Chronotropic index
Chronotropic index is calculated as follows
[A] (peak heart rate – resting heart rate) / ((220 – age)
[B] (peak heart rate – resting heart rate) / ((220 – resting heart rate)
[C] (peak heart rate – resting heart rate) / age + resting heart rate)
[D] (peak heart rate – resting heart rate) / ((220 – age) – resting heart rate)
Chronotropic index of less than ——— % is considered a sign of chronotropic incompetence
[A] 50%
[B] 60%
[C] 70%
[D] 80%
Chronotropic index – [(HRmax − HRrest) / (220 − Age − HRrest)] × 100
Failure to achieve a chronotropic index higher than 80% defines chronotropic incompetence and predicts a poor prognosis
The above formula is not useful in case of – Atrial fibrillation
The chronotropic index is a measure used in cardiology to assess how well the heart rate increases during exercise compared to what is expected for a person’s age and fitness level.
Definition
The chronotropic index (CI) quantifies the heart rate reserve used during exercise
Normal vs. Abnormal
- Normal: CI ≥ 0.80 (80%) for men and women
- Chronotropic incompetence: CI < 0.80, indicating inadequate HR response to exercise
Clinical Significance
- Low CI suggests poor autonomic or sinus node function
- Associated with:
- Increased mortality in cardiac patients
- Heart failure
- Post-MI patients
- Patients on certain rate-limiting medications (e.g., beta-blockers)
Example
Patient: 60 years old, Resting HR = 70 bpm, Peak HR = 130 bpm
Predicted Max HR = 220 − 60 = 160 bpm
1. What is the chronotropic index (CI)?
A. The fraction of heart rate reserve used during exercise: (HR_peak − HR_rest)/(HR_predicted_max − HR_rest)
B. Peak systolic blood pressure divided by resting HR
C. The ratio of stroke volume to heart rate
D. The difference between maximal and minimal QT interval
CI quantifies how much of the available heart rate reserve is utilized during maximal exercise using predicted max HR as denominator.
2. Which formula is commonly used for predicted maximum heart rate in CI calculations?
A. 200 − (0.5 × age)
B. 220 − age
C. 180 − age
D. 200 + age
The simple, commonly used predicted max HR is 220 − age (other formulas exist, e.g., 208 − 0.7×age).
3. A Chronotropic Index (CI) value < 0.80 generally indicates:
A. Excellent chronotropic competence
B. Normal HR response
C. Chronotropic incompetence (inadequate HR response)
D. Always due to anxiety
CI < 0.8 is commonly used to define chronotropic incompetence — an inadequate increase in HR with exercise.
4. Which heart rates are used in the chronotropic index formula?
A. HR at rest only
B. HR at peak only
C. Predicted max HR only
D. Resting HR, peak exercise HR, and predicted maximum HR
CI uses HR_rest, HR_peak, and HR_predicted_max to calculate the fraction of reserve utilized.
5. Example: 60-year-old, HR_rest 70 bpm, HR_peak 130 bpm. Predicted max = 160 bpm. CI = ?
A. (130−70)/(160−70) = 60/90 = 0.67
B. (130−70)/(220−60) = 60/160 = 0.375
C. 130/70 = 1.86
D. 160/130 = 1.23
Calculate heart rate reserve used: (130−70)/(160−70)=60/90=0.67, indicating chronotropic incompetence (CI <0.8).
6. Which medication class commonly reduces chronotropic index by blunting HR response?
A. ACE inhibitors
B. Beta-blockers
C. Diuretics
D. Nitrates
Beta-blockers reduce peak HR and can produce lower CI; interpretation must consider medications.
7. Which clinical condition is associated with chronotropic incompetence?
A. Well-controlled hypothyroidism
B. Acute musculoskeletal pain only
C. Sinus node dysfunction and autonomic failure
D. Isolated allergic rhinitis
Sinus node disease and autonomic dysfunction commonly underlie inadequate chronotropic responses.
8. When assessing CI, why might alternative predicted-max formulas (e.g., 208−0.7×age) be used?
A. To increase resting HR
B. To measure BP
C. To measure stroke volume
D. To provide potentially more accurate predicted max HR across ages/populations
Alternate formulas may better fit certain populations and improve accuracy of predicted max HR for CI calculation.
9. Which test commonly provides the HR_peak for CI measurement?
A. Graded exercise stress test (treadmill or bike)
B. Resting ECG only
C. 24-hour Holter only
D. Chest X-ray
Peak HR during a symptom-limited graded exercise test is used for CI calculation.
10. Which interpretation is appropriate if CI is low but patient is taking a beta-blocker?
A. Always diagnose permanent sinus node disease
B. Consider medication effect; adjust or test off drug if clinically feasible
C. Ignore medications and proceed to pacemaker
D. Start anticoagulation
Beta-blockers can cause chronotropic incompetence; consider withholding drugs or accounting for their effect before diagnosing intrinsic CI.
11. Chronotropic incompetence is associated with prognosis in which populations?
A. Healthy athletes only
B. Pediatric populations exclusively
C. Patients with heart failure and post-MI — linked to worse outcomes
D. People with no cardiac risk factors
CI correlates with mortality and adverse outcomes in heart failure and post-MI cohorts.
12. Which is a limitation of using CI clinically?
A. It measures lung volumes
B. It is independent of age
C. It does not require exercise testing
D. It can be influenced by medications, submaximal effort, and choice of predicted-max formula
Interpretation must consider beta-blockers, poor exercise effort, and which predicted max equation is used.
13. Which management may be considered for symptomatic, medication-independent chronotropic incompetence?
A. Permanent pacemaker with rate-adaptive features
B. Immediate thrombolysis
C. High-dose steroids
D. Insulin therapy
When intrinsic sinus node dysfunction causes symptomatic CI and not explained by drugs, a rate-adaptive pacemaker may be indicated.
14. How can CI be adjusted when using a more accurate predicted max formula?
A. By ignoring resting HR
B. Replace (220 − age) with an alternate (e.g., 208 − 0.7×age) in the denominator
C. Use predicted BP instead
D. Double the predicted max
Using a different predicted-max alters the denominator and may give a more accurate CI for some age groups.
15. Which value best represents normal chronotropic reserve usage?
A. CI = 0.3
B. CI = 0.5
C. CI ≥ 0.80
D. CI = 0
CI ≥0.8 is commonly used as the threshold for adequate chronotropic response.
16. A submaximal exercise test may falsely:
A. Increase CI
B. Improve resting HR
C. Change predicted max formula
D. Lower CI (appear worse) because HR_peak is reduced
If exercise effort is submaximal, peak HR will be lower, reducing the calculated CI and possibly misclassifying competence.
17. In evaluating CI, which additional measure is useful to corroborate findings?
A. Ambulatory HR monitoring, exercise tolerance, and symptoms
B. Liver function tests
C. Urine culture
D. Bone density scan
Corroborate CI with clinical symptoms, exercise capacity, and Holter or other HR monitoring when needed.
18. Which population commonly shows lower CI even with normal autonomic function?
A. Young athletes
B. Elderly — age-related decline in maximal HR can reduce CI
C. Children only
D. Pregnant women exclusively
Aging reduces predicted and achieved maximal HR, which must be considered when interpreting CI.
19. Which statement about CI and beta-blockers is correct?
A. Beta-blockers increase CI
B. Beta-blockers have no effect on HR response
C. Beta-blockers lower peak HR and therefore reduce CI; interpret with caution
D. Beta-blockers change the formula for predicted max HR
Medication effects must be considered; sometimes tests are performed after withholding drugs when safe and appropriate.
20. Best short summary of CI utility:
A. CI is the only test needed for exercise evaluation
B. CI replaces Holter monitoring
C. CI measures BP response
D. CI is a helpful, quantitative index of HR response to exercise but must be interpreted with medications, effort and patient factors
CI gives objective data on HR response but interpretation requires context (meds, effort, age, comorbidities).
| Question | Chronotropic index – Answer |
|---|---|
| What is the chronotropic index? | A measure of heart rate response to exercise, calculated as the ratio of heart rate reserve achieved to the predicted maximum. |
| Formula for chronotropic index? | (HRpeak − HRrest) ÷ (HRpredicted max − HRrest). |
| How to calculate predicted maximum HR? | Commonly 220 − age (in years). |
| What is a normal chronotropic index? | Typically ≥ 0.80 in healthy individuals. |
| What does a low chronotropic index indicate? | Chronotropic incompetence — inadequate heart rate increase during exercise. |
| Clinical significance of chronotropic incompetence? | Associated with poor exercise capacity, higher cardiovascular mortality risk. |
| What conditions can cause low chronotropic index? | Sinus node dysfunction, beta-blocker therapy, autonomic failure. |
| Impact of beta-blockers? | They blunt heart rate response; adjusted interpretation may be needed. |
| How is chronotropic index measured in stress testing? | Using HR at peak exercise and resting HR from the same session. |
| Is it useful in non-exercise settings? | Primarily used in exercise testing, but can be relevant for pacemaker programming. |
| # | infographic – Key Fact |
|---|---|
| 1 | Chronotropic Index (CI) measures the heart rate response to exercise. |
| 2 | Formula: (HRpeak − HRrest) ÷ (Predicted Max HR − HRrest). |
| 3 | Predicted Max HR = 220 − age (commonly used formula). |
| 4 | Normal CI is generally ≥ 0.80 in healthy individuals. |
| 5 | A low CI indicates chronotropic incompetence. |
| 6 | Chronotropic incompetence = inadequate HR increase during exercise. |
| 7 | It is linked to increased cardiovascular and all-cause mortality. |
| 8 | Conditions causing low CI: sinus node dysfunction, heart failure, autonomic neuropathy. |
| 9 | Medications like beta-blockers can lower CI by reducing HR response. |
| 10 | CI assessment is a standard part of exercise stress testing. |
| 11 | CI may be adjusted for patients on rate-limiting medications. |
| 12 | A CI < 0.62 in patients with coronary artery disease predicts poor prognosis. |
| 13 | CI helps in pacemaker programming to optimize rate response. |
| 14 | Both inadequate and excessive HR response can indicate pathology. |
| 15 | CI is expressed as a decimal or percentage. |
| 16 | It reflects the balance between sympathetic activation and parasympathetic withdrawal during exercise. |
| 17 | Age, fitness level, and health status influence CI values. |
| 18 | CI is more reliable when combined with VOâ‚‚ max and workload data. |
| 19 | A normal CI suggests adequate cardiovascular reserve. |
| 20 | CI is an important prognostic and functional marker in cardiology. |
