These are mechanical devices that essentially support the pumping function of the heart. As such they have a role in both acute and chronic heart failure which is refractory to optimal medical treatment. They can support either the left ventricle (LVAD), the right ventricle (RVAD), or both (BIVAD).
Can serve as a short term bridge to recovery. They are indicated in acute cardiogenic shock. The aetiology of the acute failure can be variable e.g. acute MI, viral myocarditis. In these scenarios a BIVAD device is typically required. Devices are generally short term in nature and require a sternotomy for catheter insertion, which much of the device extracorporeal. Extracorporeal membrane oxygenation (ECMO) is also an option for these patients, providing respiratory support as well. However, these are resource intensive and not really suitable for long term use. Chronic Heart Failure Heart transplantation remains the most successful intervention for patients with refractory chronic heart failure. However, the limited pool of donor organs and the long waiting time, has meant that the VAD has become used as a bridge to transplantation. This allows optimisation of donor physiology by supporting cardiac function and improved quality of life, including potential discharge from hospital. These devices are generally intracorporeal and the hardware is approximately 20-30 times the cost of short term devices. Use of VADs as a destination therapy is also increasingly being used, though not in the UK. The aim is primarily for LVAD only, as BIVAD devices are associated with a significant degree of complications. There is often secondary RV failure as part of the patient's underlying condition, and so there is often some improvement in RV function with LVAD insertion. Temporary RV support may be employed to assess for this improvement. If this doesn’t occur, then there may be consideration for a totally artificial heart.
There has been progressive development of the design of such devices, with newer generations showing notably improvements. First generation. A pumping chamber mimicked usually cardiac function. The driver for this process could be hydraulic or pneumatic. Produced pulsatile blood flow. Significant morbidity due to extent of resection for implantation. Durability of about 2 years with significant surgical risk for device exchange. Second generation e.g. Heartmate II Employ a rotary pump with continuous axial blood flow. An external magnet acts to drive the internal propeller system. Propeller system contains mechanical bearings. Third Generation e.g. Levacor VAD Contain ‘contactless bearing’ as the impeller is suspended in a magnetic field, thus reducing blood trauma. Flow is generated by rotation of the impeller, driven by the external magnet. There are several theoretical advantages to their design, including improved durability, and improved responsiveness to variations in flow.
The appropriate selection of patients, and the timing of insertion, play a central role in determining the success of the VAD. Several factors need consideration and there are a number of contraindications. Cardiovascular
Atrial arrhythmias are not a contraindication.
Ventricular arrhythmias need aggressive management perioperatively but it is possible for them to occur asymptomatically in patients with a LVAD and due to the support the device provides.
Aortic insufficiency can be very detrimental to VAD function, causing recirculation of blood. Even mild disease can be problematic and so needs assessment for and likely correction in order to proceed.
Similarly mitral stenosis must be corrected at the time of implantation. Mitral regurgitation can improve with device insertion (reduced LV dilatation).
As noted, BIVAD selection is generally undesirable, although 20-25% of patients with an LVAD may progress to RV failure. Fixed pulmonary hypertension is a contraindication.
Perioperative inotropic support may be needed.
Intracardiac defects need identification and treatment prior to insertion due to risk of shunts developing
Severe respiratory dysfunction is a contraindication e.g. FEV1 < 1L
Severe hepatic dysfunction is a contraindication e.g. liver cirrhosis and portal hypertension.
If there is doubt, a liver biopsy should be undertaken.
Renal dysfunction related to poor cardiac outcome is often improved with device insertion.
However, poor renal function (eGFR < 50ml/min/m^2) is associated with a poor outcome.
Long term dialysis is a contraindication.
Patients will need to be able to deal with the management of such devices. Therefore an appropriate degree of psychological stability and social support is needed.
Anticoagulation will be needed so there should be no contraindications to this.
This video provides a bit of an overview of LVADs: https://www.youtube.com/watch?v=EmFNvjmAmh0
Intra-aortic Balloon Pumps
The IABP remains the most widely used mechanical cardiovascular support for patients with cardiac disease. The concept was first deployed in 1968 and has had ongoing refinement over the subsequent time period.
The IABP uses the process of counterpulsation. This essentially involves the deployment of balloon inflation to assist left ventricular function. A balloon is inserted through the femoral artery and sits in the descending aorta, with the tip just distal to the left subclavian artery. It is timed with the cardiac cycle to function as follows:
In diastole, the balloon rapidly inflates, displacing the blood in the aorta.
This displaces blood proximally and distally, improving organ perfusion.
In systole, the balloon rapidly deflates, thereby decreasing the afterload that the LV has to work against
The goal of this is support the left ventricle by:
Improving myocardial oxygen delivery (via the diastolic perfusion augmentation)
Reducing myocardial oxygen demand (by improving the cardiovascular mechanics through reduced afterload and preload).
These videos are good for providing an introduction to the concepts: https://www.youtube.com/watch?v=KQxsPH3TAa4 https://www.youtube.com/watch?v=Sd8-RCgCc6I
The IABP is a 8-9.5 French catheter with a 25-50ml balloon. It has two ports; one delivers the driving gas, the other transduces a pressure waveform from the aorta. It is attached to a console that controls the balloon. The balloon is driven by helium gas, as it’s low density allows rapid movement and it is rapidly absorbed by the blood in the event of a balloon rupture. Before insertion, the correct balloon size is selected based on a provided chart using the patient’s height. When fully expanded it should not occupy more that 85-90% of the thoracic aortic diameter. The balloon is inserted into the femoral artery using a modified seldinger approach. It is threaded up the aorta, usually under fluoroscopic guidance, so that the tip is 2-3cm distal to the origin of the left subclavian artery. The balloon can then be set up to achieve the appropriate deflation and deflation with the patient's cardiac cycle. This is usually timed using the patient's ECG, but the arterial waveform can also be used. The ratio of support can be changed, including 1:1, 1:2 or 1:3. Initially it is usual for every heartbeat will be supported, but the ratio can be dropped to facilitate weaning from the machine.
Acute MI - can optimise cardiac function whilst definitive treatment is initiated
Cardiogenic shock - a class 1 indication for cases not rapidly reversed by pharmacological therapy
Acute MR e.g. following papillary muscle rupture
Ventricular failure of non-coronary aetiology e.g. myocarditis
Post cardiac surgery - can help weaning off bypass
Aortic regurgitation - IABP worsens this
End-stage disease with no prospect of recovery
Severe peripheral vascular disease
Links & References
Harris, P. Kuppurao, L. Ventricular assist devices. CEACCP. 2012. 12(3). 145-151
Krishna, M. Zacharowski, K. Principles of intra-aortic balloon pump counterpulsation. CEACCP. 2009. 9(1). 24-28