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[January 2007]  AW Khir, Brunel Institute for Bioengineering, Brunel University, Ashraf.khir@brunel.ac.uk
 
The heart is a vital organ that is responsible for circulating blood to all the other organs around the body. In effect, the heart acts as a pump and many physiologists and engineers have quantified its function in a similar way to quantifying the performance of any other mechanical pump. In order to deliver blood which carries the required nutrition to organs round the body, the heart has to generate a flow rate of approximately 5 L/min; mean flow rate of a healthy adult at rest. This flow rate has to be pumped against a head of approximately 80mmHg; mean pressure in a healthy adult.

The heart muscle itself receives blood carrying its nutrition through very small arteries, called coronary arteries whose maximum diameter is approximately 3 mm. Due to the ageing process, bad diet, lack of exercise, or arterial disease the coronary arteries start to become blocked. Eventually, this leads to the reduction of blood flow to the cardiac muscle, which in turn results in the heart muscle becoming weaker, a state that is traditionally called “heart failure”. It naturally follows that all other organs suffer as they will also have a lack of blood supply. If the cardiac muscle is deprived from blood flow altogether, severe pain in the chest is experienced, and that is the definition of heart attack.

Although advances in medical management of heart failure patients have resulted in some improvement in mortality rates and patient’s life style, approximately 80% of patients do not respond to pharmacological treatments, leaving patients requiring an alternative. When the pharmacological treatments fail to augment cardiac output and ventricular performance, heart transplantation (plant a natural heart of a recently dead person into the body of a patient) becomes the obvious choice. However, the number of heart donors is decreasing each year, and the demand is much greater than the available supply. For example, recent figures on the incidence of heart failure in the UK show approximately 300,000 new cases per annum, where the number of heart donors does not exceed 300. This great imbalance between demand and supply, presented engineers with a mighty challenge for over quarter of a century because if the patient cannot survive the waiting period, using an artificial cardiac assist device (CAD) becomes the only remaining option.

CADs are essentially mechanical pumps that assist the failing ventricle and can restore the patient’s general hemodynamics. Using the pumping principle as a criterion all of the cardiac assist devices can be generally grouped under two main categories: the pulsatile devices such as the Novacor, Thoratec and Berlin Heart. These devices pump blood in phase with the contraction of the left ventricle. The other category is the continuous blood pumps, which pumps blood continuously regardless of the actions of the heart. Those can be further classified into two sub categories, radial centrifugal pumps such as “lifeStream” and axial flow pumps such as the “Jarvik-2000”. The hemodynamics associated with using either pulsatile or continuous flow devices in vivo is greatly different, and the debate over which is best for the patient is not yet settled. CADs can also be classified into two groups according to their function. The functional category is the so-called “bridge to transplant” where the devices are designed to assist the circulation, usually for a few months, by augmenting the pumping function of the failing left ventricle until a suitable donor heart is found. A “bridge to transplant” device can also be either a pulsatile device such the “Berlin-Heart” displacement pump or a continuous device such as the Micromed Debaky axial flow pump. The other functional category of cardiac assist devices are designed to assist the heart, usually a few days, until it recovers, a so-called “bridge to recovery”. The most widely used cardiac assist device in this category is the Intra Aortic Balloon Pump (IABP), whose main benefit is to increase coronary flow and hence encourage the myocardium to recover. A recent development of the IABP made it fully implantable by sewing the balloon into the aorta.

The development procedure of almost any artificial heart requires careful consideration of several multi-disciplinary factors that will eventually affect not only the performance of the pump, but also the potential survival of the patient. Three main factors have to be considered during the design stage of any CAD:

1) Biocompatibility: It is the ability of the artificial organ to be planted without being rejected by the body, which is mainly related to the material from which the device is made of. This is a major risk factor for the failure of any device because no matter how purposeful and efficient the device is, it will clearly be rendered useless if can not be accepted by the body. So, the external surface of CADs has to be made of a material that could be accepted by the body.

2) Haemolysis: It is the damage to blood cells that are subjected to excessive sheer and possibly cyclical stresses. This is an important factor because blood is essentially a suspense fluid which carried red, white cells as well as platelets, all or some of which may not sustain the battering of the blades of a pump, for example. Therefore, whilst the design of the blades of centrifugal or axial flow pumps can and should be very efficient, high levels of sheer stresses along the path of the fluid are not acceptable.

3) Thrombosis: It is the formation of blood clots, similar to the concept of wound healing. This is a characteristic of the blood as a fluid; if it is not moving, blood cells tend to aggregate and form a clot. Formation of thrombosis in the context of CADs can occur due to the existence of stagnation areas, where the forces acting on the blood are too small to move it. Thrombosis can also be formed due to the lack of appropriate wash-out flow in some other areas. So, the design of every portion of the blood path along the CAD must be carefully considered to ensure enough wash-out flow, and to eliminate stagnation areas altogether.

Other factors that are also important to consider in the design of CADs include the size; which will determine where the device will be placed inside the body. In fact some devices are placed and remain outside the body, such as the BCM cardiac assist device, which incorporates tubes carrying the blood from the heart to the aorta where it will be circulated around the body. Expected duration of use is a factor that determines several other choices such as the selection of material from which the CAD would be made of in order to sustain a certain level cyclical stresses. Obviously a CAD designed as a bridge to transplant is different than that will be designed as bridge to recovery, and both are totally different than a CAD that is envisaged to be used as a long-term heart replacement. Also, another factor that is required to be taken into consideration is the power of the device. Most devices use conventional power leads that cross the skin to deliver the power from outside to the CAD inside the body. However, recent research has shown the possibility of delivering the power wirelessly from a power source placed at a close proximity. Most power supplies use rechargeable batteries, and hence the duration of the batteries which can remain charged will decide how long the patient can be mobile and away from the batteries charger, a social aspect that will affect the quality of life.

Techniques used in evaluating and designing a CAD will include Computational Fluid Dynamics (CFD), experimental and visualisation techniques such as Particle Image Velocimetry (PIV). CFD are usually used to investigate the flow profile along, across and the output of the CAD. PIV techniques is also used to detecting the stagnation as well as the areas suffering from high sheer stresses. The basic design of a CAD is similar to that any pump that is designed to meet certain requirements. However, because CADs deal with an unusual fluid (blood) and is placed in an unusual environment (human body), unconventional methods and creative solutions are sought and applied; making this area of research most challenging. Regardless, perhaps we should all look after our hearts in order to avoid the need to use one of those CADs!

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