IABP

IABP is the most commonly used device for mechanical circulatory support. It consists of two parts: the intra-aortic balloon itself (IAB), a double lumen catheter 7–8 French size; and a console containing the controller, pump and helium cylinder. The outer lumen of the balloon is a gas containing chamber and the inner lumen is open to the aorta and is used for direct arterial pressure monitoring. Helium is less soluble than some other gases (CO₂) and it is also less dense, which reduces travel time along the circuit and allows quick inflation and deflation of the balloon. IABP is usually positioned in the descending thoracic aorta through one of the femoral arteries using the Seldinger technique.

The balloon inflates during diastole and displaces the blood in the aorta, increasing diastolic and mean pressure and thus augmenting coronary perfusion. During systole, the balloon deflates, producing some vacuum effect and therefore reducing the afterload for the left ventricle (LV) and systolic pressure. This results in improved emptying of the heart, and therefore reduction of the wall stress of the heart. Other important haemodynamic effects of IABP include: reduction of the heart rate (by 20%), decrease in the pulmonary capillary wedge pressure (by 20%) and rise in cardiac output (by 0.5 l/min or by 20%). The magnitude of the effects can vary and depends on many factors: the volume of the IABP (bigger balloons, 50cm³, displace more volume, hence are more effective), its optimal position (the tip of the IAB should be located 2 cm caudally to the origin of the left subclavian artery which is easily confirmed by chest X-ray as 2 cm above the tracheal bifurcation, or by TOE), optimal timing of the inflation/deflation cycle, heart rate and aortic compliance.

The maximum haemodynamic effect from IABP counterpulsation is achieved when it is set to inflate just after the aortic valve closure (dichrotic notch on the arterial trace) and deflate just before aortic valve opening (upstroke part on the arterial waveform of the next cardiac cycle). Early/late inflation as well as early/late deflation could lead to suboptimal diastolic augmentation and reduced haemodynamic benefits of the IABP (Figure 21.1).

Figure 21.1 Aortic pressure waveform with IABP off and on. Inflation of the balloon corresponds to the augmented diastolic pressure. Note the decrease of systolic pressure in assisted beat.

Timing of IABP is important as too early or too late inflation or deflation can in fact worsen cardiac output and increase preload. Figure 21.2 illustrates some problems with timing of IABP.

Figure 21.2 Timing problems with IABP.

Alternative Routes for IABP Insertion

Transfemoral percutaneous method of insertion is a first choice option for IAB insertion. However, it can be technically challenging or impossible in patients with significant peripheral vascular disease or previous vascular operations on the iliofemoral segment or just small calibre arteries. Subclavian, axillary, brachial arteries have been reported to be used to access the thoracic aorta for IAB placement in different clinical settings. Direct access to the thoracic aorta can also be used as the entry site for the IABP. This technique requires an open chest and is therefore used intraoperatively during cardiac operations (Figure 21.3).

Figure 21.3 Transthoracic approach to IABP insertion.

Impella (LV to Aorta)

The Impella (Abiomed, USA) is a temporary ventricular assist device designed for percutaneous insertion through the femoral artery, which is advanced along the aorta, across the aortic valve into the left ventricle (Figure 21.4). It employs the principle of the Archimedes screw: blood enters the inflow at the tip of the catheter and is transported to the ascending aorta, generating non-pulsatile axial flow. Four versions of the pump are currently available: Impella 2.5 (flow up to 2.5 l/min), Impella CP (3.0–4.0 l/min), Impella 5.0 (up to 5 l/min) and Impella RP (designed for right ventricular (RV) support, this drains the blood from the inferior vena cava and expels it to the pulmonary artery, providing flow above 4 l/min). Most of these modifications can be inserted using a standard Seldinger technique. Impella 5.0, although providing higher flows, requires surgical cut-down to the artery due to its bigger size, making it less convenient for insertion in an emergency setting.

Working in series with the LV (or RV in the case of Impella RP) the device unloads the ventricle, reduces wall stress and oxygen demand, reduces ventricular stroke work, and increases mean arterial pressure and cardiac output. Impella 2.5 is approved for use for up to 5 hours in the USA and up to 6 days in Europe. Impella RP can provide temporary support for up to 14 days. The safety and feasibility of Impella pumps as well as their positive impact on haemodynamics were demonstrated in trials involving different patient populations: acute myocardial infarction complicated by cardiogenic shock (ISAR-Shock), high-risk PCI (PROTECT II) and postcardiotomy cardiogenic shock (RECOVER-I). However, the impact of these devices on mortality has yet to be established in larger randomised trials.

Figure 21.4 Diagram demonstrating the Impella LP2.5 axial flow left ventricular assist device sitting across the aortic valve. Reprinted with permission from Abiomed (Aachen, Germany), the manufacturer of this device.

TandemHeart (LA to Aorta)

TandemHeart (CardiacAssist, USA) is a percutaneously inserted device, which uses an extracorporeal centrifugal pump to transfer blood from the left atrium to the aorta via the femoral artery in non-pulsatile fashion (Figure 21.5). In contrast to Impella, TandemHeart requires two cannulas: the inflow cannula inserted via the femoral vein and positioned into the left atrium through the interatrial septum, and the second cannula in the femoral artery. This configuration bypasses the left ventricle, offloads it and reduces LV workload. The circuit can be reconfigured to support the right ventricle. For this purpose, the inflow cannula is positioned into the right atrium and blood is returned to the pulmonary artery. TandemHeart can generate flow of up to 5 l/min (with 19F arterial cannula). It is approved for circulatory support for up to 6 hours in the USA and up to 30 days in Europe. Preserved RV function is essential for optimal performance of TandemHeart as well as for Impella since both devices depend on LV preload.

Figure 21.5 TandemHeart consists of a 21F inflow cannula in the left atrium after femoral venous access and transseptal puncture and a 15F to 17F arterial cannula in the iliac artery. Reproduced with permission from Naidu (2011).

Similar to Impella, clinical data available suggest that TandemHeart increases blood pressure and cardiac index, decreases pulmonary capillary pressure and improves perfusion of end organs; however, evidence of influence on mortality is lacking at present.

Comparison of Percutaneous MCS Devices

Each percutaneous device for short-term mechanical circulatory support has unique characteristics and haemodynamic profile. The choice of the particular device should be guided by specific cardiovascular goals and clinical context of the individual patient (Table 21.1).

Most of the current knowledge and guidelines for percutaneous mechanical circulatory support are based on observational data, expert opinion, retrospective studies and consensus agreement. More prospective randomised studies are needed to establish evidence-based background for this fast evolving field of cardiothoracic medicine.

Learning Points

There are two main objectives of temporary circulatory support: (1) to provide perfusion for end organs and (2) to unload the heart to maximise chances of its recovery or bridge for further therapy.

Each percutaneous device for short-term mechanical circulatory support has unique characteristics and haemodynamic profile.

The choice of the particular device should be guided by specific cardiovascular and respiratory goals, and clinical context of an individual patient as well as expertise available.

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

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.

Risk Factors and Their Assessment for Implantation for VAD

Invasive Monitoring

Transpulmonary gradient (TPG) is the difference between mean pulmonary artery pressure and left atrial pressure (estimated by pulmonary capillary wedge pressure (PCWP)). Assessment of the TPG is recommended for the detection of pulmonary vascular disease in left heart conditions associated with elevated pulmonary venous pressure. Elevated pulmonary vascular resistance (PVR) is present when PVR is ≥ 3 Wood units or TPG ≥ 12 mmHg.

The optimal haemodynamic parameters of preoperative RV function indicating a low likelihood of developing RV failure include:

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  CVP ≤8 mmHg

  
  
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  PCWP <18 mmHg

  
  
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  CVP/PCWP ≤0.66

  
  
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  PVR ≤2 Wood units

  
  
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  RVSWI ≥400 l min^–2 m^–2.

  
  
• 
  

  Right ventricular stroke work index (RVSWI) = CI/HR × 1000 × (mPAP – RAP), where CI is cardiac index in l m⁻²; HR is heart rate in beats per minute; mPAP is mean pulmonary artery pressure in mmHg; RAP is right atrial pressure in mmHg.

Other intracardiac lesions such as aortic valve regurgitation or patent foramen ovale (PFO) will need to be corrected because after LVAD implantation, decompression of the left heart will increase the amount of aortic valve regurgitation and allow right to left shunt across a PFO, leading to significant oxygen desaturation. It will also increase the chance of paradoxical embolisation. Intracardiac repairs have an impact on the conduct of surgery since use of cardiopulmonary bypass will be required with or without a period of myocardial ischaemia.

Non-cardiac Factors

Presence of end organ dysfunction is a risk factor for poor outcome. Contraindication to LVAD implantation includes the following:

• 
  

  Irreversible end organ failure, especially renal, hepatic and respiratory system. Parameters such as elevated bilirubin, elevated prothrombin time suggest an auto-anticoagulated state signifying reduced hepatic synthetic function and may be signs of underlying right heart dysfunction.

  
  
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  Active and uncontrolled systemic infection.

  
  
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  Irreversible neurological injury. Recent stroke will increase risk of bleeding.

  
  
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  Contraindication to systemic anticoagulation or antiplatelet therapy.

  
  
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  History of substance abuse or non-compliance.