A Z score of 3 means he is beyond 3 standard deviations from the normal aortic diameter. This is a large size. He should restrict weightlifting and intense aerobic exercise. He needs to be followed by a knowledgeable cardiologist, preferably one at a large university hospital with high volume of patients.
Click on my screenname to learn about ARBs, a class of drugs that shows much promise for at least some people who have dilated aortas. Talk it over with a cardiologist who keeps up with the latest research and see if it's appropriate for your son (it probably is).
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A Z score of 3 means three standard deviations away from the normal aortic size mean. Depending on the normal population, that number would be greater than around 5 cm or so, and this is when we typically recommend aortic root surgery. In some patients with slowly progressing disease we wait even until 6 cm because at that time we may also recommend working on the bicuspid AV which ultimately is at risk for worsening and may require additional surgery in the future. So killing two birds with one stone makes a whole lot of sense as long as it is done in a safe manner.