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Strains on a swelling heart: Biomechanical Properties of Thoracic Aortic Dissections

Did you know that Einstein died of an aortic aneurysm that burst? An aortic aneurysm is the swelling or bulging of a section of the aorta, an artery that distributes oxygen from the heart to the other parts of the body. Such patients are also likely to develop an aortic dissection, where the inner layer of the aorta splits, causing the blood flowing within it to surge through the tear, which ultimately ends in a ruptured aorta. A crucial part of trying to predict the occurrence of this medical emergency is to know the mechanical properties of the aortic tissue. How much stress can the aorta sustain? Where is the strain greatest and at what point is the tissue likely to rupture? These are the questions that a group of scientists at the Indian Institute of Science have tried to answer.

Aortic dissections can either occur in the chest (these are called thoracic aortic dissections or TADs), or in the abdominal area (which are called abdominal aortic dissections or AADs). These rare medical conditions are difficult to study under controlled environments, since they require immediate medical attention and can be fatal if unnoticed or untreated. However, biomechanical studies on the tissues of such patients can provide vital information on how to distinguish between diseased and normal aortas. This can be critical in deciding the right time for surgical interventions that could save a patient’s life.

The herculean task of characterising the stress and strain biomechanics of diseased TAD tissues was undertaken by Anju R. Babu and Achu Byju from Dr. Namrata Gundiah’s lab. They now have evidence to prove that dissected aortic tissue, especially that from younger patients (<65 years of age) has greater stiffness compared to healthy aortic tissue from a control group. Besides this, one of the most important aspects of this study is that it is the first one that reports biomechanical statistics of aortic tissue for an Indian population. The few studies reporting such values for TAD samples have always been for European or American populations. This was mainly due to the efforts of Dr. Binoy Chattuparambil of Narayana Health hospital (Bangalore), who obtained samples of aortic tissue for the study from subjects undergoing surgery for TADs.

“The aortic tissue faces huge stresses, with turbulent blood flow at high pressures. Therefore, any change in the stiffness of this material is likely to alter its response to strain. This is why we felt that the biomechanics of diseased tissue, which is likely to tear under pressure must be different from normal tissue”, says Dr. Gundiah, one of the authors of the paper which was published in the Journal of Biomedical Engineering. The paper also hypothesises that TAD tissues are likely to contain higher quantities of collagen and lower amounts of elastin which could be causing this stiffness and decreased elasticity. Collagen is a naturally occurring protein found in the bones, muscles and skin of the human body which is responsible for maintaining rigidity in these tissues. Elastin, on the other hand, is a type of protein found in our bodies that keeps tissue flexible yet tight. “We believe that the turbulent blood flow because of conditions such as hypertension can cause higher collagen deposition in certain parts of the aorta. This would lead to a change in the constitutive properties of the tissue, which would change the geometry of the aorta, which in turn would further affect the blood flow patterns in a vicious circular loop that could ultimately lead to an aneurysm.”, Says Dr. Gundiah.

Studies such as these are a landmark in biomechanical tissue research, enabling us to understand the potential for dissections or ruptures in an aorta. “Currently, the main criterion used by doctors to decide on surgical intervention in cases of aortic dissection is a gold standard of 55 mm. If the ascending aorta size is more than 55 mm, surgery is advised. However, in many cases, tearing could happen even before this critical size. Therefore, getting to know the stiffness and the ability of the aortic tissue to withstand strain is likely to be very useful in taking decisions on surgery” Says Dr. Gundiah.

About the authors

Namrata Gundiah is an Assistant Professor at the Department of Mechanical Engineering, Indian Institute of Science, Bangalore. Anju Babu is a research scholar at her lab. Achu Byju is currently a Master’s student at Texas A&M University.

namrata@mecheng.iisc.ernet.in; 080 2293 2860/ 3366

About the study

The paper appeared online in the Journal of Biomechanical Engineering

http://biomechanical.asmedigitalcollection.asme.org/article.aspx?article...