QTc calculator with Bazett and Fridericia correction

The QT interval on an electrocardiogram measures the time from the start of ventricular depolarisation to the end of repolarisation. It is not fixed: at a faster heart rate the interval is shorter, and at a slower rate it is longer. To compare QT across people and visits, and to detect drug effects on repolarisation, it is corrected for heart rate. The corrected value is the QTc.

Both formulas start from the RR interval, the time between consecutive R waves in seconds. RR = 60 / heart rate. At 75 beats per minute, RR is 0.80 seconds. Bazett divides QT by the square root of RR; Fridericia divides it by the cube root. For QT = 400 ms at 75 bpm, Bazett gives 400 / 0.894 = 447 ms and Fridericia gives 400 / 0.928 = 431 ms.

The difference grows at the extremes. At 100 bpm (RR 0.60 s), a QT of 340 ms gives a Bazett QTc of 439 ms but a Fridericia QTc of 403 ms. At 50 bpm (RR 1.20 s), QT 440 ms gives Bazett 402 ms and Fridericia 414 ms. Bazett tends to overestimate QTc when the heart rate is high, which can produce false alarms in people who are anxious, febrile or exercising, and is why Fridericia is often preferred in drug safety work. Other formulas exist, such as Framingham (QT + 154 x (1 - RR), with QT in milliseconds and RR in seconds) and Hodges (QT + 1.75 x (heart rate - 60)); this calculator covers Bazett and Fridericia only.

QT prolongation is one of the best known reasons a medicine can be stopped or restricted, so many protocols include scheduled ECGs and QTc rules: eligibility cut-offs at screening, stopping rules during treatment, and extra monitoring for participants who cross a threshold. Because these decisions depend on the number, every site should calculate it the same way, from the same inputs, in the same unit.

In Capture, the eCRF has QTc Bazett and Fridericia as built-in calculated fields, along with other calculators such as BMI and eGFR. The site enters QT and heart rate and the corrected values appear read-only, so they cannot be mistyped or calculated with a different formula at different sites. You can add edit checks that raise a query when a value is outside a plausible range, and the change history of each entry is kept in the field-level audit trail. See the vital signs eCRF template for how a measurement form looks, and adverse event reporting for what happens when a finding becomes an event.

Think about reading as well as calculating. A central ECG reader gives more consistent QT measurement than local reading, and the heart rate used for correction should come from the same tracing as the QT. Decide in the protocol whether QTc is derived from the machine's automatic measurement, the investigator's manual measurement or a central read, and capture which one in the form. For heart-related studies, EDC for cardiology trials covers the wider data model.

An absolute QTc of 440 ms means different things in different people: it may be a normal value for one participant and a clear lengthening for another whose baseline was 395 ms. That is why ECG safety analyses also look at the change from the participant's own baseline, commonly in categories such as an increase above 30 ms and above 60 ms. To use this properly the baseline has to be defined (a single pre-dose reading or the mean of triplicate readings), recorded in the same way and calculated with the same formula as the later values.

This is one reason to capture QT and heart rate rather than only the final QTc: you can recalculate the corrected value with a different formula if the analysis plan changes. Keep the raw inputs, the formula and the reading method together in the same record. Early-phase studies, where scheduled ECGs are routine, are a typical home for this; see EDC for Phase 1 trials.

This is a reference and planning tool. It is not a medical device, is not validated software, and must not be used to diagnose a patient or to decide on treatment. It assumes the QT and heart rate you enter were measured correctly, from the same beat or averaged consistently, in sinus rhythm. In atrial fibrillation, with a wide QRS or with a bundle branch block, QT correction is less reliable and specialist methods are used. It does not apply sex-specific cut-offs automatically, and it does not model population-specific or individual correction approaches.

For studies where QTc is a primary or key safety endpoint, a cardiologist or a statistician with ECG experience should specify the correction method, the central reading arrangements and the analysis (for example, the placebo-corrected change from baseline). They will also advise on how many replicate ECGs to take and how to handle the time of day.