19 Mechanical Circulatory Support
THE NUMBER OF INFANTS and children hospitalized with heart failure resulting from congenital or acquired heart disease continues to increase each year; according to a recent analysis of 15 million pediatric hospitalizations in the United States, this number grew by 25% between 2003 and 2006.1 Failure of the myocardium, despite maximal medical therapy, to provide sufficient cardiac output for adequate support of end-organ perfusion results in the need for mechanical support of the circulation, either as an adjunct to cardiopulmonary resuscitation (CPR) or as a bridge to myocardial recovery or cardiac transplantation. Data from the Randomized Evaluation of Mechanical Assistance for the Treatment of Congestive Heart Failure Trial (REMATCH) demonstrated prolonged survival and increased quality of life in adult patients with end-stage heart failure who received mechanical cardiac support (MCS).2 Although the number and type of MCS devices available for children remains comparatively limited, particularly for children weighing less than 20 kg in the United States, the use of extracorporeal life support in this population has continued to grow, with a 32% increase in the use of ventricular assist devices (VADs) between 2003 and 2006.3 Initiatives by the National Heart, Lung, and Blood Institute (NHLBI) supporting the development of MCS devices for infants and children offer promising future alternatives for children.4
Initiation of Mechanical Circulatory Support
Indications and Contraindications
Mechanical circulatory support in children is most often necessary due to intrinsic failure of the myocardium. Preoperative cardiopulmonary stabilization may be required in children with profound hypoxemia and/or cardiovascular collapse due to hypercyanotic spells, pulmonary hypertensive crises, obstructed total anomalous pulmonary venous return, occlusion of systemic-pulmonary shunts, or cardiogenic shock (Table 19-1). Extracorporeal membrane oxygenation (ECMO) was used to bridge to surgical repair or palliation in 26 children, with 62% surviving to discharge and no observed differences in outcome between single and biventricular patients.5 ECMO has also been successful in stabilizing children with refractory dysrhythmias.6–8 In a retrospective study, ECMO was used in nine infants with a variety of tachy- or bradydysrhythmias, with all nine surviving to discharge.9 ECMO has also been used to stabilize children in the cardiac catheterization laboratory, both preemptively before high-risk interventional procedures10 and as a rescue technique for catheter-induced complications, persistent low cardiac output, or hypoxemia.11
TABLE 19-1 Indications for Mechanical Circulatory Support
ALCAPA, Anomalous origin of the left coronary artery from the pulmonary artery.
Postcardiotomy myocardial dysfunction can manifest as either early (inability to wean from cardiopulmonary bypass [CPB]) or late failure (sustained postoperative low cardiac output syndrome), with poor end-organ function, persistently increased plasma lactate concentrations, low mixed venous oxygen saturations, and escalating inotropic support. To maximize survival, it is essential to rule out the presence of residual surgical lesions, coronary insufficiency secondary to surgical manipulation, and mechanical problems (e.g., cardiac tamponade) before initiating MCS.12 Both ECMO and left ventricular assist devices (LVADs) have been used for postoperative ventricular support in infants with anomalous origin of the left coronary artery from the pulmonary artery trunk (ALCAPA), either as a bridge to recovery or transplant.13,14 The routine use of MCS after a Stage I Norwood procedure has also been advocated in order to optimize postoperative cardiac output.15
Other cardiac pathophysiologic processes, such as acute myocarditis, coronary ischemia, graft rejection after cardiac transplantation, or end-stage heart failure due to chronic cardiomyopathies, dysrhythmias, or congenital heart defects, may also warrant the use of MCS.16–18 The steadily enlarging population of adults with congenital heart disease who develop heart failure will pose special challenges due to their complex anatomy and prior surgical palliations.
Noncardiac indications for MCS include severe hypothermia, drug toxicity, and near-drowning. ECMO can also provide short-term respiratory support for tracheobronchial reconstruction in infants and children with critical airways, when conventional mechanical ventilation is not feasible or has not been successful.19 Although septicemia was initially considered a contraindication to MCS, a recent review of 45 children who required ECMO for hemodynamic support due to septic shock reported a 47% survival to discharge.20 ECMO can provide a bridge to lung transplantation or retransplantation, and posttransplantation can be used in cases of severe primary graft dysfunction, although survival in these patient groups remains less overall than for other indications.21–23
Timing and Preparation for the Initiation of Support
As experience with the use of pediatric circulatory support continues to grow, it has become increasingly evident that early institution of support better preserves end-organ function and maximizes the opportunity for recovery or bridge to transplantation. The following criteria for implementation of MCS have been used in one institution: (1) cardiac index less than 2 with inotropic dependence; (2) poor peripheral perfusion with metabolic acidosis and mixed venous oxygen saturation less than 40%; (3) signs of impending respiratory, renal, or hepatic failure; and (4) increased or rapidly increasing B-type natriuretic peptide (BNP) concentrations (Table 19-2).24
TABLE 19-2 Suggested Clinical Criteria for Mechanical Cardiac Support Implementation
CI, Cardiac index.
Modified from Hetzer R, Potapov EV, Alexi-Meskishvili V, et al. Single center experience with treatment of cardiogenic shock in children by pediatric ventricular assist devices. J Thorac Cardiovasc Surg 2011;141:616-23; and Potapov EV, Stiller B, Hetzer R. Ventricular assist devices in children: current achievements and future perspectives. Pediatr Transplant 2007;11:241-55.
During routine preparation for MCS, baseline laboratory hematologic studies should be obtained, including a complete blood cell count (CBC), platelet count, prothrombin time (PT), activated partial thromboplastin time (aPTT), activated clotting time (ACT), plasma hemoglobin, antithrombin III, fibrinogen, and thromboelastogram (TEG). A minimum of two units of packed red blood cells (PRBCs) should be available (preferably cytomegalovirus-negative, leukocyte-reduced, and irradiated), because all such children should be considered potential cardiac transplant candidates (note that the potassium concentration of irradiated PRBCs may be greatly increased, resulting in acute hyperkalemia if administered rapidly) (see also Chapter 10). In infants, ultrasonography of the head is useful before considering institution of MCS. Anesthesiologists are frequently involved during the cannulation procedure, and all arterial and central venous lines, as well as the tracheal tube, should be well secured and the ventilator easily accessible.
The emergent use of ECMO for children during in-hospital cardiac arrest with failure of conventional resuscitation methods has become increasingly common. An analysis of data from the National Registry of CardioPulmonary Resuscitation (NRCPR) database of outcomes from children less than 18 years of age who received extracorporeal cardiopulmonary resuscitation (ECPR) for cardiac arrest that was refractory to conventional CPR demonstrated a 43.7% survival to discharge. Preexisting conditions of septicemia, pneumonia, and renal insufficiency correlated with an increased risk of mortality. Children with a cardiac disease diagnosis showed slightly improved odds of survival to discharge when compared with children without cardiac disease.25 In a review of pediatric cardiac patients from the Extracorporeal Life Support Organization (ELSO) registry who received ECPR, single ventricle physiology and a history of more complex cardiac surgery were negative predictors of survival.26 Renal dysfunction, pulmonary hemorrhage, neurologic injury, and the need for additional CPR during ECMO have also been associated with increased mortality.27
Acceptable neurologic outcomes have been described in children after CPR of up to 3 hours in duration before institution of ECMO.28 The duration of CPR before ECMO cannulation did not differ between survivors and nonsurvivors.29 Of the children in the ELSO registry who received ECPR, 22% of the 682 developed an acute neurologic injury, with an in-hospital mortality rate of 89%. The risk of neurologic injury in children with cardiac disease in this cohort was reduced.30
A dry circuit can be kept ready for rapid deployment, with crystalloid prime used during initiation of support and addition of blood products (PRBCs and fresh frozen plasma) as soon as they become available. During resuscitative efforts before the institution of mechanical support, multiple doses of vasoconstrictors should be avoided, if possible, acidosis should be corrected, and ice may be placed around the head to provide cerebral protection. Ultimately, the restoration of cardiac output, even with a low hematocrit, is the most important factor for successful resuscitation and long-term survival.31,32
Mechanical Circulatory Support Devices
In pediatric practice, the size of the child and the type of support required (cardiopulmonary vs. cardiac) are the most important considerations when choosing an MCS device (Table 19-3). Secondary factors are the indication for, and expected duration of, support and the desired end point: bridge to bridge, procedure (surgery or catheterization), recovery, or transplantation. Certain devices, such as the intra-aortic balloon pump (IABP), ECMO, and centrifugal pump, are best used for short-term support (less than 2 to 4 weeks) and may occasionally serve as a “bridge to bridge” for institution of a long-term mechanical support device.33
TABLE 19-3 Mechanical Circulatory Support Devices: Advantages and Disadvantages
| Device | Advantages | Disadvantages |
|---|---|---|
| ECMO | ||
| Centrifugal VAD | ||
| Thoratec | ||
| Berlin Heart Excor and Medos HIA-VAD | ||
| MicroMed DeBakey Child VAD |
BSA, Body surface area; ECMO, extracorporeal membrane oxygenation; VAD, ventricular assist device.
From Diaz LK, Andropoulos DB. New developments in pediatric cardiac anesthesia. Anesthesiol Clin North Am 2005;23:655-76.
A VAD may be defined most simply as a mechanical pump attached between the heart and either the aorta or pulmonary artery to circulate blood when one or both ventricles are no longer capable of adequately maintaining circulation. In short, a VAD is a pump supporting a failing ventricle, whether right, left, or biventricular (BiVAD). Isolated ventricular dysfunction with adequate oxygenation, as seen in acute myocarditis, acute rejection after cardiac transplantation, or dilated cardiomyopathy, is the ideal cardiac pathology for VAD support. ECMO, on the other hand, provides full cardiopulmonary support and may be preferable to VAD support for children with pulmonary hypertension, complex congenital heart lesions involving intracardiac shunts, severe hypoxemia, or respiratory failure (Fig. 19-1).
Short-Term Support
Intra-Aortic Balloon Pump
Anatomic contraindications to the use of an IABP include patent ductus arteriosus, aortic insufficiency, aortic aneurysm, and recent aortic surgery. Technical limitations in children consist of size constraints, the increased distensibility and compliance of the aorta, and the difficulty of synchronizing pump function with an infant or child’s rapid heart rate,34 although use of M-mode echocardiography-timed IABP in infants and children to assist with cardiac synchronization has been described.35 IABP use in an adult patient with Fontan physiology and severe ventricular failure has been reported, with improvement in end-organ function and successful weaning.36 IABP support must be initiated early, while ventricular function is still capable of sustaining adequate cardiac output,37 because the balloon pump only augments, and does not replace, ventricular output. It can only support the left ventricle.
Extracorporeal Membrane Oxygenation
Whereas adults generally have isolated left ventricular failure, children more often require cardiopulmonary support due to hypoxemia, pulmonary hypertension, or concurrent right ventricular failure. For infants and children who require short-term or urgent cardiopulmonary support, ECMO remains the modality of choice. Initially reported for the treatment of cardiac failure in children in the 1970s,38 ECMO was subsequently used for mechanical support during interhospital transport.39 Since the ELSO registry began in 1989, ECMO has been the MCS modality with the most pediatric usage, with more than 7500 neonates and children using it for cardiac indications.40
A typical ECMO circuit is composed of a pump (either a roller pump with a servoregulatory mechanism for controlling circuit flow or a centrifugal pump); a hollow fiber or membrane oxygenator; a heat exchanger; and cannulas (either venoarterial or venovenous). A modified ECMO circuit composed of a heparin-coated circuit, Bio-Medicus centrifugal pump (Medtronic, Minneapolis), hollow fiber membrane oxygenator, flow probe, and hematocrit/oxygen saturation monitor, allowing the circuit to be set up and primed in 5 minutes for rapid resuscitation, has been described.41 Most hospitals supporting such a service have readily available trained personnel to assist with implementing and maintaining ECMO therapy. Versatility is one of the advantages of ECMO; venoarterial cannulation in postcardiotomy patients may be either transthoracic via the right atrial appendage and aorta, transcervical via the right internal jugular vein and common carotid artery, or femoral via the femoral artery and vein in larger patients. Heparin-bonded circuitry is often used to minimize surface-induced complement activation, platelet dysfunction, and anticoagulation requirements.42
Other advantages of ECMO include the lack of patient size limitations, the ability to institute support either in the pediatric intensive care unit or the operating room without the need for CPB, the ability to provide ultrafiltration or hemodialysis for children during mechanical support, and the ability to provide biventricular cardiopulmonary support even in very small children. In a review of 27 children who underwent venoarterial ECMO for cardiac indications, both nonsurgical and postcardiotomy, the overall survival rate was 59%; of these, 56% were undergoing CPR at the time ECMO support was instituted. Of the latter group, 73% survived.43 Hemodynamic benefits of ECMO include decreased right ventricular preload and pulmonary artery pressures. Due to reentry of blood into the aorta an increase in afterload often occurs and may require pharmacologic afterload reduction therapy, such as milrinone, nitroprusside, hydralazine, or phenoxybenzamine.
Several management options exist for children with single-ventricle physiology and shunt-dependent pulmonary circulation who require ECMO support. The survival rate of 10 children who underwent single-ventricle palliation and subsequently required ECMO support was greater in those in whom the aortopulmonary shunt was left open during ECMO.44 Adequate alveolar ventilation must be provided, however, and greater ECMO flow rates are generally required to maintain adequate pulmonary and systemic circulations. In children with low pulmonary vascular resistance, pulmonary blood flow may prove to be excessive and limitation of shunt flow with surgical clips may become necessary. Children with single-ventricle physiology have comparable survival rates after ECMO support compared with other cardiac patients.43 ECMO has been particularly successful in treating children with single-ventricle physiology who develop acute shunt thrombosis or transient depression of ventricular function.45 Of 44 children with shunted single-ventricle physiology who required ECMO support, the indication for support was the strongest predictor of survival to discharge, with 81% of those cannulated due to hypoxemia surviving, but only 29% of those cannulated for hypotension surviving to discharge.46 Patients with Fontan physiology who require ECMO have a significantly greater mortality rate (65%), possibly the result of longstanding ventricular dysfunction that is not easily reversible.47
Disadvantages of ECMO include complex circuitry, the need for greater levels of systemic anticoagulation than required by VADs, the necessity for both blood prime and frequent transfusions, and decreased pulmonary blood flow. Compared with other support modalities, ECMO circuitry is complex and requires full-time supervision by trained personnel. Left atrial decompression may occasionally be inadequate, requiring either the placement of a left atrial vent or an atrial septostomy. Inadequate unloading of the left atrium can lead to mitral regurgitation and pulmonary edema or hemorrhage, and can also minimize the chances of myocardial recovery when the left ventricle is not sufficiently unloaded. Moderate levels of ventilatory support must be maintained to ensure that well-oxygenated blood is provided to the coronary arteries, and the child must remain intubated and sedated throughout the period of ECMO support.48
Although effective for rapid rescue and short-term support, ECMO support is generally maintained only for 1 to 3 weeks before the increasing risk of significant complications limits its usefulness.49 In children requiring postcardiotomy ECMO support, the need for prolonged support, renal failure, and low pH in the first 24 hours of support have been associated with a higher mortality.50 A review of combined data between 2004 and 2009 from the ELSO registry and the Organ Procurement Transplant Network database showed that over half the children bridged with ECMO to heart transplant failed to survive to hospital discharge, illustrating that ECMO is not able to reliably provide intermediate to long-term mechanical support to bridge children safely to transplantation.51 Survivors of ECMO support also have greater rates of neurologic impairment than those supported with VADs, with poorer outcomes noted in younger children with more complex disease.52
Centrifugal Pumps
Currently used centrifugal pumps include the Bio-Medicus Bio-Pump (Medtronic, Minneapolis), CentriMag (Levitronix GmbH, Zurich), RotaFlow (Maquet Cardiovascular, Wayne, N.J.), Capiox (Terumo Cardiovascular Systems Corporation, Ann Arbor, Mich.) and TandemHeart (CardiacAssist, Inc., Pittsburgh). The CentriMag pump is available as an investigational device only in the US. The term centrifugal pump is not always synonymous with VAD, because centrifugal pumps may also be used with an oxygenator to construct an ECMO circuit. Without an oxygenator, they may be used for right, left, or biventricular support (except in infants, when size limitations can preclude the presence of two pumps53) and offer the advantage of excellent ventricular unloading and decreased wall stress, optimizing the chances of myocardial remodeling and recovery. Unloading the left ventricle can also decrease left ventricular cavity size and improve septal configuration, resulting in improved tricuspid valve function and right ventricular inflow.54 When used as a VAD without an oxygenator and heat exchanger, reduced systemic anticoagulation is required, compared with ECMO usage.
Centrifugal pumps offer the advantage of decreased trauma to red blood cells and a less pronounced systemic inflammatory response compared with roller pumps.55 A centrifugal VAD spins, creating a vortex, with negative pressure at the inlet drawing blood into the cone and positive pressure at the outlet allowing nonpulsatile ejection at the base. The RotaFlow pump (Fig. 19-2) has a rotating mechanism that is levitated in three magnetic fields with one point bearing, allowing laminar flow and reducing mechanical friction, heat production and clotting potential compared to the Bio-Medicus pump.56 Cardiac output from a centrifugal pump depends on preload, afterload, and the rotational speed of the pump. Because increases or decreases in preload and afterload can affect pump flow without changes in rotational speed, a flow probe is necessary. Excessive negative inlet pressures (hypovolemia) must be avoided because air can be entrained into the circuit.
The TandemHeart system is a low-prime centrifugal pump capable of flows of 0 to 5 L/min, with a hydrodynamic fluid bearing supporting the spinning rotor (Fig. 19-3). Although size requirements (greater than 40 kg) preclude its use in most children, it is advantageous in that it can be placed percutaneously through the femoral vessels, with a transseptal extended flow cannula allowing entry from the femoral vein into the left atrium. The arterial cannula may be placed into the femoral artery, or in patients less than 80 kg, a vascular graft to the femoral artery may be cannulated to avoid lower extremity vascular compromise.57 A pediatric TandemHeart pump with a priming volume of 4 mL for children from 2 to 40 kg is currently undergoing in vitro testing.58
Compared with ECMO, centrifugal pumps are less expensive and can be set up more rapidly, they have reduced priming volumes, and reduced anticoagulation requirements. Disadvantages of centrifugal pumps include the potential for circuit thrombus formation, nonpulsatile flow, and limited duration of usage (usually less than 3 weeks). Excellent outcomes with centrifugal VAD support have been reported in children who required postcardiotomy ventricular MCS of limited duration, such as those with ALCAPA, or ventricular failure due to cardiomyopathy.59 In addition, fewer post-support neurologic complications were noted in children who received VAD support compared with those who received ECMO for similar indications.60
Intermediate- to Long-Term Support
Pulsatile Pumps
Pulsatile pumps are VADs that facilitate chronic support of the circulation while also allowing tracheal extubation, the use of enteral nutrition, and ambulation for the child. They are paracorporeal and either pneumatically or electromechanically driven. Like centrifugal pumps, pulsatile VADs enjoy several advantages over ECMO: they are simpler in design, less expensive, require lower levels of anticoagulation, and may be used for left, right, or biventricular support of the circulation. Unlike the previously discussed devices, pulsatile pumps are suitable for intermediate- to long-term mechanical circulatory support, but until the advent of the Berlin Heart Excor, their use in infants and children was severely limited by patient size constraints. Internationally, the Berlin Heart Excor and the Medos HIA-VAD pulsatile systems (Berlin Heart GmbH, Berlin, and Medos Medizintechnik AG, Stolberg, Germany, respectively) have been successfully used in children of all ages.61,62 Insertion of a pulsatile VAD requires the use of CPB and closure of any existing septal defects.
Thoratec
The Thoratec pulsatile ventricular assist system (Thoratec Corporation, Pleasanton, Calif.) is a pneumatically driven pump with a 65-mL blood sac, Björk-Shiley tilting disc valves, and exteriorized inflow and outflow cannulas (Fig. 19-4). Inflow may be from either the left atrium or left ventricular apex, with outflow to the aorta. Pump output, depending on the cannula size and length, may vary from 5 to 7 L/min.63 The Thoratec pulsatile VAD may be operated in three modes: synchronized to the child’s underlying heart rate, asynchronous, with a set rate programmed into the device, or fill-to-empty, in which the device fills to a set volume before ejection. Although use of the Thoratec has been described in children as small as 17 kg,64 the risk of thromboembolic events is greater in smaller children, because lower blood flow in the relatively oversized device can lead to stasis and thrombus formation.65 Children with congenital heart disease and/or left atrial cannulation are at greater risk for neurologic complications during Thoratec support.66
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