Disturbed blood flow can damage the vessel wall in cases of aortic dilation

All blood in the body passes through the heart about once a minute. With every heartbeat, blood from the heart is pumped up into the aorta.

Dilation can occur in all parts of the aorta. It is unclear exactly how this happens, but one explanation may be weakening of the connective tissue in the vessel wall. Factors such as elevated blood pressure, age and various hereditary conditions increase the risk. A person affected by aortic dilation usually does not notice anything. But more and more people with aortic dilation are being discovered by healthcare -- often by chance.

"In healthcare, we're doing more and more imaging where we look at, for example, the heart or lungs, and then we can find that the patient's aorta is a little larger than normal in some parts. There's a need for more tools to guide doctors on how to handle these patients. In a minority of them, the aorta will unfortunately dilate more, and, in the worst case, the aorta will rupture, which is usually fatal," says Filip Hammaréus, PhD student at the Department of Health, Medicine and Caring Sciences at Linköping University and intern physician at Ryhov County Hospital in Jönköping.

Most people with aortic dilation are offered regular surveillance of the aortic diameter -- the larger the diameter, the higher the risk. Sometimes the aorta is operated on preventively to avoid acute complications, but it can rupture before dilation has become so pronounced that surgery is considered appropriate. At the same time, many examinations are made of people whose aortic diameter does not change. New methods for early identification of patients at elevated risk of growth could contribute to more individualised and cost-effective care. Therefore, the researchers behind the current study, published in the journal European Heart Journal -- Cardiovascular Imaging, are investigating new methods for understanding both the risk and the condition itself.

"We can be onto something new, which can say something more about the severity of the disease than what the aortic diameter shows," says Petter Dyverfeldt, professor at Linköping University and affiliated to the Center for Medical Image Science and Visualization, who has led the study.

When blood is pushed out of the heart into the aorta, a frictional force occurs between the flowing blood and the vessel wall. This is important for a type of cell that is located in the vessel wall and can sense blood flow. When the frictional force is normal, the cells tend to be healthy. However, if the friction significantly changes direction, or becomes very low or abnormal in other ways, there seems to be a signalling into the vessel wall that can eventually lead to a weakening.

Blood flow through the body's large vessels and in the heart can be measured and visualised with magnetic resonance imaging, using an advanced technology called 4D flow MRI that is available in a small number of hospitals. This gives the researchers a picture of how blood flow affects the vessel wall.

In the current study, the researchers have carried out such measurements and also measured various proteins in the blood. It turned out that there existed interesting relationships between the effect of blood flow on the aortic wall and various proteins that relate to inflammation and the build-up and degradation of connective tissue.

"We see that in patients with dilated aorta, abnormal blood flow dynamics are associated with increased inflammation and turnover of connective tissue -- something we believe can reflect processes in the vessel wall. This seems reasonable based on the mechanisms that have been shown in previous research, but it's completely new to show the connections in the way we do now using a combination of magnetic resonance imaging and blood samples," says Filip Hammaréus.

The findings strengthen previous research, but also bring new insights.

"What's interesting about the findings in our study is that the measures of how blood flow affects the aortic wall were not linked to the diameter of the aorta. So, the traditional measure that's often used in healthcare was not part of the relationship we see in the study between abnormal blood flow, inflammation and breakdown of the vessel wall," says Petter Dyverfeldt.

The study was conducted on 47 men and women who participated in the Swedish CArdioPulmonary bioImage study (SCAPIS) and whose aortic diameter was over 40 mm. They were compared with 50 control subjects who were matched by sex and age.

The research has been funded with support from, among others, ALF Grants, Region Östergötland, Linköping University, Region Jönköping (Futurum) and the Swedish Research Council. The main funder of SCAPIS is the Swedish Heart-Lung Foundation.

Facts:

• The aorta is shaped like a walking stick with a curved handle. From the heart, the aorta goes up into a bend (the "handle") and from there the long part of the vessel passes down through the chest to the abdomen.
• It is easier to find dilated aortas, or aneurysms, on the part of the aorta located in the abdomen. In Sweden, all men over the age of 65 are offered screening for abdominal aortic aneurysm. In the current study, the researchers focused on the dilation of another part of the aorta -- the very first part after the heart -- that is difficult to find with ultrasound and is not included in the screening programme.