Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
Mechanical Circulatory Support in Pediatrics
Introduction
Mechanical circulatory support (MCS) for pediatric patients has advanced enormously, since its first reported use in the 1950s by Gibbon, Kirklin, and Lillehei in the form of a cardiopulmonary bypass machine. Kirklin and others were the first group to develop a pump oxygenator with a much smaller priming volume. The rapid evolution of cardiopulmonary bypass continued for the next 20 years, including the refinement of deep hypothermic circulatory arrest in infants by Barratt-Boyes and Casteneda in the 1970s. Over the ensuing decades, continued iterative improvements in cardiopulmonary bypass technology and operative techniques contributed to a significant reduction in mortality associated with pediatric cardiac surgery and paved the way for expanded indications for ventricular support in children with heart failure. While MCS is a remarkably diverse field with a multitude of devices, ventricular assist devices (VADs) are now a key component in the management of advanced pediatric heart failure. The early use of adult VADs in older, adult-sized adolescents demonstrated encouraging outcomes, but the development of the Berlin Heart EXCOR (Berlin Heart AG, Berlin, Germany) was the pivotal event that allowed for mechanical support in children of all sizes. Nonetheless, key technical challenges specific to pediatrics remain to this day, including pump design and the management of anticoagulation. Our understanding of the outcomes of MCS has also continued to grow, as there are now over 700 device placements at more than 40 hospitals documented in the Pediatric Registry for Mechanically Assisted Circulatory Support (PediMACS) database, allowing for detailed evaluation of this important group of patients.
Heart failure in children
Although, worldwide, the major etiologies of heart failure in children include rheumatic heart disease, Chagas disease, cardiomyopathies, and congenital heart disease (CHD), in the United States, the most common causes of heart failure in children are dilated cardiomyopathy and CHD. Epidemiological studies estimate that pediatric dilated cardiomyopathy has a case incidence of ~ 400 cases per year in the US, an estimated case prevalence of 6000 pediatric patients, and a median survival ranging from 7 to 18 years. It is important to note that not all of these patients have severe heart failure. According to the Organ Procurement and Transplantation Network, roughly 300 pediatric patients are listed annually for heart transplantation due to severe heart failure from dilated cardiomyopathy. CHD constitutes a much more heterogeneous group of patients as the probability of heart failure varies greatly by specific lesion ( Fig. 18.1 ).
Single-ventricle lesions are a large contributor to the incidence of heart failure in the CHD population. It is estimated that over 2000 children are born annually with single-ventricle CHD, with approximately 300 CHD children undergoing heart transplantation. Using national datasets, investigators have estimated that 14,000 pediatric heart failure hospitalizations occur annually with a 7.4% unadjusted mortality rate. Of these, 60%–70% are attributed to CHD. Despite the greater prevalence of heart failure admissions for CHD, the vast majority of VADs (80%) are implanted in patients with cardiomyopathy. High mortality associated with pediatric end-stage heart disease stems from directly related sequelae, such as low cardiac output, respiratory failure, malignant arrhythmias, stroke, thromboembolism, irreversible end-organ dysfunction, and infection. This mortality has continued to improve over time with better medical care and advancing surgical repairs, including the use of MCS.
Heart transplantation has become the standard of care for children with end-stage heart disease. Although survival has steadily improved after pediatric heart transplantation, donor availability leads to prolonged waitlist times, and mortality while listed continues to be a major challenge, with mortality rates approaching 20%. However, recent studies have demonstrated a 50% reduction in waitlist mortality in pediatric patients receiving VAD support. Zafar and colleagues found VADs to be an independent predictor of survival, whereas weight < 10 kg, CHD diagnosis, extracorporeal membrane oxygenation (ECMO), mechanical ventilation, and renal dysfunction were all independent predictors of mortality on the waitlist. Evaluating the impact of age on this phenomenon, Law and colleagues showed that the VAD waitlist survival advantage was most profound in adolescent (> 11 years) patients as compared with medical therapy. This is likely an effect of the current stages of device development, with older patients being supported with third-generation VADs while pulsatile support or ECMO are the primary available options for smaller children. In this context, the National Heart, Lung, and Blood Institute (NHLBI) is funding development of the Jarvik 2015, a continuous-flow pediatric-specific device, which is beginning a clinical trial in patients 8–15 kg.
Current devices for pediatric cardiac support
Extracorporeal Membrane Oxygenation
ECMO remains a common method of MCS for pediatric patients due to institutional familiarity and ability to rapidly initiate support. This familiarity has developed over 30 years, but improved outcomes have been limited, with nearly half of cardiac ECMO patients dying prior to discharge. Although individual institutions have shown success and ECMO equipment has improved significantly, widespread success nationally as bridge-to-transplant support has not been realized. In addition, the incidences of mechanical, hemorrhagic, and neurologic complications all exceed 30%.
ECMO was initially used in pediatrics for primary respiratory failure with considerable success, in contrast to adult ECMO for respiratory failure, in which a National Institutes of Health trial was halted due to the absence of a survival benefit. The successful use of ECMO in pediatric respiratory disease is important for two reasons: (1) this finding highlighted the need for pediatric-specific investigations of new devices and (2) successful neonatal respiratory ECMO likely altered the course of development of mechanical cardiac support in children. With clinicians encouraged by its success in the area of respiratory support, ECMO for cardiac support became increasingly common despite its challenges, since it was all that was available for children with acute heart failure.
Initial reports of pediatric ECMO described its application primarily in postcardiotomy patients. Survival rates for all pediatric patients supported for cardiac indications were nearly 50%, with significant complications of bleeding, thrombosis, and neurologic injury in over 30% of cases. In 1992, Del Nido and colleagues described the use of ECMO in 33 patients for cardiac support after cardiac surgery. Eleven of these patients were placed on support following cardiac arrest after an average of 65 minutes of cardiopulmonary resuscitation. Patients supported in this “rescue” fashion had a 64% early and 55% long-term survival rate despite the long duration of cardiopulmonary resuscitation. In addition, survival in this group was similar to that of the entire group. These findings led most centers with significant volumes of pediatric cardiac surgery to develop ECMO “rapid deployment” units.
In cardiac surgery, ECMO has been used to maintain cardiorespiratory function until other cardiopulmonary derangements have been adequately treated, as a bridge to transplantation or as a bridge to long-term device placement. Despite advancing VAD technology, ECMO remains the best support option for patients in cardiogenic shock requiring rapid support for cardiac arrest or to provide cardiopulmonary support as long as a pulmonary component is necessary. If a VAD is initially used, extracorporeal centrifugal pumps, such as the Pedi/CentriMag (Abbott, Lake Bluff, IL) or Jostra ROTAFLOW (MAQUET, Inc., Rastatt, Germany), lend themselves to the “splicing” of an oxygenator into the VAD circuit tubing if respiratory failure ensues. Other novel techniques have been used to place a gas exchange membrane within an existing VAD circuit. The most frequent potential contraindications to ECMO support are severe neurologic injury, prematurity and small size (< 2 kg), active bleeding, irreversible disease, and noncardiac major structural and chromosomal abnormalities. In general, successful arterial-venous ECMO support is usually for periods of less than 2 weeks, whether being used as a bridge to recovery or transplantation. Furthermore, ECMO support at time of heart transplantation is well recognized as one of the highest risk factors for posttransplant mortality, with 1- and 5-year survival reported at 61% and 35%, respectively, significantly lower than that of other pediatric heart transplant recipients whose median survival is > 10 years.
Left Ventricular Assist Devices
In addition to pulmonary function, ECMO supports both the right and left heart using a single cannulation strategy of systemic venous and arterial cannulation. Therefore, in the current era, most centers still rely on ECMO for all patients needing rapid or postcardiotomy support.
Nonetheless, it is clear that unchanged survival rates since 1985, high incidences of complications, and the inability to reliably and safely support children for longer than 2 weeks leave significant room for improvement. Over the same period of development of ECMO technology for children, ever-improving VAD technologies were being developed for long-term adult cardiac support. This technology has been carefully integrated into the care of children with heart failure as we await the development of better pediatric-specific devices.
Temporary Support Strategies
Regardless of device, temporary support has come to mean a quick and simple cannulation strategy for stabilization of cardiac function in (1) a patient thought to have an inflammatory etiology of heart failure (i.e., myocarditis or acute graft rejection) with an expectation to recover within 2 weeks and (2) as support for someone in cardiogenic shock as a bridge to a more durable device. Furthermore, temporary VAD support may provide a period to assess neurologic status or etiology of heart failure prior to placement of a durable VAD or performance of a heart transplant. Typically, this has been accomplished with extracorporeal, centrifugal pumps such as the Centri/PediMag or the ROTAFLOW connected to temporary bypass cannulas placed centrally and off cardiopulmonary bypass. It is the cannulation strategy rather than the choice of pump that best defines the type of support, as Berlin Heart cannulas placed on cardiopulmonary bypass can be used with extracorporeal, centrifugal pumps for extended periods of time as a bridge to transplant. This strategy also allows transition to other devices (e.g., Berlin Heart EXCOR) externally at the bedside and for pump exchange if thrombosis develops. This strategy is becoming increasingly popular, particularly in smaller patients presenting challenges with anticoagulation during their postoperative inflammatory state and for single-ventricle patients with pulmonary artery bands, large aortopulmonary collateralization, or shunted physiology. In these patients, the centrifugal mechanics of the pump make it ideal for rapidly accommodating changes in preload and flow without constant adjustment to the device parameters. Additionally, an oxygenator can be spliced into the circuit relatively easily to simulate ECMO, as can an ultrafiltration/dialysis system to provide renal support. As these techniques in management and others are honed, evidence is accumulating that these historically “short-term” devices can safely support patients for longer periods of time than previously thought.
Two other less commonly used temporary options are delivered percutaneously and are therefore less invasive. These include the TandemHeart (CardiaAssist, Pittsburgh, PA) and the Impella (Abiomed, Danvers, MA). Both devices require large vessels to accommodate the catheter dimensions, generally necessitating a body surface area (BSA) > 1.3 m 2 . The Impella is placed across the aortic valve under echocardiographic guidance to augment cardiac output with axial, continuous-flow support, while the TandemHeart has a venous cannula percutaneously placed through the atrial septum into the left atrium with arterial output to the femoral artery. Although little data exist regarding these devices in pediatrics, a single center reported that 10 Impella devices with a median implant time of 8 days (range, 1–21) supported eight patients (median age, 17 years; range, 6.5–25 years), with all discharged from the intensive care unit. Five patients had posttransplant rejection or allograft vasculopathy causing heart failure, and four patients also required ECMO. Because the pediatric MCS field includes many children with multiple prior sternotomies, there was initial enthusiasm for the TandemHeart percutaneous system, but the fact that most movement, even for daily care (i.e., chest roentgenogram), often causes a change in location requiring repositioning has limited its use in the pediatric community.
Durable Strategies
As “adult” VADs became available over the past 20 years, they have been increasingly used in adolescents. Although covered in detail elsewhere in this text (see Chapter 10 ), durable VADs can be grouped broadly into two separate categories, pulsatile and continuous-flow pumps. Pulsatile pumps were the first VADs developed but have since given way to second- and third-generation continuous-flow pumps in the great majority of cases, with the exception of the Total Artificial Heart (SynCardia, Tuscon, AZ) and the pediatric-specific Berlin Heart EXCOR. In the PediMACS registry, among patients receiving a durable VAD, 94% of patients < 20 kg receive a pulsatile, extracorporeal VAD, while 85% of those > 20 kg get an intracorporeal, continuous-flow VAD, which in the past several years has increased to approximately 95%.
You're Reading a Preview
Become a Clinical Tree membership for Full access and enjoy Unlimited articles
If you are a member. Log in here