Aortic Compliance – Implications of Loss of Aortic Compliance after TEVAR

The GOOD – Short and Medium Term

The NOT So GOOD – the Long Term – we need to look at this to assess damages….

I recently worked on a case where a 20 year old male had multiple severe internal injuries from automobile inflicted trauma. One of these injuries was an aortic pseudoaneurysm. A pseudoaneurysm is a collection of blood between layers of the aortic wall and this is caused by a tear in the internal aortic wall – the tear is caused by forces of impact during the crash. The blood in the aorta pushes through the tear and fills the newly formed space created by the separation of the layers in the aortic wall. The client had a TEVAR (Thoracic Endovascular Aortic Repair) repair with resolution of this problem. Good story so far – however, the future is not so great for this young male – with an injury to a kidney and a family history of hypertension. So, what do we do with this? The kidney injury and predilection for hypertension sets him up for much earlier onset of hypertension accompanied by ASCVD (Atherosclerotic cardiovascular disease) and its consequences (heart attacks and strokes). The TEVAR repair superimposes another set of risks more rapid progression of hypertension and ASCVD. I will introduce the issue with a discussion of Aortic compliance. The aortic compliance gets irreversibly altered by the inserted [TEVAR] stent.

Arterial walls have an elastic component that allows them to stretch. This is important for the aorta as the aorta receives the entire stroke volume (about 120 ml or 4 ounces) of the Left Ventricle with each heartbeat. If the aorta was a rigid pipe, the left ventricle would have to pump this volume directly into the aorta and into the distribution arteries – this would make for a lot of work. Aortic compliance [elasticity], allows the aorta to expand as the Left Ventricle pumps each stroke volume into it and thus reduces the work that the left ventricle has to do. The aorta then recoils to its normal size and pushes the blood into the distribution arteries throughout the body. This cycle is repeated with each heartbeat and changes the pulsatile flow of blood to a steadier flow.

As people get older, the aorta (and other arteries) loose some of their elasticity. This makes the heart work harder to pump blood through the arteries. In some instances, calcium builds up inside the wall of the aorta and this causes it to get stiffer. When the left ventricle has to work harder to pump the blood into a rigid aorta, this leads to the left ventricle muscle becoming larger (Left ventricular Hypertrophy).  Additionally, the systolic blood pressure rises as the compliance of the aorta decreases. Unfortunately, medications cannot reverse the stiffness of the aorta and this type of systolic hypertension is very difficult to control. We sometimes see this picture in older folks with a stiff aorta. Even with medication, their systolic blood pressure may range above 180 mmHg. This leads to faster onset of ASCVD for that individual.

One way to understand this concept is by imagining a little play car with a piece of straw mounted on it to which you attach a balloon filled with air. The balloon recoils and pushes the air out of the straw propelling the car forward. The steady flow of air keeps the car going. Imagine now that you had a long tube attached to the straw instead of the balloon. Each time you blow out your breath, the car moves a little. You are going to get tired pretty quickly. It is much more efficient to blow up the balloon and let the recoil do the work.

I am talking about the elasticity of the aorta because it is a normal component of our circulatory physiology. When it is altered, things don’t work so well. This is directly applicable to the case I referenced above. (The aortic wall had a tear and blood accumulated around the tear but the mediastinal contents were able to limit the loss of blood – thus the client did not exsanguinate). The client had multiple injuries which took precedence and once they were stabilized, the pseudoaneurysm needed to be addressed. Generally, there are two methods of repair – 1.) open resection of the torn aorta and replacement of that section with a graft of some type. This is major surgery and has a long recovery period. 2.) Insertion of a “sleeve” along the inside of the aorta to put a patch over the torn part and seal it. This is a TEVAR procedure. Essentially a compressed stent is placed around a deflated balloon and inserted into the artery in the groin. Under X Ray imaging, the stent is placed along the aorta so that it covers the area of the aorta which has the tear. The balloon is then inflated and the stent expanded to sit right onto the inside of the aortic wall. The stent is long enough to stabilize the aorta before and after the tear. The stent then becomes part of the aortic wall and the pseudoaneurysm is stabilized and aorta now has integrity. There is a quick recovery from a TEVAR insertion procedure. The patient has yearly follow up and they generally do well. All is good and well until we look into the future and consider the altered physiology of the aorta – and cardiovascular system. This is where aortic compliance comes in.  If we consider the descending aorta to be about 30 cm in length and we place a 10 cm rigid stent in it, what do we do to aortic compliance?

The classic definition of compliance is the change in blood volume relative to a given change in distending pressure. For the aorta, the distension is a change in diameter – i.e. the aorta expands in a radial fashion as blood is pumped into it. The central Aorta contributes most of the compliance of the arterial tree since it is the largest. Now we consider the length of the central aorta to be 30 cm and we have just introduced a TEVAR device which is 10 cm in length into the central Aorta. The TEVAR device has minimal elastic properties (It is essentially a wire metal cage covered with special material) and it becomes part of the aortic wall. Thus, 10 cm of the aorta has lost most of it’s compliance because it can no longer expand to fill with each heartbeat – the diameter here is largely fixed. In essence, we have accelerated the effective stiffness of the aorta to that of a 70 – 80 year old. And yet, the client is still a 21 year old. So now we have a 21 year old male with cardiovascular dynamics of a 80 year old person.  His heart is normal but it will work extra hard, all the time, because of the loss of aortic compliance.  The net effect will be earlier onset of ASCVD related issues. It is very difficult to control the high blood pressure because we cannot alter the compliance of the aorta. This will lead to earlier onset of strokes, heart attacks and worse – systolic heart failure. Additionally, the same high blood pressure will adversely affect the kidneys leading to earlier kidney failure. Considering all of these things, we are predicting that this once very healthy and athletic 21 year old male will probably get into kidney failure in 20 – 25 years.  He will be about 45 years old. He will most likely need kidney transplant at that time. Additionally, the constant increased workload on his heart, coupled with high blood pressure will lead to heart failure – probably in 25-35 years. Again, he will be about 50-55 years old and may require a heart transplant at that time. Modern technology is marvelous. We are able to do things which were not possible 10-15 years ago and we are able to rescue more people after serious injuries. However, if we do not look at the long term consequences of these interventions, we fail to fully assess damages in legal cases.