What the QT interval represents
The QT interval spans ventricular electrical activation and recovery. Finding its start and end can be difficult when the waveform is noisy, the T wave is small, or the tracing contains unusual beats.
Professional guidance recommends visually checking computer-generated QT measurements. A watch recording provides one lead, while clinical assessment normally uses a high-quality ECG and the wider medical context.
Why QTc formulas differ
Heart rate affects the measured QT interval. QTc formulas adjust the value in different ways. None turns a single measurement into a clinical conclusion.
Using QT and R-R in seconds:
- Bazett:
QTc = QT / √RR - Fridericia:
QTc = QT / ∛RR - Framingham:
QTc = QT + 0.154 × (1 - RR)
Using QT in milliseconds and heart rate in beats per minute:
- Rautaharju:
QTc = QT × (120 + heart rate) / 180 - Hodges:
QTc = QT + 1.75 × (heart rate - 60)
Because the formulas make different assumptions, they can produce different results from the same recording. The selected formula should be named alongside the value. No formula is universally best for every clinical context.
ECG Tracker+ offers all five formulas plus an Auto option. Auto uses Framingham from 60 to 100 beats per minute and Fridericia outside that range. This app choice is intended to make comparison easier. It is not a clinical recommendation for an individual.
Why a threshold is not enough
A QTc value from a short single-lead recording should not be used by itself to label a result as normal, prolonged, safe, or dangerous. Formula choice, heart rate, waveform quality, beat selection, medications, clinical history, and the ECG leads reviewed can all affect professional interpretation.
ECG Tracker+ lets you compare several correction formulas. That comparison is informational and does not confirm or rule out long QT syndrome or any other condition.
Why can QTc change between recordings?
Heart rate, the estimated QT and R-R intervals, correction formula, waveform quality, and beat selection can all change the result.
Can QT be estimated from a watch ECG?
Research has studied QT measurement from Apple Watch recordings. Feasibility does not make a single-lead watch estimate equivalent to clinician-reviewed 12-lead measurement.
What should I save for a clinician?
Keep the original waveform, recording time, heart rate, selected QTc formula, estimated QT and QTc values, signal-quality information, and notes about symptoms or medications. A clinician can decide whether the watch tracing is useful and whether a standard ECG is needed.
Do not start, stop, or change a medicine because of an app-generated QT or QTc value. Ask the prescribing clinician or a pharmacist if you are concerned about a medication. Contact local emergency services for chest pain or pressure, fainting, severe shortness of breath, or another possible medical emergency.