If you are having heart symptoms (chest pain or shortness of breath, especially with exertion), your doctor may order an exercise stress test to see if the symptoms are related to underlying heart disease. Approximately 10 million exercise stress tests are done in the US each year. What is an exercise stress test? What is it used for? What information does it provide and how can that information be used?
With the exercise stress test, the patient is attached to an EKG machine and a blood pressure cuff is placed on the arm. The patient then walks on the treadmill according to protocol. The most commonly used protocol is the Bruce protocol, developed by Dr Robert Bruce and in use since around 1963. The Bruce protocol starts at 1.7 mph and a 10% incline. Every three minutes, the treadmill goes higher and faster. Stage 2 of the Bruce protocol is 2.5 mph with a 12% incline, Stage 3 is 3.4 mph, 14%, and so on. During the stress test, the patient is monitored for symptoms (chest pain, shortness of breath), the blood pressure is checked every stage and an EKG is done at the end of each stage. The test is terminated if the patient has chest pain, can’t exercise further, develops changes on the EKG (ST segment depression), develops a significant arrhythmia or the blood pressure goes too high.
Let’s look at a typical report (shown above). Under “Exercise Test Summary” is the raw data; stage, time in stage, treadmill speed (mph), grade (%), heart rate (HR) and blood pressure (BP). This patient exercised for 7 minutes 23 seconds of a Bruce protocol, 3 minutes each in stages 1 and 2 and 1 minute 23 seconds in stage 3. The person achieved a work load of 10.4 METs. The metabolic equivalent of task (MET) is an estimation of the person’s exercise capacity, based on how many minutes of the Bruce protocol is achieved. The resting heart rate was 81 BPM and rose to 166 BPM or 92% of the predicated maximal heart rate. The heart rate should increase as the work load increases (work load increases with each progressive stage). Before the test, each person’s maximal heart rate is calculated. During the test, exercise continues until the patient achieves at least 85% of their predicated maximal heart rate. A rough estimate of the maximal heart rate is:
Max HR = 220 – age
More precise estimates are used based on age and sex:
Men: Max HR = 208- (0.7 x age)
Women: Max HR= 206 – (0.88 x age)
The blood pressure was 128/80 mmHg at rest and 166/80 mmHg at peak exercise. The systolic blood pressure increases linearly with each stage and returns to baseline or, more often, below the baseline in the recovery period. The average increase in systolic blood pressure with exercise is about 30 to 40 mmHg. A blood pressure over 200 to 210 mmHg represents exercise induced hypertension. Diastolic blood pressure usually stays about the same during exercise. This test was terminated due to fatigue and the person had no chest pain. There were occasional PVCs (premature ventricular contractions, extra beats from the lower chamber of the heart). There were some ST segment changes, 0.5-1.0 mm horizontal ST segment depression. ST segment changes of more than 1.0 mm are considered abnormal. Horizontal ST segment depression is worse than upsloping depression. Downsloping ST segment depression is worse than horizontal. The test was interpreted as normal.
The main indication for an exercise stress test is to see if there is myocardial ischemia (lack of blood flow to the heart muscle due to blockage in a heart artery). Ischemia is present if the patient has chest pain or if there are diagnostic ST segment changes. However, the stress test is not ideal. The diagnostic accuracy for heart artery blockage is only about 70%, so it misses about 30% of the time. Other tests, such as a nuclear stress test (accuracy 85-90%), CT imaging of the heart arteries or cardiac catheterization are more accurate in picking out significant heart artery disease. Despite this, there is a lot of useful information to be gleaned from the exercise stress test. Exercise or functional capacity is a strong predictor of mortality and heart disease. The better the exercise capacity, the lower the risk of cardiac outcomes. Functional capacity is measured in METs. As we saw, METs are calculated and shown on the stress test report. To determine if the number of METs achieved is good, here are the formulas for average METs for men and women:
Men: Predicted METs= 18 – (0.15 x age)
Women: Predicted METs= 14.7- (0.13 x age)
If the METs on your stress test are higher than the predicted, your functional capacity is good or high. If lower, then functional capacity is fair or poor. The data from an exercise stress test may also be used to formulate a personalized exercise prescription. For more information about exercise prescription see: http://sportscardiology.blogspot.com/2017/08/an-exercise-prescription-to-get-fitt.html
The exercise stress test can also predict prognosis. In general, the longer on the treadmill and the better the exercise capacity, the lower the risk for heart outcomes. Functional capacity is the most important parameter. Other prognostic variables include heart rate achieved, heart rate and blood pressure response to exercise, exercise induced chest pain and ST segment changes. To quantify prognosis, the Duke Treadmill Score was developed and has been in use since the early 1990’s. The score uses three variables: 1- exercise time, 2- ST segment changes, 3- presence or absence of chest pain. The formula is:
Duke Treadmill Score= exercise time – (5 x ST segment deviation) – (4 x CP index)
ST segment deviation is 0.5 mm, 1.0 mm, 1.5 mm, 2.0 mm, 2.5 mm, etc. CP index is 0 if there is no chest pain during exercise, 1 if there is chest pain, but the patient can still exercise through it, 3- if chest pain causes the test to be stopped. The Duke Treadmill Score ranges from -25 (highest risk) to + 15 (lowest risk). The one-year mortality for the low-risk category (score >=5) is only 0.25% but in the high-risk category (<= - 11) the mortality at one year is 5.25%.
In addition to all of the above, the exercise stress test is used to determine the heart rate response to exercise (whether the heart rate increases appropriately), the blood pressure response (whether the blood pressure goes up too high or drops during exercise, either scenario being a high-risk characteristic) and to see if there are exercise induced rhythm problems.
Aside from diagnosing heart artery disease, the exercise stress test provides a wealth of information that is important both for the physician and patient. Armed with the formulas above and a copy of your stress test report, you can have lots of fun. You can calculate your maximum heart rate (then take 85% of that to determine your target heart rate during exercise). You can determine your functional capacity (METs) and see how you stack up against the general public. You can calculate your Duke Treadmill Score. So sharpen your pencil, get your calculator out and get to work.
