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Wednesday, December 22, 2010

Aortic Stenosis in Boxers: Physiology

ASImage by HeartBabyHome via Flickr
This is the first part of a series of articles on aortic stenosis in Boxers. Further articles will be posted shortly.
 
Aortic stenosis, a narrowing of the aorta, is one of the most common cardiac diseases in all dogs. This narrowing may lead to exercise intolerance, fainting, and in severe cases death. Aortic stenosis is not uncommon in Boxers, but screening of breeding dogs has decreased the incidence of the disease in the breed.

Physiology of Aortic Stenosis

A fibrous ridge that typically forms just below the aortic valve causes the actual narrowing, or stenosis; accordingly, the disease is also called sub-aortic stenosis, or SAS. Because of the narrowing, the heart has to pump harder to circulate an adequate amount of blood; the stenosis also causes the blood to pass through the aorta at a higher pressure and faster flow rate. (A similar phenomenon can be observed by pressing in on a garden hose with the water running; the more you squeeze the hose together, thus narrowing the opening through which the water can go, the faster and more strongly the water comes out.) The extra work required to circulate blood through the stenosis can lead to a thickening of the left heart wall, while the increased pressure can lead to ballooning of the aorta.

In addition to increased pressure and flow rate, aortic stenosis causes increased turbulence as the blood passes through the aortic valve. This turbulence creates a heart murmur, which can be heard by a cardiologist when the heart is ausculted (listened to with a stethoscope). Not all heart murmurs result from by aortic stenosis, especially in Boxers. Research conducted by Dr. Kathryn Meurs, then at Ohio State University, found that Boxers have narrower aortas, in general, compared with other breeds of similar size. This narrowness may result in what is called a "flow murmur" or "innocent murmur", which does not have any effect on the dog's health or length of life.


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