Summary
The number of children in need of mechanical circulatory support has increased substantially over the last two decades, due to the technological progress made in surgery and intensive care, leading to improved survival of patients with congenital heart disease. In addition, primary myocardial dysfunction related to myocarditis or dilated cardiomyopathy may cause end-stage cardiac failure in children or infants, although not as frequently as in adults. The need for mechanical circulatory support may be either temporary until spontaneous myocardial recovery, as in postcardiotomy cardiac failure, or prolonged until heart transplantation in the absence of recovery. Two types of mechanical circulatory devices are suitable for the paediatric population: extracorporeal membrane oxygenation for short-term support; and ventricular assist devices for long-term support as a bridge to transplantation. The aim of this review is to describe the specific issues related to paediatric mechanical circulatory support and the different types of devices available, to report on their rapidly growing use worldwide and on the outcomes for each indication and type of device, and to provide a perspective on the future developments and remaining challenges in this field.
Résumé
Le nombre d’enfants ayant besoin d’une assistance circulatoire augmente régulièrement depuis 20 ans, en raison des progrès techniques de la chirurgie cardiaque et de la réanimation qui ont permis d’améliorer la survie des patients atteints de cardiopathie congénitale. De plus, les enfants peuvent être atteints de dysfonction myocardique primitive liée à une myocardiopathie dilatée ou une myocardite, responsable d’insuffisance cardiaque terminale. L’indication d’assistance peut être de courte durée en cas de dysfonction myocardique transitoire comme après une chirurgie cardiaque par exemple, ou prolongée jusqu’à une transplantation cardiaque en l’absence de récupération myocardique. Deux types de dispositifs d’assistance principaux sont utilisables en pédiatrie : l’oxygénation extracorporelle par membrane pour une assistance de courte durée et les dispositifs d’assistance ventriculaire pour un support prolongé en pont vers la transplantation. Le but de cette revue est de décrire les difficultés spécifiques de l’assistance circulatoire pédiatrique, les différents dispositifs disponibles, l’augmentation rapide et généralisée du recours à ces techniques, les résultats pour chaque indication et chaque type d’assistance, et enfin de donner une perspective sur les futurs développements et les défis restant à relever.
Background
The number of children being hospitalized for end-stage heart failure, secondary to congenital heart disease (CHD) or primary myocardial disease, is increasing. This number is expected to rise even higher in the coming years, as the number of patients with CHD reaching adulthood is increasing steadily. While heart transplantation remains the mainstay of treatment for refractory heart failure, with generally good long-term survival approaching 70% at 10 years, the mortality rate while awaiting a suitable organ exceeds 20%. Indeed, the number of heart transplantations worldwide has remained stagnant for the last 10 years. Durable mechanical circulatory support (MCS) for the failing heart has been used extensively as a bridge to heart transplantation in adult patients, as numerous ventricular assist devices (VADs) have been available for several decades to provide both temporary and long-term support. Paediatric patients, because of their smaller size and their often complex anatomy and physiology, present a unique set of challenges that has slowed the development of MCS in this population. The use of extracorporeal membrane oxygenation (ECMO), which has long been the sole means of mechanical support for paediatric patients with end-stage cardiac failure, has increased steadily since the 1980s and has contributed to improve survival significantly . However, the short duration of support provided by ECMO (typically 10–20 days) is a major limitation, considering the current waiting times on the transplant list. The use of long-term support with VADs in children as a bridge to transplantation remained sporadic until the early 2000s. However, thanks to the development of suitable devices for infants and small children, mainly the Berlin Heart EXCOR Paediatric VAD (Berlin Heart AG, Berlin, Germany), implantation of VADs in children has grown exponentially in recent years .
Paediatric specificities
There are several critical issues to be considered for the successful support of children. The first issue is the miniaturization of the device to make it suitable for a child’s size, requiring a good understanding of the flow devices with regard to haemolysis, thrombogenesis, immuno-activation (activation of an inflammatory cascade) and effective energy transmission (specifically continuous compared with pulsatile). For geometric reasons, miniaturization of the device results in increased surface area per blood volume, which, in combination with lower flow rates than in adults, increases the risk of thrombogenesis. In addition, the narrow size of the openings and cannulae causes high shear stress during the passage of red blood cells through the device, promoting haemolysis. Finally, miniaturization of all components of the system reduces energy transmission efficiency. Owing to all these challenges, paediatric patients are more likely to benefit from pulsatile operation mode than adults.
Another aspect of paediatric MCS is that the pathophysiology of heart failure is different in children than in adults. Isolated left ventricular dysfunction is rare in children, in whom the need for circulatory support is often due to a combination of right ventricular failure, hypoxaemia and pulmonary hypertension. In this setting, ECMO is the preferable option. Left VAD is used in patients with predominantly left ventricular failure and normal lung function. Biventricular support is more commonly necessary in children with heart failure secondary to CHD. Children with CHD also have intrinsic anatomical variations that can pose significant difficulty in cannulation for MCS (e.g. single ventricle, abnormal systemic venous return, etc.). From a physiological standpoint, previous surgery may further jeopardize the application of MCS (e.g. systemic-pulmonary shunts, disconnected vena cavae in Glenn or Fontan circulations, etc.).
Indications
The two main indications for MCS in children are cardiac medical failure and postcardiotomy (post-surgical) cardiac dysfunction.
Medical indications
Although not as common in children as in adults, dilated cardiomyopathies are the leading cause of heart failure in children without CHD and are the most common indication for non-surgical MCS. According to the Paediatric Cardiomyopathy Registry, the annual incidence of dilated cardiomyopathies in children aged < 18 years is 0.57 cases per 100,000 per year overall, with a much higher incidence in infants than in children (4.40 vs 0.34 cases per 100,000) . The majority of children have idiopathic disease. The most common known cause is myocarditis (46%). As the myocardium may fully recover from viral injury, the presence of myocarditis has been described to be a predictive factor of better outcome in several studies. Indeed, the Paediatric Cardiomyopathy Registry reported a 5-year transplantation-free rate as high as 81% in individuals with myocarditis. In these patients, ECMO can be initiated to rest and unload the heart, allowing the myocardium to recover from injury, as a bridge to recovery, with excellent results. Other factors indicating better prognosis in dilated cardiomyopathy are younger age at diagnosis and higher left ventricular ejection fraction. If the myocardium does not recover, ECMO may then be switched to a long-term means of MCS until heart transplantation.
The optimal timing for initiating MCS is before circulatory collapse, avoiding end-organ injury, particularly neurological damage. As experience with this therapy has increased, the threshold for initiating MCS has been lowered. Currently, MCS should be considered in patients in heart failure requiring a progressive increase in inotropic support (e.g. epinephrine > 0.3 μg/kg/min or requirement of a second inotrope), cardiac index < 2.0 L/min/m 2 , decreased mixed venous saturation (< 40%), lactic acidosis, poor end-organ perfusion evidenced by oliguria (< 1 mL/kg/h), altered mental status, mechanical ventilation requirement, inability to tolerate enteral feeding, rising liver enzymes, rising creatinine, immobility or extreme fatigue. A promising alternative strategy in these potential candidates for MCS has been proposed by some centres, consisting of the use of levosimendan combined with milrinone and nesiritide, while minimizing catecholamine use as much as possible and keeping MCS as back-up. MCS was delayed or avoided in a small series of seven patients managed with this strategy .
ECMO is also used as a rescue therapy during cardiac arrest refractory to conventional cardiopulmonary resuscitation (CPR). In this indication, ECMO is referred to as extracorporeal cardiopulmonary resuscitation (ECPR). The current American Heart Association paediatric advanced life support guidelines recommend consideration of ECPR for in-hospital paediatric cardiac arrest patients failing to respond to initial resuscitation attempts ‘if the condition leading to cardiac arrest is reversible or amenable to heart transplantation’ .
The other indications for ECMO are intoxications with cardiodepressive drugs, life-threatening arrhythmias and hypothermic cardiac arrest, usually due to cold-water drowning.
Surgical indications
The most common indication for ECMO is failure to wean from cardiopulmonary bypass after repair or palliation of CHD. The reported frequency of ECMO use after cardiopulmonary bypass in children is 3–5%. Left VAD may be used as an alternative to ECMO for circulatory support after failure to wean from cardiopulmonary bypass in patients suspected to need circulatory support for a long duration (> 2 weeks), if they do not have pulmonary hypertension or respiratory dysfunction. Patients with anomalous origin of the left coronary artery from the pulmonary artery are a typical example, as the recovery of left ventricular function may be delayed for months after coronary artery surgical reimplantation.
Contraindications
Contraindications to MCS include extreme prematurity, very low birth weight (< 1.5 kg), significant neurological injury and extracardiac malformations with poor prognosis. Relative contraindications are multisystem organ failure, as organ function may be expected to improve with restoration of haemodynamic stability, and chromosomal aberrations. Decisions about the initiation of MCS are made on a case-by-case basis in such patients.
Devices
Extracorporeal membrane oxygenation
ECMO circuits are composed of a centrifugal or roller pump with a hollow-fibre or membrane oxygenator, an oxygen blender, a pump console, a heat exchanger and a pump cart. The site of cannulation varies with the indication for ECMO. Patients requiring support in the immediate postoperative period are cannulated through a sternotomy, via aortic, right atrial and often left atrial cannulae, the last being for decompression of the left ventricle. Peripheral cannulation, via the neck in infants and small children or femoral vessels in older children, is preferred in non-surgical patients. The circuit may be primed with crystalloids or blood products, keeping the prime volume to an absolute minimum to decrease the effect of haemodilution. After initiation, ECMO flow is increased to achieve a goal of 100–150 mL/kg/min. Anticoagulation is provided using heparin infusion to maintain an activated clotting time of 180–240 seconds. Mechanical ventilation support should be continued with low settings while on ECMO to prevent atelectasis. Regarding the duration of support, ECMO provides only short-term support, with a maximum duration of 15–21 days.
Ventricular assist devices
VADs are composed of inflow and outflow cannulae, a pump system, a power source and a system controller. The inflow cannula is attached to the left atrium (short-term devices) or to the apex of the left ventricle (long-term devices) and blood is pumped by the device into the outflow cannula, which is sutured to the ascending aorta. When a right VAD is indicated, the inflow cannula is attached to the right atrium and the outflow cannula into the main pulmonary artery. In contrast to ECMO, the implantation of VAD devices requires central cannulation via sternotomy and, in the case of long-term devices, cardiopulmonary bypass.
The VAD devices can be classified as short- or long-term, with 2 weeks being the usual limit for short-term VADs. The ejection can be achieved with centrifugal, pneumatic pusher plate or axial flow pumps ( Fig. 1 ). In centrifugal pumps, blood enters in the centre of a rotor and has direct contact with the motor parts ( Fig. 1 A). In the pneumatic pusher plate systems, blood runs through a closed polyurethane pouch with an inlet and an outlet valve ( Fig. 1 C). The blood-filled pouch is separated from an air-filled chamber by a thin membrane. Compressed air drives the blood out of the pouch in systole, and negative pressure drives the blood into the pouch in diastole. The resulting flow is pulsatile. In the axial design, a rotating impeller is suspended in a narrow housing and driven by an electrical motor ( Fig. 1 B). As blood flows through the narrow chamber, it is continuously accelerated by the blades of the impeller. These devices have a low profile and may be implanted in a large blood vessel.
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