Interpret the measured QT in context: rhythm, rate, QRS duration, QTc formula variation, serial change, and bedside next steps.
Adults / post-pubertal
ECG context and measured values
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Optional below QRS 120 ms; no wide-QRS adjustment is applied.
Use the ECG/device documentation. If the type is not stated or you are unsure, select “Type not documented / not sure.”
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Baseline
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Reading 1
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Reading 2
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Reading 3
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Use comparable ECGs when possible: same lead, device or measurement approach, rhythm/QRS context, and correction method.
ECG measurement cues
QT intervalQRS onset to T-wave end. At 25 mm/sec, one small box is 40 ms and one large box is 200 ms.
QRS durationWidth of the QRS complex. Three small boxes is about 120 ms at 25 mm/sec.
RR intervalR wave to next R wave. Use seconds when measured directly from the strip.
Heart rateIf RR is not entered, TheraCALC uses HR to estimate RR. Use values from the same ECG whenever possible.
For licensed clinicians only. Confirm ECG measurements and rhythm before interpreting QTc.
Calculated QT assessment
⚠ Inputs changed — press Assess QT/QTc to update
Enter the measured QT, rate/RR, QRS duration, rhythm, and clinical question to generate a context-aware QTc interpretation.
Learn QT/QTc Assessment
Published formulas, practical method selection, and the clinical context TheraCALC uses to decide what should be emphasized.
Core Inputs
- QT: milliseconds from QRS onset to T-wave end. Measure in a lead where the T-wave end is clear, and visually confirm automated QT when the result could change therapy.
- RR: seconds between ventricular beats. If RR is not entered, TheraCALC derives it from heart rate; in atrial fibrillation or ectopy, representative beats matter because RR changes beat to beat.
- QRS: ventricular depolarization time. QRS 120 ms or longer usually triggers wide-QRS caution. QRS 100-119 ms is below that usual threshold, but morphology still matters if the tracing looks abnormal.
- T-wave end: low amplitude, notching, artifact, or T-U/U-wave fusion can move the measured QT and lower confidence.
Published QTc Formulas
Bazett: QT in milliseconds; RR in seconds.
Fridericia: QT in milliseconds; RR in seconds.
Framingham: QT in milliseconds; RR in seconds.
Hodges: QT in milliseconds; HR in beats per minute.
How TheraCALC Chooses A Method
- Medication question, regular narrow-QRS rhythm: Fridericia is usually emphasized because it is less pulled by heart rate than Bazett and is commonly used in drug-QT assessment.
- Serial monitoring: baseline fields open automatically, and the result prioritizes whether the same-method QTc change reaches the 60-ms warning threshold.
- Toxicology / overdose: conventional QTc is shown, but the interpretation stays cautious because rate, exposure, and clinical status can change quickly.
- Wide QRS from bundle-branch or bifascicular block: modified-QT and JT/JTc checks help separate depolarization delay from repolarization risk.
- Ventricular paced rhythm: paced-QT context is shown when QRS is wide; a pacemaker does not reliably remove torsades risk, so drug burden, electrolytes, pauses, premature beats, and effective pacing rate still matter.
- Irregular rhythm or uncertain beat selection: the tool lowers confidence instead of treating one QT/RR pair as definitive.
Why The Formulas Behave Differently
- Bazett: familiar and often printed by ECG systems. It is easy to recognize, but it is the most rate-sensitive of the common methods, especially when heart rate is far from 60 bpm.
- Fridericia: cube-root RR correction. It often stays more clinically stable across common medication-assessment heart rates, so it is the usual preferred displayed method for regular narrow-QRS drug decisions.
- Framingham: linear RR correction. It is useful as a transparent comparator when rate effects are driving disagreement between methods.
- Hodges: linear heart-rate correction. It gives another rate-focused comparison and is especially useful when the clinician has heart rate but not a directly measured RR interval.
- JT/JTc and modified QT: specialized wide-QRS checks. They are not a general replacement for QTc; they become relevant when bundle-branch block, QRS prolongation, or ventricular pacing is actually documented.
Scenario Map
- Medication start or continuation: prefer the less rate-distorted regular-rhythm estimate, then review electrolytes, drug burden, and serial change.
- EHR QTc looks alarming: compare the printed value with the preferred method and ask whether the machine used Bazett or an unknown formula.
- QRS 120 ms or wider: interpret QTc alongside wide-QRS context; confirm morphology before leaning on modified-QT or JT-style numbers.
- Paced rhythm: use paced-QT context only when pacing is present on the tracing, not just because the clinical question was selected.
- Borderline or consequential result: improve confidence first: confirm QT endpoint, use a representative RR from the same ECG, and compare with baseline when available.
How To Read The Output
- Calculated result: the measured inputs, named correction method, and numerical result or serial change. The result does not establish medication suitability or absence of risk; review the tracing and clinical context.
- Additional calculations: comparison estimates that help explain differences between correction methods, particularly when the EHR-reported QTc is elevated, heart rate is fast, or estimates fall on opposite sides of a clinical threshold.
- Measured inputs: a collapsed audit trail of the QT, HR/RR, QRS, and derived RR used for the result. It sits near the calculation table so the user can quickly confirm the arithmetic inputs before reading the interpretation.
- Status chips: short action-changing reasons for caution, such as a 60-ms serial increase, QTc at or above 500 ms, wide-QRS or paced-QT context, or limited measurement confidence.
Wide-QRS Output Cards
- Primary conventional estimate: shows what the usual QTc formula gives before adjusting the interpretation for a wide QRS.
- Why QTc can overstate risk: in documented bundle-branch block or ventricular pacing, the measured QT includes delayed ventricular depolarization/activation as well as repolarization.
- Method selection: no preferred QRS-adjusted estimate is provided because the current context choices do not distinguish the exact conduction subtype or pacing site. Conventional QTc is heart-rate corrected; not adjusted for wide QRS. Supporting formula calculations remain available with their limitations.
- Modified-QT estimate: the Bogossian formula calculation subtracts 50% of QRS from QT before rate correction. This arithmetic is not proof that the method applies to the selected pattern, and a lower estimate does not establish safety.
- JTc interpretation: These JTc estimates apply heart-rate correction to JT (QT minus QRS duration). They measure a different interval from the primary QTc or modified-QTc result, so the numbers are not directly interchangeable. A lower JTc does not by itself indicate lower risk; interpretation requires reference limits specific to the JTc method.
- Unspecified ECG context: the current choices group bundle-branch patterns and do not identify pacing sites. QRS width alone cannot distinguish LBBB, bifascicular block, isolated RBBB or a pacing type; no specialized method is automatically preferred.
Wide QRS: separating activation delay from repolarization
This conceptual schematic is for orientation only, not a diagnostic ECG tracing. It shows why a wide QRS can lengthen measured QT because ventricular activation/depolarization takes longer; JT and modified QT help focus interpretation on the repolarization portion.
Narrow QRS
Wide QRS from delayed activation
Teaching point
Wide QRS can lengthen measured QT through delayed ventricular activation. Repolarization may also be altered.
Why TheraCALC shows modified QT and JT
JT is QT minus QRS duration. Modified QT subtracts part of QRS before heart-rate correction.
Illustrative shapes and durations, not a diagnostic tracing or a specific bundle-branch pattern. The slightly longer JT on the right illustrates possible additional repolarization prolongation; it is not an inevitable effect of wide QRS.
Literature anchors: AHA/ACCF/HRS ECG standardization recommendations; Bogossian wide-QRS modified-QT review; AHA/ACCF torsades-prevention statement.
Clinical Scenarios
- Fast heart rate with EHR QTc above 500 ms: many ECG/EHR systems report Bazett or do not clearly state the method. Bazett tends to overcorrect QTc at elevated heart rates because of its square-root RR formula; FDA/ICH E14 guidance supports QTcF as the usual fixed-formula comparator when corrected QT data are reported, with Hodges and Framingham kept as cross-checks when formulas disagree.
- Irregular rhythm: use a representative stable beat or direct ECG review when the decision is consequential; a single long or short RR interval can swing the corrected value.
- Wide QRS or paced rhythm: decide whether the concern is ventricular repolarization or simply prolonged depolarization from conduction delay.
- Serial monitoring: compare current and baseline ECGs only when the measurements are clinically comparable; same-method QTc change of 60 ms or more should change the action even if the current QTc is below 500 ms.
Clinical Thresholds
- Common adult reference cutoffs are about 450 ms for male and 460 ms for female in the AHA/ACCF/HRS ECG standardization recommendations.
- QTc at or above 500 ms, or a comparable increase of 60 ms or more after a QT-risk exposure, is treated as a major warning signal in drug-QT and torsades-prevention guidance.
- A 60-ms increase is not just a rule of thumb; it is an evidence-supported warning threshold that should prompt review of measurement quality, reversible risk factors, drug exposure, and monitoring needs.
- Medications differ in their effects on the QT interval and their propensity to cause torsades de pointes. Use CredibleMeds as a supplemental drug-risk resource alongside prescribing information and clinical guidance. Review label warnings, route, dose, duration, renal/hepatic clearance, interacting medicines, and whether the QT change is new versus chronic.
- Potassium below the local lab range should be corrected when safe. In high-risk inpatient torsades-prevention situations, AHA/ACCF guidance notes potassium 4.5-5.0 mmol/L may be considered, with limited evidence.
- Maintain serum magnesium within the normal range in hospitalized patients at risk for torsades de pointes. Intravenous magnesium sulfate is used to treat torsades even when serum magnesium is normal. (Drew et al., AHA/ACCF 2010, pp. 1053 and 1055.) Clinically relevant hypocalcemia means calcium below the lab reference range, especially if severe, symptomatic, persistent, or paired with other torsades risks.