Calculate chronotropic response index from exercise heart rate, resting heart rate, age, and beta-blocker use, or find target peak heart rate.
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Chronotropic Response Index Formula
The chronotropic response index, or CRI, compares the heart rate reserve you achieved during exercise with the heart rate reserve predicted from your age and resting heart rate.
CRI = 100 * (HRmax - HRrest) / ((220 - age) - HRrest)
- CRI = chronotropic response index, expressed as a percentage
- HRmax = maximum heart rate reached during exercise, in beats per minute
- HRrest = resting heart rate, in beats per minute
- age = age in years
- 220 - age = age-predicted maximum heart rate
- (220 - age) - HRrest = predicted heart rate reserve
- HRmax - HRrest = achieved heart rate reserve
For the target peak heart rate mode, the calculator rearranges the CRI relationship to find the exercise heart rate needed to reach a selected CRI percentage.
Target Peak HR = HRrest + (Target CRI / 100) * ((220 - age) - HRrest)
- Target Peak HR = heart rate needed during exercise, in beats per minute
- Target CRI = selected chronotropic response threshold, as a percentage
- HRrest = resting heart rate, in beats per minute
- age = age in years
The CRI function calculates your achieved percentage and compares it with the selected interpretation threshold. The target peak HR function calculates the peak exercise heart rate needed to meet a standard, beta-blocker, 85%, or custom CRI threshold.
Common Chronotropic Response Thresholds
| Situation | Common threshold | General interpretation |
|---|---|---|
| No beta-blocker or not specified | CRI < 80% | May suggest chronotropic incompetence |
| Taking a beta-blocker | CRI ≤ 62% | May suggest chronotropic incompetence using the beta-blocker threshold |
| Higher exercise target | 85% of predicted HR reserve | Often used as a practical stress test target |
Example Chronotropic Response Calculations
Example 1: CRI from a stress test
You are 50 years old, your resting heart rate is 70 bpm, and your maximum exercise heart rate is 145 bpm.
CRI = 100 * (145 - 70) / ((220 - 50) - 70)
CRI = 100 * 75 / 100 = 75%
A CRI of 75% is below the standard 80% threshold, so it may suggest chronotropic incompetence if you are not taking a beta-blocker.
Example 2: Target peak heart rate
You are 60 years old, your resting heart rate is 65 bpm, and you want the target peak heart rate for an 80% CRI.
Target Peak HR = 65 + (80 / 100) * ((220 - 60) - 65)
Target Peak HR = 65 + 0.80 * 95 = 141 bpm
The target peak heart rate is 141 bpm.
Chronotropic Response Index FAQ
What does a low chronotropic response index mean?
A low CRI means your heart rate did not rise as much as expected during exercise compared with your predicted heart rate reserve. This can suggest chronotropic incompetence, but the result should be interpreted with the full stress test report, symptoms, medications, rhythm findings, and clinical history.
Why does beta-blocker use change the threshold?
Beta-blockers intentionally slow heart rate response. Because of that, a lower CRI threshold is commonly used when beta-blocker therapy is present. This calculator uses ≤62% as the beta-blocker threshold and <80% as the standard threshold.
What if my resting heart rate is higher than my age-predicted maximum heart rate?
The formula cannot be used if resting heart rate is greater than or equal to 220 minus age, because the predicted heart rate reserve would be zero or negative. In that case, the inputs should be checked, and the result should be interpreted using clinical judgment rather than this formula alone.
