New Scientist, 7-28-04
A new type of pump to help failing hearts will undergo clinical trials in autumn 2004 in the UK. Its design
is intended to solve the problems of mechanical failure and blood clotting that have bedevilled artificial hearts and pumps since they were invented.
The pump also has a curious side effect: people implanted with the device have no pulse.
Support for the heart
The VentrAssist, which is made by Australian company Ventracor, is of a type known as left ventricular assist devices. LVADs are not designed to replace the heart but are implanted alongside it under the rib cage. They augment the pumping action of the left ventricle - the heart's main chamber and the one that is responsible for 90 per cent of heart failure cases.
Most LVADs attempt to mimic the way the heart works, but their complicated design makes them prone to failure, and they have a tendency to make blood pool and clot, leading to strokes. That means LVADs are usually only used as a last resort for patients waiting for heart transplants.
What makes the VentrAssist different is that it only has one moving part, a spinning impeller that drives a continuous stream of blood. That means the pulse is replaced by a gentle whirling noise that patients describe as similar to the sound of a washing machine. More importantly, the device prevents blood from stagnating, reducing the risk of clotting.
The device is also six times smaller than standard LVADs, with a diameter of just six centimetres. "These kinds of devices will be the way of the future," says Robert Kormos, head of the University of Pittsburgh Medical Center's Artificial Heart Program.
The VentrAssist has two tubes, one drawing blood in from the left ventricle and the other sending it out to the aorta, the body's main artery. A cord emerges from the abdomen, where patients connect it to a rechargeable battery.
Six copper coils within the device's titanium walls generate magnetic fields that make the magnet-cored impeller blades spin (see graphic). The blades push blood out to the body while forming a high-pressure, liquid cushion that levitates the impeller and holds it steady.
Most other attempts to use continuous rotary pumps in LVADs have relied on bearings to hold the impeller blades in place, making them susceptible to wear and failure.
Red blood cells
"There is no predicted lifespan for VentrAssist because there are no wearing parts,"
says co-inventor of the device and company founder John Woodard. "It could be a hundred years, we don't know."
VentrAssist also has an advantage over its one competitor, a device called INCOR made by the German company Berlin Heart, which has a similar design, and it is already approved for use in Europe. According to Steven Tsui, who is director of mechanical assist heart services at Papworth Hospital in Cambridge, and will run the UK trials, the VentrAssist is less likely to damage red blood cells because it moves the blood more slowly with a bigger impeller.
Since June 2003, David Kaye and his team at the Alfred Hospital in Melbourne, Australia, have implanted the VentrAssist in seven patients with end-stage heart failure. All of them were too frail for heart-transplant surgery and were not expected to survive longer than a year without artificial assistance.
Three have since died but the others are still going strong, one after almost 13 months. "They have picked a particularly difficult group of patients to treat, and yet the initial results are astounding. These people were very sick before the implant and now they are well and at home," says Tsui.
Ventracor announced on 27 July that it will extend the trial to include patients on the heart transplant list.