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?

1. A normal stress test means you won’t have a heart attack.
2. An abnormal stress test means you have coronary artery disease.
3. 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. My group’s web site has some good general information on treadmill, echo and nuclear stress testing so I won’t go into detail here.

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.

1. 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.
2. 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).
3. As part of the hospital evaluation for chest pain. Please see my recent post on CT angiography for a summary of how this is used. 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:

1. 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).
   
2. 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.
   
3. 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.

Let’s say you come to the emergency room with chest pain.  Pretend you are a 40-something-year-old male with a history of high blood pressure but no previous heart problems.  After a few tests the doctor comes into the room to tell you that your EKG and lab work are normal but that you’ll be staying in the hospital overnight anyway.

The next morning, with your repeat EKG and blood work showing no abnormality, you are ushered to the treadmill room for a test called a stress echocardiogram.  You walk, then run, on the treadmill until your pulse reaches about 85% of your maximum rate and the echocardiogram technician takes pictures of your rapidly beating heart.  The doctor supervising the test interprets the pictures and declares you free of significant blockage in the arteries of the heart.  A few hours later you’re discharged—about 24 hours after you arrive—without a firm diagnosis and with the hopeful anticipation that your hospital bill won’t be too big.

This scenario describes the way we go about evaluating most patients who come through the ER with chest pain.  I say “most” patients because there are often exceptions.  Someone with an abnormal EKG, elevated cardiac enzymes, or features to their chest pain that we consider “high risk” would likely bypass the above evaluation and proceed directly to the cath lab.  Other patients whose symptoms are so clearly noncardiac might get dismissed with instructions to seek out their primary physician’s guidance for further evaluation.

The vast majority are admitted and treated as described above.  In the business we refer to this method as a “rule-out,” as in “we’ll rule out a heart attack (with the enzyme tests) and then do a stress test.”  Doctors are quite comfortable with this for good reason.  For many years it has been a safe and effective way to figure out who is really having heart problems and who is just still trying to digest last night’s Mexican dinner.

You can turn to demographic studies to get a sense of what might be happening with the people who fall into this category (we call them “moderate risk”).  Out of every hundred people admitted for a “rule out” only a handful—five or ten—will end up having coronary disease as the cause for their symptoms.  If you were to take all 100 of them to the cath lab and look at their arteries you would find absolutely no narrowing in the majority.  Using the current method described above we admit an overwhelming preponderance of patients with normal coronary arteries just to find those few with blockage.  Most patients coming in with chest pain can plan on at least a few meals of hospital food before they get their answer.

What we really need is a test that can reliably exclude those patients with clearly normal coronary arteries and that can do it quickly with minimal risk and expense.  When you come in the ER with chest pain you need a test that can tell you with as much certainly as scientifically possible whether your coronary arteries are normal or not.  If your test is normal you can be discharged with the knowledge that your chest pain is not your heart.  If the test is abnormal you can then come into the hospital for further testing.

In statistical parlance what you need is a test with an extremely high negative predictive value.

A little primer on medical tests is in order.  The screening tests we use (such as stress tests, mammograms, blood PSA, etc) are not perfect.  Each test has various limitations that make it so we cannot conclude that an abnormal test definitively signifies the presence of disease (or that a normal test predicts that absence of disease).  We rate the accuracy of a test using complex principles called sensitivity and specificity.  These values (which are not particularly intuitive and continually vex new medical students and residents) can be translated into the more useful terms positive (PPV) and negative predictive value (NPV) by incorporating into the equation the prevalence of a particular disease in the population being tested.

PPV  is defined as the likelihood that a positive (or abnormal) test correctly identifies the presence of disease (if a stress test has a PPV of 80%, then you, as a patient, have an 80% chance of actually having significant coronary blockage with an abnormal stress test).  Just the opposite, the NPV is the likelihood that a normal test will prove the absence of disease (a NPV of 90% means that you have a 90% chance of being normal if you have a normal test).

This may alarm some people.  We are accustomed to believing that our medical tests are precise all the time, but most are not, and we doctors have to, in effect, decode the test results (interpret the interpretation, if you will) based on the individual patient’s clinical profile.

At least one test is changing that, and if you plan to head to an ER anytime soon with chest pain you should know about it.  The technology has now advanced so far that we are now able to use computed tomography (CT, or “CAT” scans) to obtain beautiful images of the heart and its arteries—images good enough to be able to immediately determine if a patient with chest pain has normal coronary arteries or not.

The coronary CT angiogram, or CCTA, is a study where the patient undergoes a chest CT scan with intravenous contrast injection.  The test is noninvasive (only an IV line is needed), low risk and quick, and it produces beautiful picture (see linked images).

Best of all CCTA has an astoundingly high sensitivity for the presence of coronary disease.  In the moderate-risk population referred to above, this translates to a near-perfect NPV.  Two recent papers (in Circulation and JACC) assessed CCTA in the ER and determined that the sensitivity and NPV for this test is 100%.

There are downsides to CCTA.  It involves radiation (about the same amount you’d get with a nuclear stress test) and so we obviously don’t want to do this in anyone who might be pregnant.  It also doesn’t do as good a job of proving the presence of significant coronary disease as it does the absence thereof, and you might need more testing if the study is not normal.

So, if you’re planning to come to the ER with chest pain you may find that we are able use CCTA to get you in and out a little more quickly than we used to.  That’s good news for all involved (unless you were hoping to enjoy a few more hospital meals).