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.

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).

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).

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.

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