ECG criteria
- Measure the RR interval (ms/s) preceding the complex with the measured QT
- Measure the QT interval from the onset of the Q wave (or R wave if Q is absent) to the end of the T wave in the same lead
- Convert RR to seconds for calculation: RR(s) = RR(ms) / 1000
- Calculate QTcF = QT(ms) / [RR(s)]^(1/3)
- Evaluate the result against threshold values: normal, borderline, prolonged QTc
- In cases of irregular rhythm (including atrial fibrillation), measure QT and the preceding RR over a series of consecutive complexes, calculate QTc for each, and average the values; there is no single standard number of cycles, but usually 10 or more are targeted when possible. The calculation method and reliability limitations of the result should be indicated separately
What it looks like on ECG
The Fridericia formula is not an ECG pattern, but rather a calculation method for heart rate correction of the QT interval. Therefore, the formula itself is not seen on the tracing, but rather its initial components: the QT interval (from the onset of QRS to the end of the T wave) and the RR interval (from R to R of the preceding cycle). Measurement is performed in lead II or V5–V6, where the T wave is clearly demarcated from the isoelectric line.
Formula: use QT in milliseconds and RR in seconds — QTcF(ms) = QT(ms) / [RR(s)]^(1/3). If both intervals are entered in seconds, the result is obtained in seconds and must be multiplied by 1000 to convert to milliseconds.
| Parameter | Significance |
|---|---|
| QT | measured interval, ms |
| RR | interval preceding the current cycle, s |
| QTcF | QT(ms) / [RR(s)]^(1/3) |
Example: QT = 400 ms, RR = 0.8 s (HR 75 bpm). RR^(1/3) ≈ 0.928. QTcF ≈ 400 / 0.928 ≈ 431 ms.
At high heart rates (short RR), the cube root of RR decreases more slowly than the square root in Bazett's formula, so QTcF is less prone to overestimating the true QT prolongation during tachycardia.
Mechanism
The QT interval reflects the total duration of ventricular depolarization and repolarization. As heart rate increases, the RR interval shortens, and the QT interval physiologically shortens in response — repolarization accelerates due to the shortening of the cardiomyocyte action potential. To compare QT in patients with different heart rates, it is "normalized" to a conventional RR = 1 s (HR 60 bpm).
Bazett's formula divides QT by the square root of RR. It typically overestimates QTc at high heart rates and underestimates it at low heart rates; this is especially noticeable at heart rates significantly different from 60 bpm. Therefore, Fridericia or other validated methods are often preferred during tachycardia. The Fridericia formula generally overestimates QTc less than Bazett's during tachycardia, though no fixed formula provides ideal correction at extreme heart rates; the choice of method depends on the clinical objective.
Clinical indications
- Tachycardia, when Bazett's formula yields an excessively high QTc
- Drug cardiotoxicity studies and clinical trials (QTcF is often used as a standard in QT prolongation protocols)
- Serial reassessment of QTc in the same patient during heart rate fluctuations
- As an alternative or adjunct to Bazett when the clinical picture and calculated QTc diverge
Differential diagnosis
- Bazett's formula — QTc = QT / √RR; the most common in clinical practice, but overestimates QTc during tachycardia and underestimates it during bradycardia.
- Framingham formula — linear correction QTc = QT + 0.154 × (1 − RR); a different mathematical approach that also performs well over a wide range of heart rates.
- Hodges formula — QTc = QT + 1.75 × (HR − 60); uses heart rate directly rather than RR.
All formulas are different methods for correcting the same measured QT; the discrepancy between them increases when the heart rate is far from 60 bpm. Therefore, when evaluating prolonged or shortened QT, the formula used should be specified.
Clinical significance and management
QTcF, like QTc by Bazett, is used to assess the risk of polymorphic ventricular tachycardia of the torsades de pointes type in congenital and acquired (drug-induced, electrolyte) long QT syndrome. Threshold values in guidelines are most often given for Bazett; the Fridericia formula is indicated as a more accurate alternative, especially at heart rates outside the 60–100 bpm range, but specific established thresholds for QTcF alone are not always consistently cited in clinical guidelines — some sources use the same figures (~450/460 ms), while others emphasize that they are approximate.
During interpretation, it is important for the clinician to: specify the formula used to calculate QTc, measure QT in the lead with the least distorted T wave, exclude artifacts (fusion with U wave, low T wave amplitude), correlate with clinical data (use of QT-prolonging drugs, electrolytes — potassium, magnesium, calcium), and in case of doubt, repeat the ECG and/or refer the patient for a cardiology/genetics consultation if congenital long QT syndrome is suspected.
Common interpretation errors
- Unit confusion: forgetting to convert RR to seconds or QT to ms, resulting in absurd QTcF values
- Using Bazett and Fridericia interchangeably without specifying which formula was used — this alters interpretation during tachycardia and bradycardia
- Measuring QT in a lead with a poorly distinguishable T wave end (fusion with U wave) without cross-checking in another lead
- Taking a single QT and RR measurement during an irregular rhythm (atrial fibrillation) instead of averaging over a series of cycles and specifying the calculation method
- Making clinical decisions based on a single borderline QTc value without considering clinical dynamics, electrolytes, and medications
1. What is the key mathematical difference between the Fridericia formula and the Bazett formula?
Bazett: QTc = QT/√RR; Fridericia: QTc = QT/RR^(1/3) — the cube root performs better during tachycardia.
2. In which clinical scenario is the Fridericia formula preferable to the Bazett formula?
At high heart rates, Bazett's formula systematically overestimates QTc, whereas the Fridericia formula yields a more stable result.
3. What intervals must be measured on the ECG to calculate QTcF?
QTcF is calculated from the measured QT interval and the preceding RR interval: QTcF = QT(ms)/[RR(s)]^(1/3).
Frequently asked questions
How does Fredericia's formula differ from Bazett's?
Bazett divides QT by the square root of RR, while Fridericia divides by the cube root. Consequently, during tachycardia, QTcF is lower than QTc by Bazett and is considered a more accurate reflection of true QT prolongation.
Which QTc formula is better — Bazett's or Fredericia's?
There is no consensus: Bazett is historically accepted and used in most clinical thresholds, while Fridericia is more accurate at high heart rates and is frequently used in drug cardiotoxicity studies. In practice, it is important to indicate which formula was used.
Can Fredericia's formula be used in atrial fibrillation?
Technically yes, but the result is less reliable due to RR irregularity. It is recommended to measure QT and the preceding RR over a series of consecutive complexes, calculate QTc for each, and average them, making sure to indicate the calculation method, as there is no single standardized number of cycles for all situations.
What QTcF is considered dangerous?
Clear validated thresholds specifically for QTcF are cited in guidelines less frequently than for Bazett's formula. In practice, the most robust marker of increased risk for torsades de pointes is a QTc of approximately 500 ms or more; values of 480–499 ms are regarded as marked prolongation requiring the search for reversible causes and serial monitoring, but not as an independent threshold for immediate danger. These figures were developed primarily for Bazett's formula and do not transfer unconditionally to QTcF.
How to calculate QTcF manually from an ECG?
Measure QT in ms and RR in ms in a single lead, convert RR into seconds, raise it to the 1/3 power (cube root), and divide QT (ms) by the resulting number.
ECG Course
Interpretation from basics to myocardial infarction — learn to read any ECG using a systematic algorithm.
- Video lessons and theory for every topic
- Simulator case studies under every lesson
- Unlimited ECG simulator practice
- Study guide: "All ECGs in 25 minutes"
ECG Strip Simulator
ECG, radiology, histology, pathomorphology, pharmacology — thousands of flashcards with detailed explanations.
- Real ECG strips, not textbook drawings
- Instant answers and explanations
- Streaks and progress tracking in your dashboard
Related terms
More in the section "Criteria, Formulas, Algorithms"
Sources
- Goldberger's Clinical Electrocardiography, current edition
- Surawicz B., Knilans T. Chou's Electrocardiography in Clinical Practice
- AHA/ACCF/HRS Recommendations for the Standardization and Interpretation of the ECG, 2009
- Orlov V.N. Guide to Electrocardiography
Educational material for medical students and physicians. Does not replace clinical guidelines and physical examination; the ECG must always be interpreted in conjunction with the clinical picture.