Abstract
Mechanical circulatory support (MCS) can be used in a setting of either acute or chronic heart failure in selected patients refractory to medical management, to augment the failing circulation due to pump failure and to avert organ failure. There are a variety of devices available, which may be employed for short- or long-term support. Longer-term support may only be afforded by a ventricular assist device (VAD) or cardiac transplantation. VADs are used most commonly for left ventricular support (LVADs), but may also be used for the RV (RVADs) or for biventricular support (BIVADs).
Introduction
Mechanical circulatory support (MCS) can be used in a setting of either acute or chronic heart failure in selected patients refractory to medical management, to augment the failing circulation due to pump failure and to avert organ failure. There are a variety of devices available, which may be employed for short- or long-term support. Longer-term support may only be afforded by a ventricular assist device (VAD) or cardiac transplantation. VADs are used most commonly for left ventricular support (LVADs), but may also be used for the RV (RVADs) or for biventricular support (BIVADs).
Over recent years there have been considerable technological advances in the various devices available, making them smaller and less traumatic once implanted, with improved safety profiles and reliability. The principal driver has been the gap between the supply and demand of organs for cardiac transplantation, which remains the ‘gold standard’ of treatment for eligible patients with non-recoverable heart failure.
VADs may be implanted as destination therapy for those ineligible for transplant (age or co-morbidities) or as either a bridge to recovery of heart function after a period of ventricular unloading; or as a bridge to transplantation (BTT) which represents the bulk (80–90%) of implants. Destination therapy is not currently approved in the UK.
The main aims of LVAD therapy are to improve heart failure symptoms, which are a huge burden to affected patients in terms of quality of life and prognosis. Other than transplantation, LVADs are the only option that can significantly improve the symptoms of advanced NYHA classes 3–4 heart failure. In addition, LVADs allow stabilization and indeed reversal of organ dysfunction (renal and liver in particular), PHT and PVR, which if high may preclude transplantation. Published in 2001, the Randomized Evaluation of Mechanical Assistance for the Treatment of Congestive Heart Failure (REMATCH) trial showed better survival (more than double at 1 year) and quality of life compared with optimal medical management using a first-generation LVAD (HeartMate V). With current devices there is >70% survival at 2 years.
Devices and Design
VADs comprise an inflow cannula, a pump (which may be implantable or extracorporeal), a power supply and an outflow cannula. The pump is connected to a controller unit by a driveline. In the case of an LVAD, blood from the LA or LV is pumped to the aorta, maintaining the circulation and decompressing the LV. In an RVAD, blood from either the RA or RV is pumped to the PA to support the failing RV. For biventricular support, two separate pumps are required; in the case of the Total Artificial Heart this may be a fully implantable system. Devices differ in their flow characteristics (non-pulsatile or pulsatile), implantability and ability to support the left or right heart. Newer devices generate a continuous non-pulsatile flow, so generate no pulse, and are designed to be small, easily implantable and cause minimal damage to the blood. LVAD patients may or may not have a pulse depending on intrinsic LV function and VAD flow.
Indications and Patient Selection
For MCS devices, the Interagency Registry for Mechanically Assisted Circulatory Support (INTERMACS) profile is a useful guide that helps classify patients with heart failure at the time of implantation (Table 16.1). The management strategy and choice of device largely depends on urgency and risk. Other factors involved in decision-making include anatomical barriers to device placement and function, perioperative risk (sepsis, bleeding risk, organ failure, etc.), the likely need for additional temporary or long-term RV support (RVAD or BIVAD) and the estimated time a patient might spend on the transplant waiting list.
Table 16.1 INTERMACS® patient profiles
| Profile | Description/patient characteristics | Time frame |
|---|---|---|
| 1 | Critical cardiogenic shock ‘Crashing and burning’, life-threatening hypotension and rapidly escalating inotropic and vasopressor support, with critical organ hypoperfusion often manifested by worsening acidosis and lactate levels | Hours |
| 2 | Progressive decline ‘Dependent’ on inotropic support but deterioration of nutrition, renal function, fluid retention or other major status indicator | Days |
| 3 | Stable but inotrope-dependent Clinically stable on, but unable to wean off, mild–moderate doses of IV inotropes (or with a temporary MCS device) | Days–weeks |
| 4 | Symptoms at rest Patients can be managed at home on oral therapy | Weeks–months |
| 5 | Comfortable at rest but limited by exertion Patients can be managed at home on oral therapy | Months–years |
| 6 | Comfortable at rest but exertion intolerant Patients can be managed at home on oral therapy | |
| 7 | NYHA class 3 equivalent Patients can be managed at home on oral therapy |
Acute Heart Failure and Cardiogenic Shock
In the situation of critical cardiogenic shock refractory to inotropic support, or with progressive organ dysfunction despite the use of inotropes (INTERMACS profile 1), temporary MCS is indicated. The IABP is commonly used as a temporizing measure especially in the setting of cardiogenic shock following acute MI and coronary intervention; however, outcomes have not been shown to be significantly different to standard care. Veno-arterial ECMO is usually the first-line treatment in this setting as it is can be instituted relatively quickly (especially when inserted percutaneously) and provides rapid support of the failing circulation.
There are a variety of other short-term MCS devices that may be used (Table 16.2), some designed for a few days (e.g. Impella); others suitable for longer. The aim is to bridge the patient to recovery, or to the next ‘decision’ – definitive reparative surgery, additional medical therapy, long-term VAD implantation or transplantation. The most common indications for acute VAD implantation outside of the setting of the cardiac catheter laboratory are post-cardiotomy heart failure (i.e. failure to wean from CPB) and acute myocarditis, where there is potential for recovery. An LVAD has particular advantages over ECMO in that it will unload a failing LV and reduce the LV wall tension, which may better facilitate recovery. A common indication for a temporary RVAD is RV failure after LVAD implantation. Long-term implantable VADs are very rarely used in the acute setting, particularly when the patient’s neurological status is unknown.
| Device | Mechanism | Notes |
|---|---|---|
| CentriMag® (Thoratec Corporation, St Jude Medical) | Impeller, centrifugal flow | LVAD, RVAD or BIVAD support An oxygenator may be incorporated into circuit |
| Thoratec pVAD | Pneumatic, pulsatile flow | Capable of LVAD, RVAD or BIVAD support Bulky Paracorporeal >30 years’ clinical experience |
| Protek Duo catheter (CardiacAssist Inc, USA) | Connect to blood pump such as CentriMag to work as a continuous flow RVAD | Percutaneously placed dual-lumen catheter, diverts blood from RA to PA |
| Abiomed Impella (Abiomed Inc, USA) | Sits across AV into LV, flow using microaxial blood pump Up to 2.5 and 5 l min–1 blood flow capability versions available | Percutaneously placed in catheter laboratory Impella 5.0™ can support blood flow up to 5 l min–1, licensed up to 6 days Can also be placed surgically (Impella LD™) |
| HeartMate PHP (Thoratec Corporation, St Jude Medical) | Similar to Impella Flows around 4 l min–1 | Percutaneous, catheter based |
A commonly used device in the emergent setting is the CentriMag® system (Figure 16.1). This comprises a high durable magnetically levitated centrifugal (impeller) pump, capable of generating flows of up to 10 l min–1 with minimal damage to blood. This system may be used as an LVAD or RVAD, with or without an oxygenator. Tunnelling the inflow and outflow cannulae through the epigastric skin permits chest closure and aids nursing care.
