Researchers have developed a novel method of modelling blood flow through the heart, which should improve artificial heart valves, writes Dick Ahlstrom
Surgeons are getting very good at mending broken hearts by replacing faulty heart valves, but there is always room for improvement. A NUI Galway research team has helped with a new method for measuring blood coursing through artificial valves, providing information that could help prevent dangerous blood clots.
The implantation of prosthetic or artificial heart valves is now commonplace, with about 275,000 patients worldwide receiving this surgical treatment each year. Yet the mechanical devices sometimes pose a risk of clots if blood cells become damaged when flowing through, says Dr Nathan Quinlan of NUI Galway.
"One of the many engineering challenges prosthetic valves present is the difficulty of avoiding clot formation," explains Quinlan, a researcher in Galway's department of mechanical and biomedical engineering and the National Centre for Biomedical Engineering Science.
"The valves can give rise to unnaturally severe fluid dynamics in the blood that flows through them. This in turn aggravates blood cells and can trigger the coagulation process," he says.
The usual medical response is to administer lifelong drug therapy to block clot formation, but Quinlan plans to use his new blood flow measuring technique to build a better replacement heart valve.
The method is known as "stereoscopic particle image velocimetry", he explains. "It came out of the aerospace industry. It is one of a range of techniques to measure flow of a liquid or gas."
It is no small feat to measure the way something like blood moves through the chambers of the heart. Cells can slow then accelerate as the valves open and close and the blood produces swirls and eddies that change in an instant.
These flows become increasingly complex when an artificial valve is inserted, something that also increases the "shear" forces acting on the blood cells. If this reaches a point where cell damage occurs unwanted and unwelcome clots can form, says Quinlan.
The team, including Quinlan, Dr John Eaton and postgraduate student, Mr Donal Taylor, developed a way to use the aerospace technique in the biomedical area. It measures blood flow very accurately in a model heart fitted with various artificial valve designs. The Higher Education Authority via its PRTLI programme provided funding for the research, which captured the bronze medal for the best bioengineering paper last January at the annual conference of the Royal Academy of Medicine in Ireland.
The method involves taking rapid-fire pictures of small particles suspended in the "blood" flowing through the model heart. It uses a laser to illuminate the particles, taking 15 pairs of images every second as they move along in the flow, says Quinlan.
"We can see how the particles move from one image to the next. We get a measure of velocity in the fluid," he says. "Velocity in principle tells us everything. There are several things of interest, vortices, swirls in the flow and the shear stresses on the blood cells."
It sounds disarmingly simple but reading the data is anything but straightforward. "The size and density of the particles are important," he says, with the goal being to match particle density with fluid density. This keeps the particles in the flow and prevents they shooting off on a tangent as the flow turns and twists.
Typically the overall flow is divided into layers moving at different speeds and exerting different shear stresses. The technique, which has been used to test valves supplied by industrial partners St Jude Medical in the US and Aortech in Scotland, can provide a detailed map of what is going on in the fluid.
"This should aid understanding of complex fluid dynamics in the next generation of replacement heart valves," Quinlan says. It may also provide a new way to understand how blood flows through a damaged heart.
