Calcula el intervalo QT corregido con las fórmulas de Bazett, Fridericia, Framingham y Hodges. Clasificación de riesgo, pasos detallados y comparación entre fórmulas.
Ingrese los valores medidos del electrocardiograma
Rango normal: 350–440 ms
Afecta los umbrales de riesgo clínico
RR en segundos. Fridericia es más precisa a frecuencias altas/bajas.
Learn more about this calculator and how to use it
Did you know that a prolonged QTc interval is responsible for thousands of sudden cardiac deaths every year, often in people who seemed perfectly healthy? The QTc calculator is a critical clinical tool that helps physicians, nurses, pharmacists, and researchers detect dangerous heart rhythm abnormalities before they turn fatal. At thecalculators.net, you can compute your QTc value instantly and accurately using multiple validated formulas.
This guide covers everything you need to know: what QTc means, how it is calculated, how to interpret the results, and when to act on abnormal values.
QTc stands for the corrected QT interval. The QT interval is the time measured on an electrocardiogram (ECG) from the start of the Q wave to the end of the T wave. It represents the total duration of ventricular depolarization and repolarization, which is simply the time it takes for the heart's lower chambers to electrically recharge between beats.
The problem is that the raw QT interval changes with heart rate. A faster heart rate naturally shortens the QT interval, while a slower heart rate lengthens it. Without correcting for heart rate, comparing QT values between patients or over time is unreliable. That is where the QTc formula comes in. By mathematically adjusting for heart rate, the QTc gives a standardized value that allows meaningful comparison.
A prolonged QTc can lead to a life-threatening arrhythmia called Torsades de Pointes (TdP), which can degenerate into ventricular fibrillation and sudden cardiac arrest. According to a 2020 review published in the Journal of the American College of Cardiology, drug-induced QT prolongation is one of the leading causes of drug withdrawal from the market worldwide.
There are several validated correction formulas. The most widely used is Bazett's formula, but clinicians also rely on Fridericia, Framingham, and Hodges corrections depending on the clinical situation.
|
Formula |
Equation |
Best Used When |
|
Bazett |
QTc = QT / sqrt(RR) |
Standard clinical use, most common |
|
Fridericia |
QTc = QT / RR^(1/3) |
Faster or slower heart rates |
|
Framingham |
QTc = QT + 0.154 x (1 - RR) |
Population-based research studies |
|
Hodges |
QTc = QT + 1.75 x (HR - 60) |
Linear correction, bradycardia patients |
RR interval is the time in seconds between two consecutive R waves on the ECG. It is calculated as: RR = 60 / Heart Rate (bpm)
Let us walk through a complete calculation using Bazett's formula.
Given values: Heart Rate = 75 bpm, QT interval = 400 ms
1. Step 1: Convert QT to seconds: 400 ms = 0.400 seconds
2. Step 2: Calculate RR interval: RR = 60 / 75 = 0.800 seconds
3. Step 3: Apply Bazett's formula: QTc = 0.400 / sqrt(0.800)
4. Step 4: sqrt(0.800) = 0.894
5. Step 5: QTc = 0.400 / 0.894 = 0.447 seconds = 447 ms
Result: QTc = 447 ms. For an adult male this falls into the borderline prolonged range and warrants monitoring and clinical evaluation.
FEATURED SNIPPET: The QTc calculator corrects the raw QT interval on an ECG for heart rate using Bazett's formula: QTc = QT divided by the square root of the RR interval. A normal QTc is under 440 ms in men and under 460 ms in women. Values above 500 ms carry a high risk of life-threatening arrhythmia.
Using the QTc calculator at thecalculators.net takes less than 60 seconds. You only need two numbers from the patient's ECG report: the QT interval and the heart rate or RR interval.
QT Interval (ms): This is the raw uncorrected QT interval measured in milliseconds from your ECG strip. Most ECG machines display this value automatically. Ensure you are using the longest QT interval from the 12-lead ECG, typically found in leads II, V5, or V6.
Heart Rate (bpm) or RR Interval (ms): Enter either the heart rate in beats per minute or the RR interval in milliseconds. If you enter heart rate, the calculator automatically converts it to the RR interval for you.
Correction Formula: Select from Bazett, Fridericia, Framingham, or Hodges. Bazett is the clinical default. Use Fridericia when the heart rate is significantly above 100 bpm or below 60 bpm, as Bazett tends to over-correct at extremes.
Sex: Some calculators offer sex-based normal ranges since women naturally have slightly longer QTc intervals than men on average.
Once you click Calculate, the tool displays your corrected QTc value in milliseconds and flags it according to standard clinical thresholds.
|
QTc Value (ms) |
Classification |
Clinical Action |
|
Less than 440 (men) / Less than 460 (women) |
Normal |
No action needed |
|
440 to 470 (men) / 460 to 480 (women) |
Borderline Prolonged |
Monitor and review medications |
|
470 to 500 (men) / 480 to 500 (women) |
Prolonged |
Evaluate for causes, adjust medications |
|
Greater than 500 (any sex) |
Critically Prolonged |
Urgent evaluation, high TdP risk |
|
Less than 350 |
Short QTc |
Evaluate for Short QT Syndrome |
Always interpret QTc results in clinical context. A single elevated value does not confirm pathology. Trending over time, reviewing the medication list, electrolyte levels, and patient history are all essential parts of a thorough assessment. For additional health metrics, you may also find the mean arterial pressure calculator and the ABG calculator useful for a comprehensive cardiovascular and respiratory overview.
The QTc calculator is used across multiple clinical and research settings. Here are two detailed scenarios.
A 58-year-old woman is being started on azithromycin for a respiratory infection. Her ECG shows a QT interval of 460 ms and a heart rate of 68 bpm.
RR = 60 / 68 = 0.882 s
QTc (Bazett) = 0.460 / sqrt(0.882) = 0.460 / 0.939 = 0.490 s = 490 ms
A QTc of 490 ms in a woman is in the prolonged range. Azithromycin is a known QT-prolonging medication. The prescribing physician reviews her medication list, notes she is also on haloperidol, and decides to switch to amoxicillin instead. This is a classic case where a pre-prescription QTc check prevented a potentially fatal drug interaction.
A 44-year-old man in the ICU is on intravenous amiodarone for refractory atrial fibrillation. His daily ECG shows QT = 480 ms and HR = 55 bpm (bradycardia).
RR = 60 / 55 = 1.091 s
QTc (Bazett) = 0.480 / sqrt(1.091) = 0.480 / 1.044 = 0.460 s = 460 ms
QTc (Fridericia) = 0.480 / (1.091)^(1/3) = 0.480 / 1.029 = 0.466 s = 466 ms
Both formulas show QTc near 460 to 466 ms, which is borderline but not yet critically prolonged. The ICU team decides to continue amiodarone with daily ECG monitoring and electrolyte optimization. This example shows why using Fridericia's formula matters when bradycardia is present, since Bazett can under-correct at slow heart rates.
6. Measure the QT in multiple leads: Use the longest QT interval from leads II, V5, or V6 for the most accurate reading.
7. Use Fridericia for extreme heart rates: Bazett over-corrects above 100 bpm and under-corrects below 60 bpm. Switch to Fridericia or Framingham for better accuracy.
8. Check electrolytes: Low potassium, magnesium, or calcium can all prolong the QTc independently of medications.
9. Audit the medication list: Hundreds of drugs prolong QTc. Antibiotics, antipsychotics, antifungals, and antihistamines are common offenders. Always cross-reference with crediblemeds.org.
10. Establish a baseline: Get a baseline ECG before starting any QT-prolonging medication so you have a reference value for comparison.
11. Consider sex-specific thresholds: Women have naturally longer QTc intervals due to hormonal differences. Use the appropriate cut-offs when evaluating female patients.
According to data from the FDA, approximately 2 to 3 percent of patients taking QT-prolonging medications will develop clinically significant QT prolongation. For that reason, routine monitoring is not just good practice, it is a regulatory expectation for many drug classes.
Mistake 1: Using a single ECG result to make a permanent diagnosis. QTc values can vary from one ECG to the next due to positioning, lead placement, and measurement technique. Always trend the values over time.
Mistake 2: Ignoring the correction formula used. A QTc of 460 ms using Bazett is not the same clinical picture as 460 ms using Fridericia, particularly at extreme heart rates. Always document which formula was applied.
Mistake 3: Treating QTc in isolation. A mildly elevated QTc in an otherwise healthy young adult with no symptoms and no QT-prolonging medications is very different from the same value in an elderly patient on six medications.
Mistake 4: Confusing QT with QTc. Raw QT without heart rate correction is not comparable across patients. Never use raw QT for clinical decision-making without applying a correction formula.
Mistake 5: Forgetting U waves. In some patients, a prominent U wave (a small deflection after the T wave) can be mistakenly included in the QT measurement, artificially lengthening it. Trained ECG readers learn to exclude U waves from the measurement.
A 2022 study in Heart Rhythm Journal found that up to 30 percent of QT measurement errors in clinical settings were related to incorrect identification of the T-wave endpoint, highlighting the importance of automated QTc calculators with expert-reviewed algorithms.
The QTc calculator works best as part of a broader clinical assessment toolkit. Here are complementary tools that pair naturally with it.
|
Tool |
Purpose |
When to Use Together |
|
Mean Arterial Pressure Calculator |
Assess hemodynamic stability |
Patients with cardiac arrhythmias and blood pressure concerns |
|
ABG Calculator |
Interpret arterial blood gas results |
ICU patients with concurrent respiratory and cardiac issues |
|
A1C Calculator |
Track long-term blood sugar control |
Diabetic patients on QT-prolonging medications |
|
BMI Calculator |
Assess weight status |
Obesity is a risk factor for cardiac conduction abnormalities |
If you are also tracking metabolic health, the A1C calculator helps monitor long-term glucose control, which is important because hyperglycemia can impair cardiac conduction. For weight-related cardiovascular risk, the BMI calculator offers a quick starting point.For clinicians tracking overall patient wellness, pairing QTc monitoring with the body fat calculator provides a more complete metabolic picture. And for educational reference on statistical tools used in clinical research settings, the normal CDF calculator and the margin of error calculator are frequently used.
The QTc calculator is one of the most important tools in cardiology, pharmacology, and emergency medicine. Whether you are a clinician reviewing a patient's ECG, a pharmacist checking drug interactions, or a medical student learning cardiac physiology, understanding the corrected QT interval can genuinely save lives.
To recap the essentials: always correct the raw QT for heart rate, choose the right correction formula for the clinical situation, compare results against sex-specific thresholds, and never interpret QTc in isolation. Context, trending, and clinical judgment are irreplaceable.
Ready to calculate? Use the QTc calculator tool above and pair it with related tools like the half-life calculator for pharmacokinetics reference, or the scientific calculator for custom calculations.
Did you know that a prolonged QTc interval is responsible for thousands of sudden cardiac deaths every year, often in people who seemed perfectly healthy? The QTc calculator is a critical clinical tool that helps physicians, nurses, pharmacists, and researchers detect dangerous heart rhythm abnormalities before they turn fatal. At thecalculators.net, you can compute your QTc value instantly and accurately using multiple validated formulas.
This guide covers everything you need to know: what QTc means, how it is calculated, how to interpret the results, and when to act on abnormal values.
QTc stands for the corrected QT interval. The QT interval is the time measured on an electrocardiogram (ECG) from the start of the Q wave to the end of the T wave. It represents the total duration of ventricular depolarization and repolarization, which is simply the time it takes for the heart's lower chambers to electrically recharge between beats.
The problem is that the raw QT interval changes with heart rate. A faster heart rate naturally shortens the QT interval, while a slower heart rate lengthens it. Without correcting for heart rate, comparing QT values between patients or over time is unreliable. That is where the QTc formula comes in. By mathematically adjusting for heart rate, the QTc gives a standardized value that allows meaningful comparison.
A prolonged QTc can lead to a life-threatening arrhythmia called Torsades de Pointes (TdP), which can degenerate into ventricular fibrillation and sudden cardiac arrest. According to a 2020 review published in the Journal of the American College of Cardiology, drug-induced QT prolongation is one of the leading causes of drug withdrawal from the market worldwide.
There are several validated correction formulas. The most widely used is Bazett's formula, but clinicians also rely on Fridericia, Framingham, and Hodges corrections depending on the clinical situation.
|
Formula |
Equation |
Best Used When |
|
Bazett |
QTc = QT / sqrt(RR) |
Standard clinical use, most common |
|
Fridericia |
QTc = QT / RR^(1/3) |
Faster or slower heart rates |
|
Framingham |
QTc = QT + 0.154 x (1 - RR) |
Population-based research studies |
|
Hodges |
QTc = QT + 1.75 x (HR - 60) |
Linear correction, bradycardia patients |
RR interval is the time in seconds between two consecutive R waves on the ECG. It is calculated as: RR = 60 / Heart Rate (bpm)
Let us walk through a complete calculation using Bazett's formula.
Given values: Heart Rate = 75 bpm, QT interval = 400 ms
1. Step 1: Convert QT to seconds: 400 ms = 0.400 seconds
2. Step 2: Calculate RR interval: RR = 60 / 75 = 0.800 seconds
3. Step 3: Apply Bazett's formula: QTc = 0.400 / sqrt(0.800)
4. Step 4: sqrt(0.800) = 0.894
5. Step 5: QTc = 0.400 / 0.894 = 0.447 seconds = 447 ms
Result: QTc = 447 ms. For an adult male this falls into the borderline prolonged range and warrants monitoring and clinical evaluation.
FEATURED SNIPPET: The QTc calculator corrects the raw QT interval on an ECG for heart rate using Bazett's formula: QTc = QT divided by the square root of the RR interval. A normal QTc is under 440 ms in men and under 460 ms in women. Values above 500 ms carry a high risk of life-threatening arrhythmia.
Using the QTc calculator at thecalculators.net takes less than 60 seconds. You only need two numbers from the patient's ECG report: the QT interval and the heart rate or RR interval.
QT Interval (ms): This is the raw uncorrected QT interval measured in milliseconds from your ECG strip. Most ECG machines display this value automatically. Ensure you are using the longest QT interval from the 12-lead ECG, typically found in leads II, V5, or V6.
Heart Rate (bpm) or RR Interval (ms): Enter either the heart rate in beats per minute or the RR interval in milliseconds. If you enter heart rate, the calculator automatically converts it to the RR interval for you.
Correction Formula: Select from Bazett, Fridericia, Framingham, or Hodges. Bazett is the clinical default. Use Fridericia when the heart rate is significantly above 100 bpm or below 60 bpm, as Bazett tends to over-correct at extremes.
Sex: Some calculators offer sex-based normal ranges since women naturally have slightly longer QTc intervals than men on average.
Once you click Calculate, the tool displays your corrected QTc value in milliseconds and flags it according to standard clinical thresholds.
|
QTc Value (ms) |
Classification |
Clinical Action |
|
Less than 440 (men) / Less than 460 (women) |
Normal |
No action needed |
|
440 to 470 (men) / 460 to 480 (women) |
Borderline Prolonged |
Monitor and review medications |
|
470 to 500 (men) / 480 to 500 (women) |
Prolonged |
Evaluate for causes, adjust medications |
|
Greater than 500 (any sex) |
Critically Prolonged |
Urgent evaluation, high TdP risk |
|
Less than 350 |
Short QTc |
Evaluate for Short QT Syndrome |
Always interpret QTc results in clinical context. A single elevated value does not confirm pathology. Trending over time, reviewing the medication list, electrolyte levels, and patient history are all essential parts of a thorough assessment. For additional health metrics, you may also find the mean arterial pressure calculator and the ABG calculator useful for a comprehensive cardiovascular and respiratory overview.
The QTc calculator is used across multiple clinical and research settings. Here are two detailed scenarios.
A 58-year-old woman is being started on azithromycin for a respiratory infection. Her ECG shows a QT interval of 460 ms and a heart rate of 68 bpm.
RR = 60 / 68 = 0.882 s
QTc (Bazett) = 0.460 / sqrt(0.882) = 0.460 / 0.939 = 0.490 s = 490 ms
A QTc of 490 ms in a woman is in the prolonged range. Azithromycin is a known QT-prolonging medication. The prescribing physician reviews her medication list, notes she is also on haloperidol, and decides to switch to amoxicillin instead. This is a classic case where a pre-prescription QTc check prevented a potentially fatal drug interaction.
A 44-year-old man in the ICU is on intravenous amiodarone for refractory atrial fibrillation. His daily ECG shows QT = 480 ms and HR = 55 bpm (bradycardia).
RR = 60 / 55 = 1.091 s
QTc (Bazett) = 0.480 / sqrt(1.091) = 0.480 / 1.044 = 0.460 s = 460 ms
QTc (Fridericia) = 0.480 / (1.091)^(1/3) = 0.480 / 1.029 = 0.466 s = 466 ms
Both formulas show QTc near 460 to 466 ms, which is borderline but not yet critically prolonged. The ICU team decides to continue amiodarone with daily ECG monitoring and electrolyte optimization. This example shows why using Fridericia's formula matters when bradycardia is present, since Bazett can under-correct at slow heart rates.
6. Measure the QT in multiple leads: Use the longest QT interval from leads II, V5, or V6 for the most accurate reading.
7. Use Fridericia for extreme heart rates: Bazett over-corrects above 100 bpm and under-corrects below 60 bpm. Switch to Fridericia or Framingham for better accuracy.
8. Check electrolytes: Low potassium, magnesium, or calcium can all prolong the QTc independently of medications.
9. Audit the medication list: Hundreds of drugs prolong QTc. Antibiotics, antipsychotics, antifungals, and antihistamines are common offenders. Always cross-reference with crediblemeds.org.
10. Establish a baseline: Get a baseline ECG before starting any QT-prolonging medication so you have a reference value for comparison.
11. Consider sex-specific thresholds: Women have naturally longer QTc intervals due to hormonal differences. Use the appropriate cut-offs when evaluating female patients.
According to data from the FDA, approximately 2 to 3 percent of patients taking QT-prolonging medications will develop clinically significant QT prolongation. For that reason, routine monitoring is not just good practice, it is a regulatory expectation for many drug classes.
Mistake 1: Using a single ECG result to make a permanent diagnosis. QTc values can vary from one ECG to the next due to positioning, lead placement, and measurement technique. Always trend the values over time.
Mistake 2: Ignoring the correction formula used. A QTc of 460 ms using Bazett is not the same clinical picture as 460 ms using Fridericia, particularly at extreme heart rates. Always document which formula was applied.
Mistake 3: Treating QTc in isolation. A mildly elevated QTc in an otherwise healthy young adult with no symptoms and no QT-prolonging medications is very different from the same value in an elderly patient on six medications.
Mistake 4: Confusing QT with QTc. Raw QT without heart rate correction is not comparable across patients. Never use raw QT for clinical decision-making without applying a correction formula.
Mistake 5: Forgetting U waves. In some patients, a prominent U wave (a small deflection after the T wave) can be mistakenly included in the QT measurement, artificially lengthening it. Trained ECG readers learn to exclude U waves from the measurement.
A 2022 study in Heart Rhythm Journal found that up to 30 percent of QT measurement errors in clinical settings were related to incorrect identification of the T-wave endpoint, highlighting the importance of automated QTc calculators with expert-reviewed algorithms.
The QTc calculator works best as part of a broader clinical assessment toolkit. Here are complementary tools that pair naturally with it.
|
Tool |
Purpose |
When to Use Together |
|
Mean Arterial Pressure Calculator |
Assess hemodynamic stability |
Patients with cardiac arrhythmias and blood pressure concerns |
|
ABG Calculator |
Interpret arterial blood gas results |
ICU patients with concurrent respiratory and cardiac issues |
|
A1C Calculator |
Track long-term blood sugar control |
Diabetic patients on QT-prolonging medications |
|
BMI Calculator |
Assess weight status |
Obesity is a risk factor for cardiac conduction abnormalities |
If you are also tracking metabolic health, the A1C calculator helps monitor long-term glucose control, which is important because hyperglycemia can impair cardiac conduction. For weight-related cardiovascular risk, the BMI calculator offers a quick starting point.For clinicians tracking overall patient wellness, pairing QTc monitoring with the body fat calculator provides a more complete metabolic picture. And for educational reference on statistical tools used in clinical research settings, the normal CDF calculator and the margin of error calculator are frequently used.
The QTc calculator is one of the most important tools in cardiology, pharmacology, and emergency medicine. Whether you are a clinician reviewing a patient's ECG, a pharmacist checking drug interactions, or a medical student learning cardiac physiology, understanding the corrected QT interval can genuinely save lives.
To recap the essentials: always correct the raw QT for heart rate, choose the right correction formula for the clinical situation, compare results against sex-specific thresholds, and never interpret QTc in isolation. Context, trending, and clinical judgment are irreplaceable.
Ready to calculate? Use the QTc calculator tool above and pair it with related tools like the half-life calculator for pharmacokinetics reference, or the scientific calculator for custom calculations.
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