Warning: This is a long and tedious post that many of you will find boring (I actually fell asleep briefly while writing it) but it contains good information that I believe is useful. If you’re the type of reader who’s looking for something lighter and wittier please skip this week’s post and look to my other inane blog articles (Better yet, turn off your computer and go out to enjoy the nice weather…sorry, forgot this is Nebraska. Read on).
Quiz. Which of the following statements are true?
- A normal stress test means you won’t have a heart attack.
- An abnormal stress test means you have coronary artery disease.
- If you have coronary artery disease you need an annual stress test.
A stress test in the cardiology world is an indirect way to assess the flow of blood through the coronary arteries that feed the heart muscle.
Stress testing is not a terribly complicated subject but it can be a bit confusing and suffers from a few misconceptions. The idea is pretty simple. First, we find some way to assess whether heart muscle is getting enough oxygen (so-called ischemia), some marker that we can follow through the course of testing. When doing the plain treadmill test we watch one particular segment of the EKG that changes in the presence of ischemia. With stress echocardiogram we examine the motion of the left ventricular wall. If the ventricle struggles for oxygen to any degree the affected area becomes flaccid and fails to contract appropriately. With the nuclear medicine study we observe where the muscle tissue absorbs the isotope (the nuclear dye) and where it doesn’t. This gives us an idea about blood flow to each region of the heart.
Then we find a way to exert stress on the heart. The treadmill raises the heart rate and blood pressure as well as the contractile force of the ventricle. The healthy heart has enormous functional reserve and can increase its output over a dozen-fold. If the patient can’t walk on the treadmill we can stress the heart with chemical infusions. Dobutamine more or less mimics exercise by revving up the heart and adenosine (and the newer Lexiscan) causes temporary expansion of healthy arteries with effective shunting of flow away from diseased areas.
We assess the heart at rest and then repeat the assessment at peak stress. If blood flow is good we should see good function at both points. If a major coronary vessel is blocked more than about 70% we’ll see normal function at rest but abnormal results at peak stress. The next test ordered will likely be a cardiac catheterization for definitive diagnosis and treatment of the affected artery.
With stress testing we are able to determine—with reasonable reliability—if the patient has any major coronary artery blocked to more than 70%.
Now let’s review how a heart attack happens. Heart attacks occur because a cholesterol-rich “plaque” in the wall of the artery becomes unstable, ruptures, and the body tries to seal the damage with proteins and cells that form a clot. The clot occludes the flow in the vessel and all downstream muscle is starved for oxygen. We’ve known for several years that the physical dimensions of a stenosis (ie. how narrow the blockage) don’t determine its risk of plaque rupture and complete vessel closure—a 50% blockage may be just as likely to result in a heart attack as a 90% blockage. A more important indicator of whether the artery will develop plaque rupture is the microscopic structure of the plaque: how much cholesterol is built up in the vessel wall and how stable the thin lining of fibrous tissue is that separates the cholesterol from the flowing blood.
While we have several tests that can evaluate the degree of narrowing caused by the plaque, there is currently no test available that can see the coronary arteries in enough detail to assess the stability of the plaque. Even cardiac catheterization can’t tell us which 60% blockage will become unstable and trigger a heart attack.
It’s for this reason that a normal stress test can’t insure against the future possibility of a heart attack. Furthermore, since stress testing is sensitive enough to detect only the worst of the blockages we can’t declare a patient free of coronary disease based on a normal test. A patient could have a handful of 30-50% blockages in their coronary tree and still pass a stress test with flying colors.
So what good is stress testing? Here are some situations where it’s helpful.
- It’s most useful in patients with exertional symptoms since those are easiest to reproduce on the treadmill. If chest pain arises from a coronary blockage then we’d expect to see the abnormalities show up reliably on our testing. Exertional chest pain in the face of a normal test is a pretty good argument for a noncardiac source of symptoms.
- Stress testing prior to major surgery is often helpful to rule out high-risk, dangerous blockage in persons who don’t regularly exert themselves to a great degree. We use it to prove that the heart can tolerate the physical demands of the surgery (which is generally equivalent to briskly climbing a couple flights of stairs).
- As part of the hospital evaluation for chest pain. As with the pre-operative testing, we are mainly looking for dangerous blockages in critical locations of the heart.
The downside to stress testing is the moderate false positive rate (the likelihood that someone with normal arteries is erroneously found to have an abnormal test result) that accompanies this screening test. It’s this reason that our current expert guidelines don’t recommend using stress testing to screen asymptomatic patients, even if they a history of stable coronary artery disease (in fairness, many cardiologists order routine stress tests for patients with previous stents or bypass surgery—the utility of this is debatable).
So, here are the answers to the questions I pose above:
- A normal stress test means you won’t have a heart attack. False. We do a better job of predicting a heart attack by looking at your risk factors and lifestyle (see the more useful Framingham Risk Calculator).
- An abnormal stress test means you have coronary artery disease. False. All we can say is that you might have blockage in the 80-99% range, but the false-positive rate of the study requires more definitive testing.
- If you have coronary artery disease you need an annual stress test. False, although some cardiologists will have a different opinion.
There you have it. For those of you who actually made it through this post you can now move on to reading War and Peace or the IRS tax code.