INDICATIONS FOR MECHANICAL CIRCULATORY SUPPORT
Children with severe cardiac or respiratory failure that is refractory to medical management may require mechanical cardiorespiratory support. Transient respiratory and/or circulatory support can be accomplished using extracorporeal membrane oxygenation (ECMO) (see Chapter 25), but prolonged use of ECMO is associated with increased morbidity and mortality due to the inflammation, thromboembolic complications, and issues with durability related to the oxygenator. Longer-term circulatory support without an oxygenator can be accomplished using a ventricular assist device (VAD) in patients with adequate pulmonary status. Currently, only the Berlin EXCOR is approved by the Food and Drug Administration (FDA) for pediatric use, but a number of adult VADs are being used in older children and adolescents.
CHOOSING A DEVICE
Duration of Therapy: The anticipated duration of therapy is an important component of device selection.
Short-Term Therapy (CentriMag/PediMag, RotaFlow, Impella, TandemHeart):
Patients experiencing an acute process (classically myocarditis) with hope of possible recovery may benefit from a short-term device that is easier to explant.
These devices typically last 2 to 4 weeks (similar duration to ECMO without the sequelae of the oxygenator).
These devices can be used while the medical team weighs options (“bridge to decision”) or as a pathway to eventual device explant (“bridge to recovery”) or conversion to a long-term VAD (“bridge to bridge”). On some occasions a donor heart may become available fast enough for the device to serve as a “bridge to transplant.”
Long-Term or Destination Therapy:
Patients requiring long-term support as a bridge to transplant will be candidates for a different set of devices.
Currently, the devices routinely available for pediatric long-term use are the Berlin EXCOR, HeartWare VAD, HeartMate II, and the SynCardia Total Artificial Heart (TAH).
Some centers are also beginning to offer VAD placement as destination therapy in those who are not candidates for transplant (ex: Duchenne’s muscular dystrophy).
Type of Flow: There are two types of flow patterns: pulsatile and centrifugal. Whereas pulsatile flow more closely mimics the normal heart function, continuous-flow devices are smaller and more durable.
Berlin EXCOR: Only available device for infants and young children
Pump system sits outside the body (paracorporeal).
The pump is composed of a blood chamber and an air chamber, which are separated by a diaphragm. A driving unit forces air into and out of the chamber, which moves the diaphragm. The diaphragm’s movement draws blood into the chamber via the inflow cannula and then pushes it out to the body via the outflow cannula.
Each pump has valves to ensure that blood can only flow forward.
The inflow cannula typically drains the patient’s atrium. The outflow cannula pumps blood back into one of the great vessels (RA, PA for RVAD vs. LA, Ao for LVAD).
Syncardia Total Artificial Heart
Native ventricles and heart valves are explanted so that artificial right and left ventricular pumps can be placed in situ.
Each half of the device consists of a chamber with a tilting-disk artificial valve in both the inflow and outflow position. Each pumping chamber is connected to the native atrium and to the corresponding great vessel.
Continuous Flow-Rotary Pump Devices (see Figure 36-2): Pump is made of a single rotating element that drives blood forward. Continuous-flow devices are smaller and more efficient than pulsatile pumps and are associated with improved mortality. Continuous-flow devices have largely replaced pulsatile pumps in adults. The long-term effects of decreased arterial pulsatility on various organ systems are not yet completely understood.
Axial Flow (HeartMate II, DeBakey VAD, infant Jarvik):
Generates flow by rotating its internal impeller.
Because the impeller is very small, it has to be operated at a very high revolutions per minute (RPM) to generate adequate flow.
TandemHeart is placed percutaneously, which limits feasibility in smaller children. Some centers use an open surgical approach, which allows use in smaller patients.
Centrifugal Flow (CentriMag, PediMag, Rotaflow, TandemHeart, HeartWare HVAD):
Generates flow by rotating a bladed disk that “throws” blood forward
Typically operates at a lower RPM than an axial flow device, which may lead to less red blood cell (RBC) shearing/hemolysis
Even the smallest long-term device is not recommended by the manufacturer for use in infants or very small children, although there are case reports of use after device modification in some centers
The Berlin EXCOR can be used successfully in infants as small as 3 kg
Continuous-flow devices are designed for adults, but have been adapted for use in smaller children
Some long-term continuous-flow devices have been used in children as small as ∼0.7 m2 in body surface area (BSA) (ex: HeartMate II) at centers with significant VAD expertise.
The infant Jarvik device can be used in even smaller children, but its use has been limited by associated significant intravascular hemolysis. The new infant Jarvik 2015 model is currently undergoing study in the “PumpKIN” clinical trial.
TandemHeart is placed percutaneously, which limits feasibility in smaller children. Some centers use an open surgical approach and placement of a recirculation bridge, which allows use in smaller patients who require lower flows and cannot accommodate large percutaneous cannulae.
The SynCardia Total Artificial Heart has specific size requirements (BSA and anteroposterior [AP] chest diameter), which currently limits its use to larger adolescents. A smaller SynCardia device is currently undergoing investigation.
Single vs. biventricular support: VAD selection must take into account the functioning of both ventricles. Many times LVAD support will offload the RV such that an RVAD is unnecessary. The most bi-VAD experience is with the Berlin EXCOR, although other devices have also been attempted. In patients who are large enough, the SynCardia TAH may be the best option for bi-VAD support.
Patients with restrictive physiology or refractory arrhythmias may benefit from explanting the native heart and placement of the SynCardia TAH.
Patients with complex congenital heart disease may be anatomically ill suited for traditional VAD and may benefit from SynCardia TAH. In these patients, complex atrial reconstructions may be necessary.
Although patients with single-ventricle physiology have increased mortality after Berlin EXCOR placement compared to patients with dilated cardiomyopathy, mortality while awaiting transplant is still improved compared to ECMO.
Many centers have extensive experience with Berlin VAD placement in single-ventricle patients.
Outcomes better after Glenn or Fontan procedure than after stage I Norwood.
Failing Fontan physiology is best supported if pathophysiology is ventricular pump failure rather than elevated central venous pressure (protein-losing enteropathy [PLE], plastic bronchitis, etc.)
Ventricular assist devices commonly used in pediatrics. (A) The adult, child, and infant Jarvik devices (Used with permission from Dr. Robert Jarvik). (B) The RotaFlow assist device (Used with permission from Getinge). (C) The PediMag assist device (Reproduced with permission of St. Jude Medical, ©2018. All rights reserved). (D) The TandemHeart assist device (Used with permission from CardiacAssist Inc). (E) The Berlin Heart EXCOR 60, 50, 30, 25, and 10 mL devices (Used with permission from Berlin Heart Inc). (F) The Thoratec assist device (Reproduced with permission of St. Jude Medical, ©2018. All rights reserved). (G) The HeartMate II assist device (Reproduced with permission of St. Jude Medical, ©2018. All rights reserved). (H) The HeartWare assist device (Reproduced with permission of Medtronic, Inc). (I) The SynCardia Total Artificial Heart (Courtesy of syncardia.com).