Mechanical circulation support systems (assist devices)

Mechanical circulatory support systems (assist devices) are available as bridging until cardiac transplantation (HTx) and as an alternative form of treatment for terminal heart failure if no donor organ is available in an emergency. A left ventricular assist device (LVAD) was employed for the first time in 1978 but controlled clinical studies have been conducted in Germany only since the mid 1990s.

Classification and functional principles of the systems

With over 50 different systems on offer, a distinction is made nowadays by definition between the ventricular assist device (VAD), where the patient’s heart is left in place, and complete heart replacement with a total artificial heart (TAH).

The pump systems can operate according to pneumatic, electromechanical, electromagnetic and electrohydraulic principles. The operating principles include displacement pumps with pulsatile blood flow and rotary pumps with continuous blood flow. With the displacement or sac pumps, the blood is squeezed out of a pump sac by movement of a membrane or between pusher plates and pump action is pulsatile. In the rotary pumps, the blood is moved using a rotor and squeezed out of the pump by centrifugal force. The position of the blood pump plays a crucial part for the different assist periods. As the illustration shows, they can be classified as:

1. extracorporeal, i.e. short-term systems located near the patient
2. pericorporeal, i.e. medium-term pump systems located directly in front of the patient’s abdomen and 
3. intracorporeal long-term systems in the patient’s chest or abdomen; these are divided into partially and fully implantable systems (see illustration).

Indication and aim of treatment

The indication is usually urgent or emergency (often in patients on the waiting list for cardiac transplantation) because of decompensated dilated or ischaemic cardiomyopathy, heart muscle inflammation (myocarditis) and heart failure on a heart-lung machine after unsuccessful medical and surgical therapy with catecholamines and IABP.  The criteria are mean arterial pressure < 60 mmHg, cardiac index < 2 l/m2, CVP and wedge pressure > 20 mm Hg and oliguria < 20 ml/h. In addition, severe pulmonary hypertension, defects of the septum and heart wall and certain valvular lesions are special indications for a TAH. Contraindications are recent stroke, cerebral haemorrhage, unclear neurological status after resuscitation, sepsis and an elevated bleeding risk. Risk factors include severe cardiac cachexia, incipient renal and hepatic failure in usually ventilated patients, recent myocardial infarction and age over 50 years.

The desired treatment concepts are usually bridging to cardiac transplantation or as a permanent heart replacement as an alternative to cardiac transplantation, e.g. as “destination therapy“ in non-transplantable elderly patients (patients over 70 years or with systemic diseases such as severe diabetes or renal impairment and other contraindications to cardiac transplantation). On the other hand, explantation of the assist device can be planned as a bridge to recovery or weaning, e.g. after myocarditis. Bridging from the short-term assist device until a decision is made on implantation of a long-term assist device is possible, known as bridge-to-bridge or bridge-to-decision, especially in resuscitated patients in whom the neurological status is unclear initially.

In practice, patients with terminal heart failure are treated in intensive care units with maximum drug therapy and also by IABP, e.g. after myocardial infarction, and are then referred to our cardiac surgery clinic via a 24-hour emergency number (089/7095-5633). The patient is then accepted directly or assessed on site by a senior cardiac surgeon so that a decision can be made immediately in the case of massive circulatory instability on whether an assist device should be implanted or an ECMO should be employed in case of doubt. In the longer term, cardiac transplantation is discussed with the patient or his relatives or a long-term assist device such as the Jarvik 2000 system may be considered for unsuitable elderly patients.