Mechanical blood pumps

Ideal treatment for chronic refractory heart failure should be reliable, cost effective, easy to implement, and capable of providing a physiological level of circulatory support. Existing cardiac support devices such as the pacemaker and implantable defibrillator are already widely accepted for patients of all ages. Within the next 10 years a user friendly miniaturised LVAD is destined to become the treatment of choice to relieve symptoms and prolong life in older heart failure patients.

Total heart replacement became virtually redundant when it was clear that more than 90% of patients could be sustained with left ventricular support alone. Only those with advanced right ventricular pathology or fixed pulmonary hypertension require biventricular support. Those LVADs currently used for bridge to transplantation have their origins in the 1970s and can be regarded as first generation blood pumps. The Novacor (Baxter Health Care, California, USA) and Thermo-Cardio Systems (Woburn, Massachusetts, USA) LVADs consist of a blood sac in series with the native left ventricle and compressed by a pusherplate mechanism, either electrically or pneumatically driven.24 25 Bioprosthetic heart valves dictate the direction of flow. This mechanism mimics the native left ventricle by providing pulsatile stroke volume with either variable or fixed pump rate. The patients own left ventricle is completely offloaded so that the aortic valve does not open. While large external pneumatic consoles have been replaced by implantable electric systems with portable control and power source, the serious problem of LVAD size, noise, driveline infection, and thromboembolism persist. These devices are unsuitably large for most female patients or children. Nevertheless, some bridge to transplant patients have survived with acceptable quality of life for up to four years.26

The new axial flow impeller pumps are the next generation of artificial hearts.27 In animal studies these compact, silent, non-pulsatile blood pumps provide up to 6 litres flow per minute, without significant haemolysis or thromboembolism. The thumb sized Jarvik 2000 heart fits within the apex ofthe failing left ventricle and pumps blood to the descending thoracic aorta (fig 11.8).28 The impeller supported by blood immersed microceramic bearings revolves at up to 18 000 rpm accelerating blood so rapidly through a narrow channel that the cellular components remain undamaged. The controller and batteries are the size of a portable telephone and fit easily onto a normal belt. While transcutaneous power induction is under development, we have devised an infection resistant skull mounted percutaneous titanium pedestal for the first human implants.29 The extracardiac NASA/de Bakey axial flow pump has already been tested for bridge to transplantation in humans with mixed results owing to mechanical problems.

Other ingenious blood pumps with magnetically suspended rotors (without bearings) are under development. These next generation, fully implantable miniature artificial hearts greatly increase the future scope of circulatory support, but mechanical reliability and freedom from complications remains to be established. We must also define the effects of chronic non-pulsatile blood flow, though the recovering native heart will transmit a pulse through the device. We anticipate that the cost of a blood pump will be less than that of multi-

Figure 11.8. The intraventricular "Jarvik 2000 Heart", an axial flow impeller pump which rotates at between 8000 and 18 000 rpm without damaging the blood.

• A realistic blood pump for long term circulatory support must be unobtrusive, silent, mechanically reliable with economic energy consumption, non-thrombogenic, free from constant infection risk and able to provide between 3-10 litres flow per minute without haemolysis. Such devices are now available for human implant.

• In the next 10 years blood pumps will be used for heart failure in the same way that pacemakers and implantable defibrillators are used for dysrhythmia.

ple hospital admissions for stabilisation during the last year of the patient's life. The device will replace most drug treatment and require only a maintenance dose of warfarin.

Blood Pressure Health

Blood Pressure Health

Your heart pumps blood throughout your body using a network of tubing called arteries and capillaries which return the blood back to your heart via your veins. Blood pressure is the force of the blood pushing against the walls of your arteries as your heart beats.Learn more...

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