Corrected QT interval (QTc)

Clinical background

The QT interval measures the total duration of ventricular depolarisation and repolarisation and varies with heart rate: it shortens in tachycardia and lengthens in bradycardia. To judge whether the QT is pathologically prolonged, a rate-corrected quantity is required, the QTc. Without correction, a normal QT at a high heart rate may be misread as prolonged, and a pathological prolongation at a low heart rate may be missed.

Assessment of the QTc is central to several clinical decisions: starting or uptitrating QT-prolonging drugs, monitoring during electrolyte disturbances such as hypokalaemia and hypomagnesaemia, evaluating syncope with suspected congenital long QT syndrome, and antiarrhythmic therapy. The choice of correction formula directly affects whether a patient is wrongly classified as high or low risk, and hence whether treatment is withdrawn or continued unnecessarily.

Calculating the corrected QT interval (QTc)

All four formulae start from the RR interval in seconds, calculated as:

$$\text{RR} = \frac{60}{\text{HR}}$$

where HR is the heart rate in beats per minute. QT is given in milliseconds and RR in seconds. The four corrections are:

$$\text{QTc}_{\text{Bazett}} = \frac{\text{QT}}{\sqrt{\text{RR}}}$$

$$\text{QTc}_{\text{Fridericia}} = \frac{\text{QT}}{\sqrt[3]{\text{RR}}}$$

$$\text{QTc}_{\text{Framingham}} = \text{QT} + 154 \times (1 - \text{RR})$$

$$\text{QTc}_{\text{Hodges}} = \text{QT} + 1{.}75 \times (\text{HR} - 60)$$

Bazett's formula, published in 1920, uses a square-root correction based on the assumption that the QT increases linearly with $\sqrt{\text{RR}}$. Fridericia's formula, from the same era, uses a cube root instead, which gives a weaker correction at extremes of heart rate. The Framingham and Hodges formulae are linear corrections developed later to reduce the rate dependence of Bazett. The 2009 AHA/ACCF/HRS standardisation document summarised and formalised the recommendations for QT measurement and heart rate correction [1].

The calculator includes sex as a field, but none of the four formulae incorporates sex in the calculation. The field is present to facilitate interpretation against sex-specific reference values, since women have a somewhat longer QTc than men on average.

Interpretation in practice

Interpretation of the QTc is governed by clinical context and sex-specific thresholds. Risk grading according to international practice is as follows:

An increase of more than 60 ms from the patient's baseline QTc marks a substantially increased risk of torsades de pointes irrespective of the absolute value [5]. An increase of 30 ms from baseline is regarded as clinically meaningful by the FDA and should prompt careful monitoring [5].

In a population-based cohort of adults over 55 years of age (the Rotterdam Study, 7,983 subjects), an abnormally prolonged QTc (>450 ms in men, >470 ms in women) was associated with a 2.5-fold increased risk of sudden cardiac death after adjustment for age, sex and cardiovascular comorbidity (HR 2.5, 95% CI 1.3 to 4.7) [6].

Validation and performance

Bazett's formula is the oldest and the most widely used in clinical practice, but also the most criticised. In a comparative study of 22,063 healthy individuals (mean age 34.9 years, 16,170 men and 5,893 women), Bazett showed the poorest agreement between QTc and heart rate, with a Pearson correlation of r = 0.483 and a regression slope of b = 1.12. Fridericia performed considerably better with r = 0.018 and b = 0.04, followed by Hodges (r = −0.182) and Framingham (r = 0.200) [2].

Another study analysed 452,440 ECG measurements from 539 healthy volunteers (259 women, mean age 33.3 years) and found that the Bazett correction gave significantly higher intra-individual variability than all other formulae (p < 0.00001). Fridericia and Framingham had the lowest variability and did not differ significantly from each other. The conclusion was unequivocal: the Bazett correction should no longer be used in clinical practice, and Fridericia or Framingham should replace it [3].

A modelling study based on 751 healthy subjects (mean age 34.2 years, 335 women) examined in thorough QT studies showed that Fridericia gives errors below 8 ms for $\Delta$QTc provided the heart rate does not change by more than $\pm 10$ beats per minute. The error margins of Framingham were in practice equivalent to those of Fridericia. The other formulae, including Bazett, gave larger errors [4].

In a clinical population of 920 patients treated with antipsychotics, the proportion with a prolonged QTc was 12.0% with machine-measured Bazett versus 2.2% with manually measured Fridericia (p < 0.001). The mean QTc was 435 ms with Bazett versus 394 ms with Fridericia. The overestimation was particularly marked in patients with tachycardia, a common adverse effect of antipsychotics with anticholinergic and adrenergic blocking properties [5].

Limitations

Bazett's systematic error. Bazett overcorrects at high heart rates (>100/min) and undercorrects at low ones (<60/min), which can lead both to false alarms and to missed risk. Many ECG machines use Bazett by default, and a healthy patient with tachycardia may be wrongly classified as high risk [2, 3, 5]. Fridericia is the correction recommended for drug safety assessment and should generally be preferred.

Bundle branch block and ventricular pacing. With a wide QRS due to bundle branch block or pacing, the QT is prolonged by definition through the prolonged depolarisation, not through abnormal repolarisation. The QTc then reflects abnormal depolarisation rather than a repolarisation disturbance and exaggerates the risk of torsades de pointes. Alternatives are the JT interval (from the end of the QRS to the end of the T wave) or a QT correction that adjusts for QRS duration, for example Bogossian's formula. No method is universally accepted, but the data support the use of the JT interval or the Hodges/Framingham correction after adjustment for QRS [7].

Rate stability. All fixed formulae presuppose that the heart rate is reasonably stable at the time of measurement. With large fluctuations in rate, as in atrial fibrillation or autonomic instability, the correction becomes unreliable. Individual correction based on the patient's own QT/RR profile is superior but rarely available in clinical practice [4].

Manual versus automatic measurement. Automatic QT measurement by ECG machines can be imprecise, particularly when the T wave is biphasic or flat or overlaps a U wave. The AHA recommends that the QT be measured manually in the lead showing the longest QT interval, using the median of repeated measurements in several leads [1, 5].

Sex and age. Women have on average a 10 to 20 ms longer QTc than men, and sex-specific thresholds take this into account. The sex field in the calculator does not affect the calculation but serves as a reminder that the threshold for a prolonged QTc is higher for women (>470 ms) than for men (>440 ms).

References

1. Rautaharju PM et al. AHA/ACCF/HRS recommendations for the standardization and interpretation of the electrocardiogram: part IV. J Am Coll Cardiol. 2009;53(11):982–91. PMID: 19281931
2. Hoek LJ et al. A comparison of the four most commonly used formulae to adjust the QT-interval for heart rate in 22,000 healthy subjects. J Electrocardiol. 2025;92:154091. PMID: 40829441
3. Andršová I et al. Influence of heart rate correction formulas on QTc interval stability. Sci Rep. 2021;11:3720. PMID: 34253795
4. Hnatkova K et al. Errors of fixed QT heart rate corrections used in the assessment of drug-induced QTc changes. Front Physiol. 2019;10:635. PMID: 31275152
5. Andric T et al. Estimation of cardiac QTc intervals in people prescribed antipsychotics: a comparison of correction factors. Ther Adv Psychopharmacol. 2022;12:20451253221104947. PMID: 35747226
6. Straus SM et al. Prolonged QTc interval and risk of sudden cardiac death in a population of older adults. J Am Coll Cardiol. 2006;47(2):362–7. PMID: 16412861
7. Funk MC et al. Assessment of QTc and risk of torsades de pointes in ventricular conduction delay and pacing: a review of the literature and call to action. J Acad Consult Liaison Psychiatry. 2021;62(5):501–510. PMID: 34489062