ECG criteria
- Measure QT in ms from the beginning of QRS to the end of the T wave; the end of T is defined as the intersection of the isoelectric line with the tangent to the downslope of T, or by clear visual return to the isoelectric line; the U wave is not included; if T and U merge, the QT measurement is considered unreliable
- Select the lead with the clearest QRS onset and T wave end, comparing several leads (often II or V5–V6)
- Measure RR immediately preceding this complex (in ms) and convert to HR = 60000 / RR(ms)
- Plug into the formula: QTc = QT + 1.75 × (HR − 60), all values in ms
- During tachycardia (HR > 60), the adjustment is positive — QTc increases relative to the measured QT
- During bradycardia (HR < 60), the adjustment is negative — QTc decreases relative to the measured QT
- In irregular rhythms, especially atrial fibrillation, measure several consecutive complexes (usually at least 5–10, if the tracing allows), pre-define the averaging method for QT and RR, and interpret the result with caution
- Compare the obtained QTc with thresholds recommended in a specific guideline for the respective correction method, taking gender and QRS width into account; in wide QRS, RBBB/LBBB, or ventricular pacing, consider JT/JTc
What it looks like on ECG
The Hodges formula is not a visual ECG pattern, but a calculation tool applied to already measured QT and RR intervals. On the tracing, the physician measures the QT interval from the onset of the QRS complex to the end of the T wave. The end of T is best determined using the tangent method: a tangent is drawn along the steepest part of the terminal downslope of T and its intersection point with the isoelectric line is found; for a distinct T wave contour, a simple visual landmark — the point where the curve returns to the isoelectric line — is also acceptable. The U wave is not included in the measurement; if T and U are fused, the end of the T wave cannot be unambiguously identified, and the measurement in this lead should be considered indeterminate.
The lead for measurement is chosen by comparing several leads — often II or V5–V6, where QRS onset and T wave end are clearly visible and the U wave interferes less. If QT varies between leads, primary reliance is placed on the lead where the interval is measured most reliably; if quality is comparable and QT differs, the longer interval is taken into account, but one should not choose a visually dubious "long" QT solely due to its duration if the prolongation may be explained by artifact, a U wave, or an error in determining the end of T. The preceding RR interval is then measured and converted to HR.
Important caveat: in the presence of a wide QRS complex (bundle branch block, ventricular pacing, pronounced intraventricular conduction delay), the QT interval includes prolonged depolarization and poorly reflects repolarization per se. In these cases, QT and its correction by any formula become less reliable, and it is preferable to assess the JT interval (from the J point to the end of T) or JTc, or to use a special QRS-width correction.
Unlike Bazett's formula, where the correction is based on the square root of RR, the Hodges formula uses a linear dependency directly on HR:
| Parameter | How to obtain |
|---|---|
| QT, ms | measure on ECG |
| HR, min⁻¹ | 60000 / RR(ms) |
| QTc, ms | QT + 1.75 × (HR − 60) |
At an HR of 60 bpm, the correction is zero, and QTc equals the measured QT. The further HR deviates from 60, the greater the correction — upward during tachycardia, downward during bradycardia.
Mechanism
QT duration reflects the total duration of ventricular depolarization and repolarization. As HR increases, the diastolic interval between contractions shortens, and the QT interval also shortens electrophysiologically — this is due to the complex dynamics of repolarizing and depolarizing ion currents and their incomplete adaptation to cycle length changes, rather than a single mechanism. In bradycardia, QT conversely prolongs. Since this HR dependency is not perfectly linear, but rather hyperbolic, different correction formulas (Bazett, Fridericia, Framingham, Hodges) approximate this relationship differently. The Hodges formula is an empirical linear correction designed to reduce the systematic error inherent in Bazett's formula at high and low heart rates.
When to use
The Hodges formula can be used as an alternative to Bazett's formula when calculating QTc: the linear HR correction reduces overcorrection during tachycardia and undercorrection during bradycardia in certain ranges, which are characteristic of Bazett's formula. At the same time, at extreme HR values, no simple correction formula yields an ideal result. In protocols evaluating drug-induced QT prolongation, especially during tachycardia, Fridericia's formula is often preferred — the choice of formula in each case is tied to a specific guideline or study protocol rather than being a fixed rule. The Hodges formula is also encountered in clinical and research QTc calculations, including automated continuous ECG monitoring.
Differential diagnosis
Bazett's formula (QTc = QT / √RR in seconds) is historically the most common and is still widely found in automated ECG analyzers, but it provides excessive correction during tachycardia and insufficient correction during bradycardia. Fridericia's formula (QTc = QT / RR^(1/3)) is also power-based, but with a different exponent; during tachycardia (including assessment of drug-induced QT prolongation), it is often preferred precisely because of lesser overcorrection compared to Bazett. The Framingham formula (QTc = QT + 0.154 × (1 − RR)) is also a linear correction, but is structured using RR in seconds rather than HR directly. The main difference of the Hodges formula is the use of HR (rather than RR) as a variable and a coefficient of 1.75.
Clinical significance and management
The choice of QT correction formula is important where clinical decisions depend on the QTc value: continuing or stopping a drug that prolongs QT, assessing the risk of torsades de pointes, or diagnosing congenital long QT syndrome. Discrepancies between formulas are most noticeable at the limits of normal and at extreme heart rates, so a physician should know which formula was used by the automated ECG analyzer and, in doubtful cases, recalculate QTc manually using a second formula. Bazett's formula is historically widespread and more common in automated systems, but during tachycardia in assessing drug-induced QT prolongation, Fridericia's formula is often preferable; Hodges and Framingham formulas are considered potential alternatives, while the choice of a specific formula depends on the guideline, clinical context, and HR range. There are no strictly fixed independent QTc threshold values specifically for the Hodges formula in clinical guidelines — thresholds must be interpreted in the context of the specific guideline and correction method used, taking gender, measurement method, and clinical presentation into account, rather than relying on an isolated figure alone. With a wide QRS, it is preferable to rely on JT/JTc or a validated complex-width correction.
Common interpretation errors
Mixing formulas: using thresholds established for Bazett when calculating with Hodges without considering that the figures may not match. Inaccurate measurement of the T wave end — determining the end of T "by eye" instead of the tangent method, especially when fused with the U wave, distorts the result of any correction formula. Choosing the longest QT from several leads without considering tracing quality — prolongation may be an artifact or the result of a U wave rather than true prolongation. Calculating from a single complex in irregular rhythm (e.g., atrial fibrillation) without averaging over several cycles. Using standard QT instead of switching to JT/JTc in wide QRS, bundle branch block, or ventricular pacing. Automated QTc calculation by the device without physician verification, especially in the presence of artifacts or complex T waves.
1. Which variable is used directly in the Hodges formula for QT correction?
The Hodges formula is constructed as QTc = QT + 1.75 × (HR − 60), meaning the correction depends on HR rather than directly on RR.
2. At which HR is the correction by the Hodges formula equal to zero?
At an HR of 60 bpm, the multiplier (HR − 60) equals zero, therefore QTc matches the measured QT.
3. Which method for determining the end of the T wave is considered the most standardized when measuring QT?
The tangent method reduces the error in determining the end of T, especially with a shallow, biphasic T wave or its fusion with the U wave.
Frequently asked questions
How does the Hodges formula differ from Bazett's formula?
Bazett divides QT by the square root of RR, while Hodges adds a linear correction of 1.75 × (HR − 60) to QT. At heart rates far from 60 bpm, Bazett's formula provides overcorrection during tachycardia and undercorrection during bradycardia, whereas Hodges' linear correction behaves more stably in some cases, although it is not ideal at extreme heart rate values.
Which QT correction formula is considered the most accurate?
There is no single consensus: Bazett's formula is historically the most widespread and used in automated analyzers, but during tachycardia, especially when assessing drug-induced QT prolongation, the Fridericia formula is often preferred. The Hodges and Framingham formulas are considered possible alternatives — the choice depends on the guidelines, population, and heart rate range. The formula is indicated in the report if the calculation was performed manually.
Is it necessary to recalculate QTc manually if the ECG machine has already calculated it?
Yes, for doubtful or borderline values, as well as in the presence of artifacts when measuring the end of the T wave, it is worth verifying the calculation manually, comparing the result using two formulas if possible.
Can the Hodges formula be used in atrial fibrillation?
Technically it is possible, but due to RR irregularity, the result is less reliable. It is recommended to measure several consecutive complexes, determine the method for averaging QT and RR in advance, and interpret the final value with caution.
How to properly determine the end of the T wave when measuring QT?
The most standardized approach is the tangent method: a tangent drawn to the steepest part of the terminal descending limb of the T wave is extended until it intersects the isoelectric line; this point is considered the end of the T wave. A simple visual return of the curve to the baseline is acceptable if the T-wave contour is distinct. The U wave is not included in the measurement, and when the T and U waves fuse, the measurement result becomes unreliable.
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Related terms
More in the section "Criteria, Formulas, Algorithms"
Sources
- Surawicz B., Knilans T. Chou's Electrocardiography in Clinical Practice
- Goldberger A. Clinical Electrocardiography: A Simplified Approach
- AHA/ACCF/HRS Recommendations for the Standardization and Interpretation of the Electrocardiogram, 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.