A unique bicycle or “cycle ergometer" designed to allow patients to perform exercise tests during an MRI examination is providing new, previously unobtainable information on heart abnormalities under exercise-induced stress.
A team of researchers from the Auckland Bioengineering Institute (ABI) developed the ergometer to help cardiologists better assess the heart’s function and effectiveness of treatments for cardiovascular diseases. As part of a HRC-funded project led by ABI Principal Investigator Professor Alistair Young, the first cardiology patient trials using the ergometer have now been successfully completed in collaboration with cardiologists from the Auckland District Health Board.
The ergometer is a major engineering achievement because it doesn’t contain any ferro-magnetic materials such as steel, which would interfere with the high magnetic fields in the MRI (magnetic resonance imaging) scanners. It is also made to shield external interference that could affect the signals that make up the scanned images.
Professor Young says the ergometer was first conceived by ABI researchers Professor Poul Nielsen and Associate Professor Andrew Taberner, together with Liggins Institute Professor Paul Hofman. Since then, ABI PhD student Paul Roberts has worked with him and Associate Professor Brett Cowan and Professor Ralph Stewart at the University of Auckland to refine the ergometer for clinical use and subsequently used it in a study of 40 healthy volunteers and 60 patients with vascular disease.
Exercise testing is an essential tool for evaluating heart disease. However, although cardiac MRI produces excellent images of heart structure and function, it has up until now been difficult to take images of the heart during exercise.
“We know that you have to stress the heart to reveal its pathology because it may function normally at rest. Our results show that exercising on the ergometer while lying on a table in the MRI scanner provides a good stress test for the heart, with heart and work rates comparable to the traditional treadmill exercise tests. We achieved heart rates of over 120 beats per minute and cardiac outputs of over 12 litres of blood per minute,” says Professor Young.
The ergometer has significant advantages over the treadmill test.
“After hopping off the treadmill, the patient has to be moved and positioned in the MRI scanner. This can take up to a minute, during which time the heart rate can drop dramatically. Our device enables imaging of the heart immediately after the patient has stopped exercising,” says Professor Young.
For the patient trial, Professor Young’s team developed new fast imaging methods that can capture images while the heart is beating faster during exercise. As MRI is slower at taking images than ultrasound, they had to find clever ways of using less data to build up the image, while still getting all the necessary information.
Patients at risk of heart and aortic abnormalities, including aortic aneurysms (where an area of the aorta, the blood vessel which takes blood from the heart to the rest of the body, becomes very large or balloons out), were tested on the ergometer inside a MRI scanner to see which two blood pressure lowering drugs – Metoprolol and Losartan – were more effective at reducing aortic stiffness at rest and during exercise. Patients were scanned three times: a month after being on each drug and again after stopping treatment.
The team was able to detect abnormalities in cardiac function, and used pulse wave velocity to measure the effect of the two drugs on the stiffness of the aorta.
“The aorta is quite elastic. As blood pumps through the aorta, it expands. When you’re young your aorta is very elastic, but as you get older it stiffens. This stiffening makes it harder for blood to flow through, which in turn increases blood pressure.”
Professor Young says while the data from this study is currently being analysed in preparation for a journal submission, early signs are that there was a difference between the two blood pressure lowering treatments. The Losartan drug resulted in lower pulse way velocity than the Metoprolol drug, which suggests it was more effective at reducing aortic stiffness.
“The data from this study are unique in that the effects of different blood lowering agents have never been compared before, and never compared during exercise,” says Professor Young.
The ergometer and the new MRI scan methods developed as part of this project are now being used in clinical evaluations at Auckland City Hospital, where clinicians have been keen to combine echocardiography or MRI examinations with exercise for patients with coronary artery disease.
Paul Roberts, who recently submitted his thesis related to this project, has also received several enquiries for the ergometer from overseas institutions. He is now starting a research fellowship with the newly established Medical Devices CoRE (national Centre of Research Excellence), led by ABI Director Professor Peter Hunter, on medical devices and facilitating links between hospitals and bioengineering. The ergometer will be one of his first case studies.
Professor Young and his colleagues are currently developing an ultrasound lab for University teaching and research, including applications of the ergometer in echocardiography, adjacent to the Centre for Advanced MRI at the University of Auckland’s Grafton campus.