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Dissecting the forces of aneurism

The study of the aorta tensions moves on using the device of an Indian engineer

Research into aneurism of the aorta done during autopsies since 1992 by doctor Erasmo Simão da Silva has just received an important contribution. During a stay of one month in Brazil, in July, the Indian biomedical engineer Madhavan Lakshmi Raghavan exchanged ideas with the Brazilian researcher and brought with him a device that he developed to measure the pressure on the walls of aneurism. This illness, very serious and little known, consists in the dilation of the area of the abdominal aorta, which can rupture without warning, causing serious hemorrhaging and many times leading to death. In the United States, aneurism of the aorta is mentioned in medical statistics as the thirteenth cause of death of men over 60 years of age. In Brazil it hits close to 4% of the men in the same age bracket.

The studies of Dr. Raghavan, who worked for ten years at the Iowa University in the United States, met those of Dr. Simão da Silva, a researcher in the discipline of Surgical Techniques and Experimental Surgery at the Medical School of São Paulo University (USP). The Brazilian researched into the bodies of dead patients who had developed aneurism but had not reached the rupture stage. “The autopsy,” explains the researcher, “is the only way of minutely analyzing the biomechanical properties of the aneurism wall, which has heterogeneous characteristics and properties.”

The Indian had previously been researching the aortas of animals, and in 1999, gave a lecture for doctors in the United States, demonstrating the importance of analyzing all of the portions of the aneurism wall in order to obtain better conclusions. However, the only form of carrying out this analysis, is to examine aneurisms that have not ruptured through autopsies, a practice almost non-existent in the United States. “For example, it is impossible to carry out a biopsy of all of the portions of an aneurism in a living patient, since there is no way of collecting samples of the tissue of the walls of a vital artery, that cannot be perforated”, explains Dr. Silva.

Dr. Raghavan got to know of the work of the Brazilian doctor in 1999 through the Journal of Vascular Surgery, and in the Cardiovascular Surgery in 2000. As the practice of autopsy is not very common in the American hospitals, Dr. Raghavan made contact with Silva through the Internet so that together they could research the mechanical behavior of this vascular illness, with the objective of obtaining a mathematical model that analyzes the pressure throughout its structure.

Risk factors
Aneurism of the abdominal aorta has its origin in hereditary causes which are not well defined yet. What is known is that factors such as arteriosclerosis, smoking and arterial hypertension in sixty -year old males greatly increase the development of this problem. It is easily detected by ultrasound exams, magnetic resonance or even computerized tomography. Mortality occurs in more than 50% of the cases in which there is rupture and there is not a specific medicine to avoid the progressive closure of the walls of the aorta or to stop their shrinkage.

Presently, the only solution is to detect the aneurism before the walls rupture. Once the anomaly is identified, surgery needs to be done to remove the dilated section and to substitute it with a synthetic prosthesis. The surgery can be done via an incision in the stomach, a technique celebrated for more than 50 years, or by catheterism of peripheral arteries, a recourse that has been adopted for some ten years and still in evolution, with the advantage of skipping the stomach incision.

However, there is a dilemma within the doctors’ circle. To submit or not to submit people of an advanced age to the risks of surgery. “Once aneurism occurs in elderly people, who already have related diseases, the ideal, above all would be to identify the patient with a higher risk of aorta rupture. In this way, a significant number would be saved from a highly delicate operation”, appraised Dr. Simão da Silva.

The specialists have already determined that, in general, the risk of rupture becomes evident when the dilation reaches 5 centimeters of diameter. However, there are many cases in which the rupture only happened with 10 or 12 centimeters of dilation. “Up to now, we don’t know why such a factor happens. Often, for this reason, surgery becomes the option, since factors associated with the natural evolution of aneurisms have to be counterbalanced with the surgical risks for each individual.”

Pressure calculation
Computerized tomography images can be used, through a color scale, to determine the pressure levels in the wall of a typical aneurism. In these images, the red color represents the highest levels and the blue color the lowest, with the intermediate values being in yellow and green. The calculation of the pressure is based on a computerized biomechanical model. However, this model has its limitations, since one cannot obtain data about certain biomechanical properties of aneurisms. “Wrongly, the present models assume that these anomalies have the same pressure on all of the wall and that the mechanical properties are isotropic and uniform” informs Simão da Silva.

A detailed study of all of the wall of the aneurism, detecting the thickness and the force necessary to cause rupture, can only be obtained if all of the wall were to be thoroughly analyzed. From now onwards, starting out from a mathematical model, Silva and Raghavan hope to obtain the individualized calculation of the pressure to which an aneurism would be subject. “With this model we may understand better why the rupture of an aneurism occurs at different levels of dilation. In this way, it would be easier to determine if a patient should be submitted to surgery or not”, completed the Brazilian doctor. Since the walls of the abdominal aorta are heterogeneous, one needs to specify their thicknesses, as well as their morphology and the forces that act upon them such as arterial pressure. “With this analysis, it would be easier to study the mechanical behavior and to project the mathematical models in order to calculate the stress produced on the tissue”, states Silva.

Raghavan’s apparatus was temporarily brought in and then returned to the United States. “However, it was of great value for the development of a pilot project.” reports Silva. “In order to advance in the work, we now intend to purchase the equipment which costs around US$ 13,000.00 and to establish a means of permanent cooperation between the Department of Bioengineering of Iowa University and the Department of Surgery of the Medical Faculty of USP.”

The Project
Study of Tension in the Wall of Aneurisms of the Abdominal Aorta Obtained in Autopsies (nº 01/05030-8); Modality Assistance – visitor; Coordinator Dr. Erasmo Simão da Silva – Medical Faculty of USP; Investment R$ 5,089.15