22 – Ventricular Assist Devices (VAD)




22 Ventricular Assist Devices (VAD)


Harikrishna M Doshi and Steven SL Tsui



Introduction


The UK National Heart Failure Audit 2013–2014 estimated that there are 2 million people in the UK suffering from heart failure, a figure that is likely to rise due to improved survival following heart attack and more effective treatment. Heart transplant has remained a therapeutic cornerstone for patients with end stage heart failure when optimum medical therapy has failed. It offers an excellent short and long term outcome with median survival of 11 years. Major limitation in expansion of heart transplantation is the limited availability of donor organs. According to the 2015 Annual Report on Cardiothoracic Transplantation in the UK, only 30% of non-urgent heart patients were transplanted within 6 months of their listing, during which time 9% died awaiting a transplant.


Exciting new developments are being trialled to increase the donor pool, for example transplanting heart from donation after circulatory death. However, donor heart numbers will never satisfy the rapidly expanding cohort of patients who require cardiac replacement therapy. Many such patients are over 75 years of age and with comorbidities which make them unsuitable for heart transplant. Besides, many patients on the list deteriorate and cannot survive the wait for a transplant without other intervention. In 1978, Dr Denton Cooley reported the first successful bridging to transplant using mechanical heart support, and since then rapid strides have been made in the field of mechanical circulatory support devices (MCSD).


Ventricular assist devices (VAD) are mechanical blood pumps that work in parallel or in series with the native ventricle(s). Most commonly, a left ventricular assist device (LVAD) draws oxygenated blood from the left atrium or ventricle and returns it to the aorta; a right ventricular assist device (RVAD) draws venous blood from the right atrium or ventricle and returns it to the pulmonary artery. VADs provide life support for patients with failing ventricle(s) by maintaining perfusion to vital organs. The basic components of a VAD system are as follows.




  1. 1. Inflow cannula – for draining blood from the patient to the blood pump.



  2. 2. Blood pump – usually consists of a motor and an impeller for driving the blood.



  3. 3. Outflow cannula – returns blood from the pump to the patient.



  4. 4. Driveline – a composite cable for the transfer of electrical signal and power between the blood pump and the console.



  5. 5. Controller – controls blood pump operation and displays pump performance parameters.



  6. 6. Rechargeable battery pack – to power the motor of the blood pump.



Temporary MCSD



Indications


For patients who require mechanical circulatory support (MCS) over days to weeks, temporary devices may be used as a bridge to recovery, bridge to transplant or bridge to a longer term device, i.e. bridge to bridge. Temporary MCS (Table 22.1) may be indicated in patients with decompensated heart failure, acute cardiogenic shock or arrested patients with uncertain neurology, to assess recovery of end organ function before deciding on the next step (bridge to decision). They may also be considered for patients with severe symptomatic acute heart failure where myocardial recovery is anticipated, for example fulminant myocarditis. Occasionally it may be indicated in postcardiotomy cardiogenic shock where weaning from cardiopulmonary bypass fails. The aim is to provide short term support while other medical problems such as infection, renal failure and neurological assessment can be dealt with and prognosis of the patient can be better defined.




Table 22.1 Indications of temporary MCSD










  • Cardiogenic shock due to acute myocardial infarction



  • Postcardiotomy shock



  • High risk percutaneous coronary intervention, ventricular tachycardia ablation



  • Acute rejection post cardiac transplant with haemodynamic compromise



  • Bridge to LVAD or transplant



  • Right ventricular failure



Types of Temporary Mechanical Circulatory Support




  1. 1. Extracorporeal life support (ECLS) or cardiac ECMO (see Chapter 23):




    1. (a) Peripheral cannulation;



    2. (b) Central cannulation.



  2. 2. Percutaneous devices:




    1. (a) Intra-aortic balloon pump (IABP) (see Chapter 21);



    2. (b) Impella devices: 2.5, CP, 5.0, LD (Abiomed Inc., MA, USA), transaortic intraventricular pump;



    3. (c) TandemHeart System (CardiacAssist, Inc., PA, USA), transvenous transseptal left atrium to arterial pump.



  3. 3. Non-percutaneous devices: for example CentriMag centrifugal pump (Thoratec Corporation, CA, USA).


    Characteristics of Impella devices are given in Table 22.2, and contraindications to temporary MCSD are listed in Table 22.3.



Non-percutaneous Devices: CentriMag


The CentriMag (Thoratec Corporation, CA, USA) system is an extracorporeal circulatory support device. It has a disposable polycarbonate pump (Figure 22.1) mounted on a reusable motor. The magnetically levitated pump impeller provides a friction-free blood path which minimises blood trauma and haemolysis. It is approved for use for up to 30 days and has been widely adopted with reasonable success. This device can produce flows of up to 9.9 l min−1 in vitro and uses a priming volume of 31ml. The CentriMag system can be used to provide left or right ventricular support and it can also be used in an ECMO circuit. The CentriMag system is most commonly implanted through a sternotomy incision in the operating room. Anchoring of the percutaneous vascular cannulae as they traverse the anterior abdominal wall allows the patient limited mobilisation and rehabilitation. Bedside pump changes can be performed in awake patients every 30 days, allowing patients to be supported for months on the CentriMag system.





Figure 22.1 CentriMag primary console and centrifugal pump (courtesy of Thoratec Corporation, CA, USA).




Table 22.2 Types of Impella devices and characteristics




















































Device and function Impella 2.5 Impella CP Impella 5/LD Impella RP
Access Percutaneous femoral Percutaneous femoral Surgical, axillary, femoral or ascending aorta Percutaneous femoral vein
Maximum output, litre per minute 2.5 4.0 5.0 4.6
Catheter size, F 9 9 9 11
Motor size, F 12 14 21 22
Maximum revolutions per minute (rpm) 51,000 46,000 33,000 33,000
EU approved duration of use, days 5 5 10 14



Table 22.3 Contraindications and complications associated with temporary circulatory support




























Device Contraindications Complications
All devices


  • Severe peripheral vascular disease



  • Irreversible neurological disease



  • Sepsis




  • Bleeding



  • Vascular injury



  • Infection



  • Neurological injury

Impella


  • LV thrombus



  • Moderate to severe aortic stenosis



  • Moderate to severe aortic insufficiency



  • Mechanical aortic valve



  • Recent TIA or stroke



  • Aortic abnormalities



  • Contraindication to anticoagulation




  • Haemolysis



  • Pump migration



  • Aortic valve injury



  • Aortic insufficiency



  • Tamponade due to LV perforation



  • Ventricular arrhythmia

TandemHeart


  • Ventricular septal defect



  • Moderate to severe aortic insufficiency



  • Contraindication to anticoagulation




  • Cannula migration



  • Tamponade due to perforation



  • Thromboembolism



  • Air embolism during cannula insertion



  • Interatrial shunt development

CentriMag Contraindication to anticoagulation


  • Thromboembolic events



  • Air embolism



Implantable or Durable VAD


Implantable VAD are designed to provide durable circulatory support for patients with advanced heart faiure. Patients can be supported with these devices for months or years. The main advantage of durable VAD is to allow implanted patients to return to the community and lead an active life. Currently, LVAD is the most common type of durable MCSD implanted.



Types of Long Term VAD




  • First generation LVADs such as the Thoratec HeartMate XVE, PVAD and IVAD were pulsatile devices. They consisted of a pump housing with a diaphragm and unidirectional valves to pump blood in a pulsatile manner, typically ejecting 80 to 100 times per minute. In 2001, results of the REMATCH study showed that patients with end stage heart failure not eligible for transplantation who were implanted with the HeartMate VE had a significant survival advantage at 1 year compared with patients who were managed with optimal medical therapy. However, first generation LVAD were bulky, less durable and had high complication rates including bleeding, infection, stroke, device malfunction and development of right heart failure. Over the last 10 years, outcomes with continuous flow devices have been shown to be far superior to outcomes with pulsatile devices. As a result, first generation LVADs have become obsolete.



  • Second generation VAD are axial flow pumps with a single moving impeller mounted on mechanical bearings spinning at high speed. They are smaller in size and more durable than their predecessors. These devices are silent in operation and provide continuous blood flow with reduced pulsatility. They include devices like the Thoratec HeartMate II and Jarvik 2000 amongst others.



  • Third generation pumps use hydrodynamic and/or electromagnetic bearings to suspend the impeller inside the pump housing, thereby eliminating contact between moving parts and avoiding friction. Their smaller size allows intrathoracic placement (Figure 22.2) and obviates the need for an abdominal pump pocket. Examples of third generation devices include the HeartWare HVAD and the Thoratec HeartMate 3.





Figure 22.2 (a) HeartMate 3 with its controller. (b) HeartWare HVAD with driveline. (c) Diagram of a patient with a HeartWare HVAD sited in the left ventricular apex and a percutaneous driveline going to a wearable controller and batteries. (Courtesy of Thoratec Corporation, CA, USA and HeartWare Corporation, MA, USA).



Indications and Patient Selection


Current indications for LVAD therapy can be broadly classified as follows.




  1. 1. Bridge to transplant (BTT): For patients eligible for heart transplant but deteriorating before a donor heart is available.



  2. 2. Bridge to candidacy (BTC): For patients with contraindication(s) to heart transplantation secondary to advanced heart failure, such as renal dysfunction or pulmonary hypertension, that is potentially reversible after a period of LVAD support.



  3. 3. Destination therapy (DT): As a permanent treatment for patients who are not eligible for heart transplant.


The Interagency Registry for Mechanically Assisted Circulatory Support (INTERMACS) is a North American registry established in 2005 to collect clinical data on patients receiving mechanical circulatory support. The INTERMACS stratifies patients with advanced heart failure into seven profiles based on their clinical status as outlined in Table 22.4.




Table 22.4 INTERMACS profile descriptions








































INTERMACS level Definition Description
1: Critical cardiogenic shock Crash and burn Haemodynamic instability in spite of increasing doses of catecholamines and/or mechanical circulatory support with critical hypoperfusion of target organs (severe cardiogenic shock)
2: Progressive decline Sliding on inotropes Intravenous inotropic support with acceptable blood pressure but rapid deterioration of kidney function, nutritional state, or signs of congestion
3: Stable but inotrope dependent Dependent stability Patient with stable blood pressure, organ function, nutrition and symptoms on continuous intravenous inotropic support (or a temporary circulatory support device or both), but demonstrating repeated failure to wean from support due to recurrent symptomatic hypotension or renal dysfunction
4: Resting symptoms Frequent fliers Patient can be stabilised close to normal volume status but experiences daily symptoms of congestion at rest or during activities of daily living (ADL); doses of diuretics generally fluctuate at very high levels
5: Exertion intolerant Housebound Complete cessation of physical activity, stable at rest, but frequently with moderate water retention and some level of kidney dysfunction
6: Exertion limited Walking wounded Minor limitation on physical activity and absence of congestion while at rest; easily fatigued by light activity
7: Advanced NYHA III Placeholder Patient in NYHA functional class II or III with no current or recent unstable fluid balance


Risk Factors and Their Assessment for Implantation for VAD



Cardiac Factors


Right ventricular (RV) function is one of the most important parameters to consider before LVAD implantation. Adequacy of RV function after LVAD implantation is a balance between the intrinsic RV function and the RV afterload. During RV ejection, RV afterload is proportionate to pulmonary artery pressure, which is a sum of the impedance of the pulmonary vasculature and the left atrial pressure.


A number of risk prediction models for RV failure following LVAD insertion have been described. However, these have all been derived from retrospective studies on patients supported with pulsatile devices. A validation study on continuous-flow LVAD recipients showed that none of the described risk models reliably predicted the need for RVAD support post LVAD. In any case, preoperative risk models would not be able to take account of intraoperative events.



Echocardiographic Assessment

Numerous echocardiographic parameters for assessing RV function have been described and some are listed below. However, their usefulness in predicting RV failure after LVAD implant are still debated.




  • Visual assessment: volumetric assessment of RV function is challenging and many physicians rely on visual assessment to estimate RV size and function.



  • Tricuspid annular plane systolic excursion (TAPSE): assess RV systolic longitudinal function of RV. TAPSE < 16 mm indicates RV systolic dysfunction. TAPSE correlates well with RV global systolic function.



  • Right ventricular index of myocardial performance (RIMP): RIMP > 0.40 by pulse Doppler indicates RV dysfunction; fractional area change (FAC) <35% indicates dysfunction.



  • RV diastolic function can be assessed by tricuspid inflow velocities, pulsed Doppler imaging of hepatic veins and measurement of IVC size and collapsibility. Tricuspid E/A ratio <0.8 indicates impaired relaxation.



  • Tricuspid valve regurgitation (TR) can help assessment of right atrial pressure in the absence of right ventricular outflow obstruction (RVOT). TR velocity >2.8 to 2.9 m/second corresponds to systolic pulmonary artery pressure (PAP) of approximately 36 mmHg assuming right atrial pressure (RAP) of 3–5 mmHg, and indicates elevated RV systolic and pulmonary artery pressures.

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Jan 9, 2021 | Posted by in CARDIOLOGY | Comments Off on 22 – Ventricular Assist Devices (VAD)

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