A middle-aged surgeon recently asked me to do a stress test on him. Why? I asked. Are you having problems? No, he said. He simply wanted to have his heart checked out, to be sure.
I asked him a few questions. I learned he exercised vigorously almost every day. He had no chest pain, breathing problems, or dizzy episodes. He did not smoke, nor did he have high blood pressure, diabetes or high cholesterol levels. Both of his parents lived well into their eighties.
When I told him getting a stress test was a bad idea, he looked at me funny. He said he didn’t want to die suddenly or have a heart attack.
If only testing for heart attack were that easy. It is not. Despite massive gains in medical knowledge, there is no single test that can predict when and if a heart attack will occur.
What a stress test can do is help us establish a diagnosis of heart disease, estimate future risk and learn more about specific situations, such as heart rhythm problems.
The idea of cardiac stress tests is putting the heart under stress and seeing what happens. If the heart tolerates the stress, one can feel confident there isn’t significant heart disease, which most often comes in the form of blockages in the arteries that supply blood to the heart—the coronary arteries.
A stress test is usually done with a treadmill or bike. The workload starts out slowly and is gradually increased, causing the heart to beat faster and harder and increasing the demand for blood and nutrients. A healthy heart can supply the increased demand, and shows no signs of strain. But, if there are blockages or other forms of disease, we may see signs of strain on the ECG recordings or on images taken after the test.
Signals of strain during a stress test aren’t like lab values, which are either high or low, normal or abnormal. Signs of strain of the heart during stress are less clear, more open to interpretation. For instance, many non-cardiac conditions can cause the heart to look as though it is having trouble when it is actually not – things like abnormal electrolytes, fluctuations in blood pressure, breathing too fast, and certain medications. Not surprisingly then, false positive stress tests are a common problem.
The opposite problem also occurs. False negatives happen when the stress test fails to reveal a potentially dangerous blockage. The supply/demand challenge of stress tests only identifies major blockages. But, the vexing way heart disease works is that most often it is the minor—not major—blockages that cause heart attacks or sudden death. Stress tests can’t see these minor blockages. The next sentence is not a typo: You can pass a stress test and have a major heart attack the next day.
You might wonder why we do stress tests. Here are three of the main reasons:
- Help in making a diagnosis. In someone who is having suspicious symptoms, like chest pain or breathlessness, a stress test can help establish or exclude a cardiac reason for the symptoms. Notice I used the word help. A stress test isn’t definitive, instead it adds or subtracts to the probability of heart disease.
- Predict future risk. The extremes of a stress test can provide some helpful insight. A person who has to stop exercising early in the test because of signs of strain carries a high risk of having severe blockages and future cardiac events. In comparison, a person who achieves a high level of exercise carries a much lower future risk of events.
- Observe the heart rhythm during exercise. Some abnormalities of the rhythm (arrhythmia) occur only when a patient is active. An exercise test can aid in making the diagnosis and determining the best treatment of an abnormally slow, irregular, or fast heart rhythm.
Now you can see why I did not recommend our surgeon undergo a stress test. He did not have suspicious symptoms. He had good exercise tolerance and no cardiac risk factors. His future cardiac outlook was terrific. Given the limits of stress tests, there was no useful information to be gained.
In the end, stress tests are like all other tests in medicine: their usefulness depends on the situation.