, verapamil, and (sometimes) digoxin, should be avoided or held.

Atrial Flutter

The atrial flutter rate can be up to 400 b/min in infants, but generally occurs at rates of 250 - 320 b/min. The ventricular response rate can vary but is usually brisk (1:1 and 1:2 conduction). Atrial EKG recordings are helpful in clarifying this rhythm if the P wave is obscured by the preceding T wave.

Atrial flutter can be cardioverted since it behaves as a reentry rhythm, and is the first treatment option if the flutter is of acute onset and is associated with hemodynamic compromise. Drug treatment consists of digoxin first to block the AV node, then (usually) followed by procainamide in an attempted to chemically cardiovert the arrhythmia.

Automatic Ectopic Atrial Tachycardia

Automatic ectopic atrial tachycardia probably results from a rapid atrial ectopic focus that overrides the sinus node. The EKG pattern is rapid, often irregular P wave with gradual acceleration and deceleration in rate. The AV conduction ratio is variable, and the P wave morphology is distinctly different from that in sinus rhythm.

Since ectopic atrial tachycardia behaves as an automatic focus, it cannot be interrupted with cardioversion or pacing maneuvers. Drug therapy is indicated if the patient is compromised hemodynamically, and consists of digoxin, to slow down the ventricular rate, and infrequently b -blockers or verapamil. Dilantin and procainamide have also been occasionally tried.

Atrioventricular Reciprocating Tachycardia

Atrioventricular reciprocating tachycardia refers to reentry supraventricular tachyarrhythmia that depends on the AV node and/or ventricle for its circuit. The hallmark of this arrhythmia is a 1:1 AV relationship, as opposed to other primary atrial tachycardias that have variable AV conduction.

Reciprocating tachycardias are generally treated by vagal stimulation. Atrial overdrive pacing is reliable and perhaps the best method for infants with frequent recurrences. Cardioversion is effective, but if episodes recur frequently, repeated shocks are stressful and can cause myocardial depression.

Suppressive drug therapy depends on the mechanism of supraventricular tachycardia. For WPW syndrome or concealed bypass tracts, procainamide and sometimes b -blockers are most effective. For AV nodal reentry tachycardias, digoxin, b -blockers, or verapamil can be used.

Automatic Junctional Ectopic Tachycardia

Automatic junctional ectopic tachycardia is manifested as an increase in the junctional rate, sometimes resulting in a rhythm in which the junctional rate may outrun the sinus rate, allowing AV dissociation or retrograde atrial activation. If the rates are not excessive, the rhythm is usually well tolerated, and my be treated by atrial overdrive pacing. Rapid junctional ectopic tachycardia (rate of 180 - 200 bpm) results in a critical situation, especially in the early postoperative period, when it usually occurs. The mortality from this rhythm has been reported to be as high as 50%, although therapy has recently improved these results.

The characteristics of junctional ectopic tachycardia include:

• rapid tachycardia of 180 - 200 BPM, often with variable rates
• QRS complex identical to NSR
• Atrioventricular dissociation or retrograde Wenckebach conduction
• Occasional sinus capture beats

the presence of "cannon" a-waves on the right atrial or left atrial monitoring lines, which are attenuated as waves of atrial P waves are synchronized with the junctional ectopic tachycardia rate.

When junctional ectopic tachycardia is suspected, the rhythm must be proved to be "automatic" and not reentry. Overdrive pacing, adenosine infusion while recording the EKG, and an attempt at cardioversion should be attempted. If the tachyarrhythmia is refractory to overdrive pacing, and AV dissociation is observed, then the diagnosis can be made with confidence.

Treatment with medications has generally not been satisfying, and have included digoxin, Dilantin, verapamil, b -blockers, propafenone, and procainamide. Except for digoxin, and possibly Dilantin, other drugs generally have undesirable side effects. Current treatment consists of hypothermia to a temperature of 34 - 35° C, paralysis to prevent shivering, and analgesia/sedation to prevent catecholamine surges. Digoxin may also be added, especially if the patient has not previously been on digoxin, or Dilantin if patient is on already on digoxin. Judicious use of inotropic medications is also indicated, although stopping inotropic medications altogether is rarely justified.

Atrioventricular Block

The incidence of atrioventricular block has declined substantially following repair of congenital heart defects, although do still occasionally occur, especially in complex congenital malformations in which the conduction system may be aberrant. Temporary pacing wires may be life-saving for patients at risk for developing temporary of permanent AV block.

Treatment consist of external pacing, preferably in AV sequential mode. Any infant who is pacer-dependent should have the threshold measured twice a day, and the pacemaker output set to at least twice the threshold.

If AV conduction has not returned by 7 - 10 day, then a permanent pacemaker is usually implanted. The route for insertion of a permanent pacing leads in infants is influenced by patient size, type of cardiac disease, and the patient’s growth potential. Transvenous lead systems are not used in infants with intracardiac right-to-left shunts due to the risk of systemic embolization. Transvenous systems in small infants may also be limited because of rapid growth potential and the repeated need to reoperate for lead advancement. For these reasons, direct epicardial leads are most commonly used in infants.

The choice of pacing modes varies. Generally DDD pacing provides the most physiological response, but size constraints and the need for dual wires may preclude this option in infants. Activity responsive pacing is probably the next best option in single-chamber pacing, but may not be feasible in young infants due to the preprogrammed lower rate limits. In very small infants, a VVI unit is still the most commonly used.

Patients who receive a pacemaker should remain on a pulse oximeter for the entire hospital stay (or at least for the first 2 weeks postoperatively) in order to ensure that the pacemaker current is being appropriately transmitted into a pulse and not only artifactual.

Therapeutic Techniques

Overdrive Pacing

Overdrive pacing is a quick and well-tolerated option for interrupting classic "reentry" tachycardias such as atrial flutter, AV nodal reentry, and most arrhythmias from accessory AV connections. When attempting overdrive pacing, provisions should be made for immediate cardioversion if needed.

The burst method begins with the pacemaker rate set 10 BPM faster than the atrial rate, and then a brief 4 - 10 beat burst is administered. If unsuccessful, the pacing rate can be increased in increments of 10 BPM and reattempted to a maximum safe burst rate of approximately 300 - 360 BPM. If refractory, longer pacing drive trains can be attempted, with the pacer initially reset to a rate slightly higher than the atrial rate.

An alternative method is the entrainment method, in which pacing is again commenced 10 BPM above the atrial tachycardia rate, gradually increased by an additional 50 BPM, then gradually decreased to physiologic rates over about a 30 second period.

If both methods are unsuccessful, and a programmable stimulator is available, single or multiple premature beats can be introduced into the tachycardia, either with or without 8-beat drive trains in attempts to break the reentry.

External Cardioversion and Defibrillation

Table 3: Energy Doses for External Cardioversion

Any sustained tachyarrhythmia causing acute hemodynamic deterioration should be managed with cardioversion. In infants and small children, the paddles should be anterior-posterior using a large back plate to maximize energy delivery and prevent arcing. Supraventricular tachyarrhythmias should be synchronized with the EKG. The following dosage schedule should be followed.

Antiarrhythmic Medications

The indications for individual medications are discussed in "Specific Arrhythmias", above. Recommended dosages and warnings are given in the section "Medications".

Postoperative Respiratory Care

Pressure-cycled Ventilators

These ventilators are typically used in neonatal patients and patients weighing up to 5 - 7 kg. They are generally time-cycled, pressure-limited ventilators. A valve in the expiratory limb of the tubing is occluded to a set peak inspiratory pressure (PIP) and released to a set end-expiratory pressure (PEEP). A constant flow of gas is delivered with a set FiO₂ at a set duration of inspiration. The tidal volume is determined by the inspiratory pressure, duration of inspiration, along with the compliance of the lung and resistance of the system. Typical initial settings are shown on the right. Again note that the FiO₂ needs to be adjusted according to the physiology of the lesion involved.

Table 4: Initial Ventilator Settings

Volume-cycled Ventilators

These ventilators are the typical adult-type volume ventilators and provide a preset tidal volume and respiratory rate. Typical initial settings are shown in table on right. Note that the FiO₂ is adjusted according to the physiology of the lesion being treated.

High Frequency Jet Ventilation

Table 5: Endotracheal tube sizes in children

Mechanical Support of the Circulation

History and Introduction to Extracorporeal Life Support

Extracorporeal Life Support (ELS) holds the promise of providing effective rest for injured hearts and lungs. The advent of ELS was in 1953, spearheaded by a twenty year effort in the laboratory by John H. Gibbon, Jr. With the development of the screen oxygenator. Subsequently, C. Walton Lillehei used cross-circulation, and John W. Kirklin used a mechanical screen oxygenator as a means of providing extracorporeal oxygenation. It remained for Robert H. Bartlett to synthesize all the foregoing work and bring to fruition an extension of extracorporeal oxygenation, using a functioning heart to offer immature lungs two to three weeks ‘rest’ to regain their ability to function normally. Other prominent contributors include Kolobow, Bramson, and Clowes, who independently experimented with silicone to build membrane oxygenators. Hence, only as recently as 1952, a surgeon at the bedside of a child dying from an intracardiac malformation could only pray for recovery, whereas today, with the development of extracorporeal life support systems, correction is routine in greater than 90% of cases.

Extracorporeal Life Support for Cardiac Failure

The standard ELS circuit used for pulmonary support in neonates with respiratory failure was not developed with cardiac support in mind. The setup for ELS for cardiac support is veno-arterial, so that venous drainage of the right side of the heart unloads the right ventricle, while arterial perfusion supports the left ventricle. Therefore, the left ventricle is not decompressed, the right ventricle is not directly supported, and most importantly, arterial perfusion is directed away from the aortic arch. Therefore, coronary arterial perfusion is tenuous. These considerations are of critical importance in planning extracorporeal life support following cardiac failure. Specifically, it is important to recognize that provision has to be made for right heart versus left heart failure, for myocardial ischemia as predominant factor in the etiology of the cardiac failure, for the need for decompression of the left side of the heart, for single-ventricle, physiology for the presence of intra-cardiac or great arterial shunts for the presence of arterial or ventricular valvar insufficiencies or stenoses for the proximity of the cardiac failure to the operative procedure for the etiology of the cardiac failure, and for whether extracorporeal life support is being provided for as a bridge to transplantation.

It is convenient to divide the cardiac lesions into either two-ventricle or single-ventricle repairs. Two-ventricle repairs are the most common type of repairs, in which the patient is provided with two functional ventricles, two atria with functional ventricular valves, and two outflows with functional arterial valves. Such repairs include repair for ventricular septal defects, tetralogy of Fallot, truncus arteriosus, transposition of the great vessels, double outlet right ventricle, and cardiac transplantation, among others. Single-ventricle repairs include palliation for lesions such as tricuspid atresia, hypoplastic left heart syndrome, pulmonary atresia with intact septum, double inlet single left ventricle, among others.

Extracorporeal Life Support for Cardiac Failure Following Two-Ventricle Repairs

Whereas two-ventricle repairs are the most commonly performed intra-cardiac repairs, they are the least common repairs that require postoperative extracorporeal life support. Preoperative understanding of the morphology and physiology of the lesions, intraoperative support of the circulation and protection of the heart, and appropriate postoperative management make it rare that two-ventricle repairs require postoperative ELS. However, following large operative procedures, such as repair of truncus arteriosus or transposition of the great vessels, ELS is occasionally required for postoperative cardiac stunning [1662]. In patients who require ELS following two-ventricle repairs, between 50 to 75 patients are successfully weaned off ELS, and approximately 35 to 50% leave the hospital [694, 690,1961,851,1115].

Additional uses for extracorporeal life support for two-ventricle physiology is in the preoperative management of pulmonary hypertensive crises, and in a variety of cardiomyopathies. In the latter situations, one-third of such patients will recover normal ventricular function, one-third will remain approximately the same, and one-third will deteriorate.

Cannulation Techniques for Two-Ventricle Extracorporeal Life Support

As has been alluded to previously, percutaneous neck or femoral cannulation provides right heart decompression and left heart support, but does not provide reliable coronary perfusion, left heart decompression, or right heart support. The lack of support of the right ventricle is particularly important in that many cases of biventricular failure are due to right heart failure, and not necessarily left heart failure. If the heart failure is due to left ventricular dysfunction, then coronary perfusion is of major importance. As such, it is often important to vent the left ventricle by the placement of a cannula within the left atrium. Partial LV venting is provided by draining the right ventricle as less pulmonary blood flow occurs and therefore, less blood returns to the left ventricle. A potentially useful cannula has been designed by Kolobow at the National Institutes of Health in which an open spiral-tipped cannula is placed directly into the pulmonary valve, therefore, providing free pulmonary insufficiency. This essentially decreases further the amount of blood flowing antegrade into the pulmonary circulation and back to the left side of the heart. The free pulmonary insufficiency that the cannula causes is controlled by direct systemic venous cannulation.

However, in the face of severe cardiac failure, the left ventricle may remain dilated and be subjected to progressive failure. In such situations, it is important to vent the left side of the heart. Provision of adequate coronary arterial blood flow is usually provided by allowing the left ventricle to eject. Hence, the catch-22 of extracorporeal life support for left heart failure is that while one wishes to decompress the left ventricle in order to provide it with adequate recovery, it remains important for the left ventricle to be somewhat functional in order to provide itself with adequate coronary blood flow. This intricate balance is one of the main management variables in postoperative left ventricular cardiac failure. In this regard, it is often more favorable to provide direct aortic cannulation as this provides a greater means of coronary blood flow as compared to standard neck cannulation techniques.

Management Techniques for Extracorporeal Life Support

Providing adequate perfusion while decompressing and supporting the two ventricles is the main goal of extracorporeal life support in the face of cardiac failure. Dependent on the contribution of the left ventricle to systemic perfusion, ELS usually provides between 50 and 100% of the total cardiac output. Great attention is paid to left and right atrial pressures, as these provide direct information on the filling pressures and the compliance of the left and right ventricles. In addition, the mixed venous oxygen saturation is followed as this provides on-line information of systemic perfusion. Arterial blood gases are followed carefully and particular attention is paid to the acid-base status as an indicator of systemic perfusion. Base deficits of less than -2 usually indicate inadequate systemic perfusion and should start the search for reasons for inadequate perfusion. The most common cause for inadequate perfusion is hypovolemia, but other causes include inadequate support of the circulation and sepsis.

Detailed attention is given to blood coagulation profiles and studies. The platelet count is kept above 100,000, the fibrinogen is maintained above a level of 150, the ACT is maintained between 100 and 200 seconds, the AT III is maintained around 100, and the plasma free hemoglobin is maintained below 75. It has recently been shown that the antithrombin III level is of particular importance in avoiding intravascular coagulation in the face of an adequate ACT. Aprotinin and/or Amicar may be used on ELS in the face of continued bleeding that is thought to be due to hemolysis. This is assessed on a case by case basis.

Attention is paid to the hemodynamics. Mild to moderate inotropic support and mild to moderate vasodilatation is often beneficial to the failing myocardium. However, it is often not favorable to use large amounts of inotropic agents as these overly stimulate the heart and increase myocardial oxygen consumption.

Nutritional support is of paramount importance during ELS. Between 100 and 130 kcal/kg/day and adequate protein intake is provided. Attention to infection surveillance and aggressive treatment of any infection is also important. At a minimal, the patient is cultured completely daily, including culturing of the urine, sputum and blood. In the absence of any evidence of ongoing infection, first generation cephalosporins are used as antibiotic prophylaxis. Topical antifungal agents, such as Nystatin are routinely used. Prophylaxis against gastrointestinal bleeding is also important, and often takes the form of intravenous Zantac, provided in the TPN. Finally, attention is given to skin breakdown sites, as these patients are often immobilized for prolonged periods of time with low cardiac outputs.

Weaning from Extracorporeal Life Support

Weaning from extracorporeal life support is based on recovery of the myocardium. In general, a slow wean down to approximate 50% support is provided for. In the face of improving myocardial performance, the pulse pressure will increase, the mixed venous saturation will not decrease, the patient will not become acidemic, and urine output will be maintained. When these parameters are met, a trial-off ELS is indicated.

It is important to examine myocardial performance during the trial-off period. This is best accomplished by transesophageal echocardiography at the time of weaning and during trial-off period. It is important to provide adequate inotropic support which should be started one to two hours prior to the trial-off period. This is usually provided for by an infusion Epinephrine maintained between 0.05 to 0.1 mcg/kg/min. Pulmonary and systemic resistance is often decreased by an infusion of Amrinone which is loaded with 2 mg/kg and maintained with an infusion of 5 to 10 mcg/kg/min. A slow weaning off ELS is then begun over approximately a one-hour period in which myocardial performance is carefully examined. Oftentimes, blood transfusion will be required and should be readily available during the trial-off period. At a support of approximately 20 to 25%, the ELS circuit is turned off and the patient is continued to be observed. As long a time as necessary to provide information as to the adequacy of myocardial performance is then observed with the TEE probe in place. Serial arterial and mixed venous blood gases are obtained, urine output is scrupulously followed, and tactile information as to the adequacy of perfusion is obtained. If the patient can maintain an adequate cardiac output for approximately one hour, then the ELS circuit is resumed and plans for elective decannulation are made.

Extracorporeal Life Support for Cardiac Transplantation and End-Stage Heart Disease

In general, ELS is not to be used as a bridge to transplantation because of the low likelihood of finding a suitable donor in a one to two week period. However, transplantation is considered for all patients who fail to recover cardiac function while being supported on ELS. The only indication for ELS as an intentional bridge to cardiac transplantation is when the donor heart is already available. In general, however, if such a patient requires extracorporeal support while awaiting the arrival of a donor heart, then this can be accomplished using conventional cardiopulmonary bypass. Evaluation for transplantation is best performed when the patient is stable on ELS and daily thereafter. If the patient is not a transplant candidate, then a time-table for therapy should be established. If, on the basis of this evaluation, the patient is a potential transplantation candidate, then the patient should be listed for transplantation with the highest priority possible.

If a heart donor becomes available, then heart transplantation should be considered. If clinical evaluation indicates that recovery of the endogenous heart is likely, then the donor heart is referred to another recipient. If, however, endogenous cardiac recovery is unlikely, then the donor heart is accepted with the intent of cardiac transplantation. Conditions which would contraindicate transplantation, prevent listing, or cause removal from the transplant list as a result of daily evaluation include: Infection, liver failure, neurologic damage, severe pulmonary failure and multiple organ failure. Notably, isolated renal failure and coagulopathy are not necessarily contraindications. Patients removed from the transplantation list due to the presence of any of the above contraindications, are managed as non-candidates.

Time-Table

A time-table is established for each patient and re-evaluated based on the best estimate of the likelihood of recovery of cardiac failure. ELS should be continued while there is reasonable hope of myocardial recovery, but stopped when treatment is futile. The likelihood of cardiac recovery is minimal when:

There is no left ventricular ejection while on appropriate inotropic support and with adequate filling pressures following 5 or more days on ELS. This is particularly the case when transplantation is not an option.

When the patient has some function, but is ELS-dependent after 10 days and transplantation is not an option.

Major neurologic injury is documented at any time.

Management

The decision to initiate ELS for post-cardiotomy cardiac failure includes attending staff from Pediatric Cardiac Surgery, Pediatric Cardiology and Pediatric Critical Care. All attending staff members should agree that ELS should be instituted.

In general, extracorporeal flow should be sufficiently high to allow cardiac rest with relatively low filling pressures, low after-load and minimal inotropic support. However, efforts are made to maintain left ventricular ejection with moderate levels of inotropic support and left-sided filling pressures as necessary. There should be a low threshold for direct left atrial access and left-sided venting in the presence of left ventricular dysfunction.

Although high flow veno-arterial access is used, flow rates are adjusted to assure that at least 25% of the venous return passes through the pulmonary circulation. A daily trial of decreased flow to evaluate right and left ventricular function is carried out. If a trial of decreased flow is tolerated then a full trial-off of bypass is carried out with appropriate inotropic and filling pressure support. In general, this is evaluated by hemodynamics and transesophageal echocardiography.

Vascular access and the need for isolated left ventricular support for intraoperative ELS support is made by the pediatric cardiac surgeon. Vascular access for closed chest cases is carried out by the pediatric surgery service, and is usually that of right jugular and right carotid access.

Extracorporeal Life Support for Single-Ventricle Palliation

Cardiorespiratory failure following single-ventricle palliation is associated with the highest morbidity and mortality. In contrast to ELS performed for two-ventricle repairs, ELS performed for a single-ventricle palliation is associated with an 85% mortality. ELS support for single-ventricle palliation is dependent on the stage of palliation, which is based upon the provision of pulmonary blood flow. In stage I, pulmonary blood flow is provided by an aortopulmonary shunt, and is used for patients whose pulmonary vascular resistance is high. Typically, such patients are neonates, and pulmonary vascular resistance is physiologically high. Second-stage palliation is conversion of the aortopulmonary shunt to a superior cavopulmonary anastomosis. In this physiology, approximately one half of the systemic venous return is diverted to the pulmonary artery and the remaining one half diverted back to the systemic ventricle. This stage is typically performed when the patient is about six months of age, at a time when pulmonary vascular resistance has declined, but not to the adult level. These patients remain cyanotic as only one half of the systemic venous return is returned to the pulmonary vasculature for oxygenation. Third-stage palliation entails diversion of all systemic venous return to the pulmonary vasculature. Pulmonary vascular resistance must be at the adult level, there must be no morphologic pulmonary artery abnormalities, no systemic ventricular dysfunction, and no atrioventricular valvar regurgitation. The presence of any of the above abnormalities increases the risk to third-stage palliation, as pulmonary venous pressure is increased and the transpulmonary gradient is widened.

Cannulation for First-Stage Palliation

Venous return is accomplished through a cannula placed either in the superior vena cava or in the right atrium directly. Arterial blood flow is provided by cannulation of the right carotid artery typically, or directly into the ascending aorta. Of major importance is control of the systemic to pulmonary arterial blood flow. This is a difficult clinical management decision, as complete occlusion of the aortopulmonary shunt will risk either pulmonary ischemia or development of reperfusion injury once pulmonary blood flow is restored. Conversely, if pulmonary blood flow is not controlled, the great majority of the perfused blood flow will go directly to the pulmonary circulation, leading to pulmonary edema and systemic hypoperfusion. As such, the balance of systemic to pulmonary blood flow must be regulated and assessed carefully.

Cannulation for Second-Stage Palliation

In second stage palliation, the systemic venous return is divided into that of the superior caval return, which is returned to the pulmonary artery, and the inferior caval return, which is returned to the systemic ventricle. Venous return to the extracorporeal circuit is provided by bicaval cannulation, the superior caval cannulation ensuring that superior caval blood pressure is reduced in order to provide for adequate cerebral perfusion. In this situation, cerebral blood flow is determined by the difference in the mean arterial pressure and the main pulmonary artery pressure. In the face of increased pulmonary vascular resistance and systemic hypotension, cerebral perfusion pressure may decline to a level that may lead to neurologic ischemia and injury. It is, therefore, important to cannulate both the inferior vena cava and the superior vena cava to insure both adequate venous return to the extracorporeal circuit and adequate decompression of the cerebral venous system, respectively.

Cannulation for Third-Stage Palliation

Cannulation for third stage palliation is similar to that for two-ventricle repairs. Namely, provision is made for decompression of the systemic atrium, typically, the left atrium. In the face of ventricular dysfunction, failure to decompress the systemic atrium may lead to systemic ventricular dilatation and progressive dysfunction. Systemic venous return is provided by a single cannula placed in the Fontan circuit, this being either the extra-cardiac conduit or within the lateral tunnel. Arterial blood flow is provided by a cannula placed into the carotid artery or into the arch of the aorta.

Weaning Extracorporeal Life Support Following Staged Palliation

Weaning from extracorporeal bypass following staged-palliation is similar to weaning following two-ventricular repairs. However, it is important to recognized that following first- stage and second-stage palliation, the patient will be cyanotic and saturations in the 70 to 85% range are acceptable. In this saturation, the hematocrit must be increased to at least 45% in order to provide adequate oxygen carrying capacity of the blood. In most other respects, weaning from extracorporeal circulation is analogous to a two-ventricular repair.

Other Heart-Assist Devices in Children

Support of the circulation remains a difficult problem in neonates and infants, due both to size constraints and lack of technology. Some centers have used support with varying degrees of success.