The treadmill stress test has been used for calculating short–term risk of dying. For example, Duke Treadmill Score can be used to calculate the one-year risk of death in patients with suspected heart disease. However, this score is valid only in patients with heart disease.
There was no treadmill test score that predicted long–term risk of dying. Also, the test had to be for ordinary people, not just for heart patients. So, the researchers at Johns Hopkins University came up with the FIT treadmill score.
First, a little bit about the treadmill stress test itself.
Treadmill stress test
A treadmill exercise stress test is used as a diagnostic test for heart disease.
In this test, a person is made to walk on a treadmill and his electrocardiograph (ECG or EKG) is recorded. The speed and the incline of the treadmill are increased slowly. If a person has any heart blockages, his ECG will show changes. Based on the changes, and at what treadmill speed they appear, the doctors can judge the extent and the location of heart disease.
Normal heart
The heart has four chambers. Think of the heart as a 4–cylinder car engine. The muscles of these chambers fire in a particular sequence. That helps the heart to complete one full stroke of pumping blood out into the body.
The firing of the heart muscles is electrically synchronised. Small electrical impulses are generated in a region of the heart. They flow to different heart muscles, causing them to fire in a particular sequence. This electrical activity can be recorded by attaching sensor probes to the area around the chest. This recording is plotted as a graph and is called ECG or EKG (electrocardiogram). Thus, an ECG monitors the electrical activity going on in the heart muscles.
The heart muscles, just like other muscles in the body, need a blood supply. That blood is supplied by heart arteries.
If you are resting, your heart just has to supply a regular amount of blood to the body. So, it does not have to work hard.
However, when you are walking on a treadmill, or doing any physical activity, the muscles of the body need more oxygen. So, more blood needs to flow to those areas. And, the heart has to pump that higher volume of blood.
All heart muscles have to work harder for the heart to pump more blood out to the body. These muscles, in turn, need more blood supply for themselves. As a result, the demand on the heart arteries increases.
In normal conditions, only about 30% of the blood-carrying capacity of a heart artery is used. As their workload increases, more percentage of the capacity is used.
At one point, the capacity utilisation can reach nearly 100%. Since the heart muscles cannot get more extra blood, they cannot function still harder and the heart reaches its limit. You can call this your maximum heart capacity. If there is no disease in the heart, it may be working at the highest rate it theoretically can, called your maximum heart rate (HRM, or heart rate max).
However, if any heart muscle does not get adequate blood supply from its heart artery, that muscle’s function falters. This can be noted as a change in ECG (if the muscle misfires), or as fatigue (if the muscle fires weakly). In a normal heart, these changes are not noticed almost till the heart rate reaches its theoretical peak.
In a diseased heart, on the other hand, these changes are observed much before the heart reaches its theoretical peak. That is an indication that the heart is diseased.
Peak heart rate (HRM)
In general, the theoretical (predicted) peak heart rate is a function of age and gender. There are formulas to calculate HRM for any individual. However, remember that it is a rough estimate. The actual number may vary from the calculated number by 10–20 beats per minute.
Formulas for HRM calculation
- HRM = 220 – Age
However, this formula was found to be inaccurate in many situations. It gives a fairly large standard deviation (the actual HRMs of people differ a lot from the calculated values). - So, a new formula was developed:
HRM = 205.8 − (0.685 × Age)
It had a standard deviation of 6.4 beats per minute, much less than that in the first formula. - Another non–linear formula gave a much tighter standard deviation of 2 to 5 beats per minute:
HRM = 191.5 − (0.007 × Age2) - In 2010, researchers at Northwestern University modified the second formula, only for women, as:
HRM (women) = 206 − (0.88 × Age)
The standard deviation in the measured heart rates for a specific age and gender is 10 beats per minute. So, the peak heart rates of most people will be within 20 beats per minute (two standard deviations) of the calculated values.
Blockages, or a diseased heart
A heart artery that has blockages cannot supply its full blood-carrying capacity to the corresponding heart muscle. This is because the blockage reduces its cross–section area. You can think of that artery as a partially blocked pipe.
For example, in an artery with 50% blockage, its effective capacity is 50%. When the person is resting, only 30% capacity is needed. So, that person faces no problem while resting or doing light daily activities.
However, when such a person does any strenuous activity, such as climbing stairs or walking faster on the treadmill, the 50% blood-carrying capacity limit is reached quickly. The heart artery supplying blood to that specific heart muscle can not give any extra blood. As a result, that particular heart muscle starts fatiguing, or malfunctioning.
Such a person will not be able to go faster on the treadmill beyond this limit. If that person has an ECG machine connected to his heart, the ECG wave pattern may show changes. These changes may be specific to the part of the heart muscle that is not getting adequate blood supply.
Stress test results
Seeing the changes in the ECG as the speed of the treadmill increases, a cardiologist can estimate which heart muscle has a problem. He can also estimate the rough amount of blockage in the artery supplying that muscle.
Thus, the treadmill exercise stress test is the benchmark, diagnostic test for detecting heart disease. Unfortunately, it is an expensive and cumbersome test.
Stress test as a fitness test
Over the years, scientists have found that the treadmill stress test is also an excellent fitness test.
The stress test results are measured in terms of METs (Metabolic Equivalent of Task). The maximum workload, or maximum MET, a person can handle before exhaustion during a stress test can be considered as his fitness level.
MET (metabolic equivalent of task)
One MET is roughly equal to 1 calorie (kilocalorie, in scientific terms) of energy consumption per kg of body weight per hour. A person sitting idle will be burning 1 MET.
The act of sleeping will consume 0.9 MET, so MET is different from BMR (Basal Metabolic Rate).
Here is the Compendium of Physical Activities that gives MET values for hundreds of daily activities. Desk work in the office consumes 1.5 MET, light yoga exercises involve 2.3 MET, walking for pleasure at 5 km/hr is a 3.5 MET activity, and running at 10 km/hr is a 10 MET task.
Light activities require less than 3 METs. Moderate activities consume between 3 and 6 METs. Vigorous activities consume more than 6 METs.
For a more detailed discussion on METs, read on this website: This simple fitness test can predict if you will die early. While the focus of that article is to discuss many medical studies that used treadmill tests to evaluate fitness, it also offers a good read on METs. It gives MET values for many daily activities.
A typical treadmill fitness test
The treadmill test follows something called the Bruce Protocol, which consists of three-minute segments that increase in speed and incline gradient (angle of slope). There are seven such segments, or stages.
Stage 1: 1.7 mph or 2.7 kmph and 10% incline = 5 METs
Stage 2: 2.5 mph or 4 kmph and 12% incline = 7 METs
Stage 3: 3.4 mph or 5.5 kmph and 14% incline = 10 METs
Stage 4: 4.2 mph or 6.8 kmph and 16% incline = 13 METs
Stage 5: 5.0 mph or 8 kmph and 18% incline = 15 METs
Stage 6: 5.5 mph or 8.6 kmph and 20% incline = 18 METs
Stage 7: 5.5 mph or 8.6 kmph and 22% incline = 20 MET
The person walks on the treadmill. Every three minutes, the speed and gradient settings of the treadmill are changed to those of the next stage. They make the test harder and increase the MET.
At some point, the test is stopped. Either the person gets fatigued, develops symptoms of chest pain, or the ECG starts showing bad changes. The supervising physician decides when to stop the test.
The highest MET reached is recorded, based on the stage completed. The highest heart rate reached is also recorded.
In the exercise stress test for detecting heart disease, one must reach 9 minutes (complete 3 stages), and at least 85% of the peak heart rate. Otherwise, the stress test is considered ‘abnormal’.
The stress test for heart disease is either normal or abnormal. The person either has heart disease (abnormal) or does not (normal). There are no shades of grey.
However, scientists could extract different grades of fitness from the same stress test. Based on the highest MET and heart rate levels reached, they could calculate various levels of fitness.
Sensitivity of stress test
Sensitivity of a test is its ability to detect the problem, when there is a problem. So, if there is a blockage, a stress test should come out as ‘abnormal’. However, 32% of the time, it shows a ‘normal’ test result.
This is called a False–negative result — the test is falsely showing a negative or normal result. This is dangerous because the person may go away thinking that he has no problem when he actually has heart disease.
Thus, the sensitivity of the exercise stress test is 68% (= 100% – 32%). It can rightly detect heart disease only 68% of the time.
32% of the people who get a ‘normal’ stress test report may be having heart disease.
Specificity of stress test
The specificity of a test is its ability to rule out a problem when there is no problem. So, if there is no blockage, a stress test should come out ‘normal’. However, 23% of the time, it shows an ‘abnormal’ test result, indicating that there is heart disease, when actually there is none.
This is called a False–positive result — the test is falsely showing a positive or abnormal result. It is problematic because it may lead the person to undergo further, more expensive tests and needless treatments until the problem is ruled out. This is besides the mental anguish caused.
Thus, the specificity of the exercise stress test is 77% (= 100% – 23%). It can rightly rule out heart disease 77% of the time.
23% of the people who get an ‘abnormal’ stress test report may not be having any heart disease.
Imaging tests for heart disease
This article has nothing to do with heart disease testing. However, while learning about stress tests, we ended up discussing their use for detecting heart disease.
So, for the sake of comparison, let me mention some other imaging tests for heart disease. Table 1 below gives a summary of various imaging tests used for detecting heart disease. The table gives the sensitivities and specificities of the tests. So, you can judge how much to rely on their results.
Table 1. Summary of tests for detection of heart disease| Test | Sensitivity (%) | Specificity (%) | Advantages | Limitations |
|---|---|---|---|---|
| Exercise stress testing | 68 | 77 | Less expensive, limited equipment required, | Requires normal baseline ECG |
| Stress echocardiography | 79 | 87 | Assesses cardiac structure, function at rest and during stress, relatively inexpensive, does not require radiation | Image quality affected by body build and dependent on operator, limited time for imaging post–exercise |
| Exercise SPECT | 85 | 85 | Assesses passage of fluids in myocardium and function at rest and during stress | Cannot assess myocardium or valves, heart rhythm irregularities may affect results, requires radiation |
| Cardiac catheterization or angiogram | 98 | 82 | Preferred test, allows for detection and intervention | Invasive, requires radiation |
Some comments about the stress test
- Exercise stress tests cannot detect heart artery blockages smaller than 70%. Thus, if you get a ‘normal’ stress test, you could still have blockages smaller than 70%.
Often, heart attacks are caused by more minor blockages that can rupture. So, you may have a normal stress test, and still be vulnerable to heart attacks. - An exercise stress test is more useful to rule out heart disease than diagnose it, since it has more specificity (77%) than sensitivity (68%).
Remember, specificity helps rule out the disease, while sensitivity helps detect the problem. - Exercise stress test is no more considered a good test for normal people, in whom heart disease is not suspected. That should be logical. Can you guess why it would be so? Here is the reason:
If you take a random bunch of normal people, roughly 3.5% will have heart disease. So if you take 1000 such people, 35 will have heart disease, and 965 will be normal. Of course, you won’t know, beforehand, who is normal or who is not.
Now, the exercise stress test has 32% false–negatives, and 23% false–positives.
So, 22 (= 32% x 35) people will be falsely shown as normal, and 222 (= 23% x 965) people will be falsely shown as abnormal.
In other words, though the false–negative percentage is higher than the false–positive percentage, the false–negative numbers are lower than the false–positive numbers by an order of magnitude.
Thus, a huge 22% of normal people (222 of 1000) will be shown as having heart disease, adding to their testing and treatment costs.
Hence, the exercise stress test is not a good screening test. - Exercise stress test is a good, but not great, diagnostic test.
Note that screening tests are used for normal people, and diagnostic tests are used for people who are suspected to have a problem. In a crude way, screening tests say ‘the person is normal until found abnormal’, while diagnostic tests say ‘the person is abnormal until found normal’.
The study
In 2015, scientists at Johns Hopkins University published an article in the journal Mayo Clinic Proceedings that gave a formula for one’s chances of survival over the next 10 years, based on one’s fitness levels obtained from treadmill test MET values.
Generally, the stress test mentions results as ‘normal’ or ‘abnormal’. However, the scientists found that the results showed varying degrees of fitness among those with ‘normal’ stress test results.
Based on the results, the scientists came up with the following formula:
FIT treadmill score = (12 x METs) + (% of predicted MHR) – (4 x age) + (43, if female; and 0, if male).
Maximum predicted heart rate (predicted MHR) is calculated as given earlier in this article. The maximum heart rate achieved during exercise should be divided by predicted MHR to get the percentage of predicted MHR in the formula.
In the study, FIT treadmill scores ranged from negative 200 to positive 200.
Interpretation of the scores
- If your FIT score is 100 or higher, you have a 2% chance of death in the next 10 years.
- If your FIT score is between zero and 100, your chance of dying is 3% in the next decade.
- If your score is between negative 100 and 0, your chance of dying will be 11% in the next 10 years.
- Scores lower than negative 100 have a 38% chance of death in a decade.
Back–of–the–envelope calculation
A person who has a normal stress test has to complete at least 9 minutes (completion of stage 3), and have at least 85% of peak heart rate (MHR). So, the lowest qualifier to get a normal stress test will have 10 MET and 85% predicted MHR. So, his FIT treadmill score would be
(12 x 10) + 85 – (4 x age)
= 205 – (4 x age)
For a 50–year old male, the FIT score would be 5, and for a 50–year old female, the FIT score would be 48. So their chance of dying in the next decade would be 3%.
Go 2 years higher, and the males would have negative FIT scores. So 52–year old males, who barely passed (normal) a treadmill stress test, will have 11% chance of death over the next decade.
Of course, there are no such abrupt changes in real life. So, one can say that males beyond the age of 50 years should be careful even if they have barely passed a normal exercise stress test.
Another article on this website referred to earlier in this article—This fitness test can predict your chances of dying— shows that 10 MET is sort of the cutoff for fitness, below which the risk of death in the next 5–6 years rises significantly.
This is also the cutoff below which people fail the exercise treadmill test (‘abnormal’ result). As per the above FIT score formula, people who cannot sustain 10 MET for 3 minutes will have an 11% higher risk of dying in the next 10 years.
In conclusion
FIT treadmill score can be used for checking the 10–year mortality risk in normal people.
For a person who barely passes the treadmill test, the chance of dying in the next 10 years would be 3%. For a person failing the treadmill test, the chance would be between 3% (women) to 11% (men).
MET of 10 seems to be cut–off for fitness, below which 10–year mortality risk rises sharply.
First Published on: 14 February 2019
Image credit: tawatchai07 on Freepik