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

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

• Primary recommendation: the number or serial change TheraCALC thinks best fits the entered rhythm, rate, QRS duration, and clinical question. This is the fast bedside anchor, not a substitute for reviewing the tracing.
• Additional calculations: comparison numbers that explain disagreement. They are useful when the EHR-reported QTc is high, the heart rate is fast, or a colleague asks why one formula crosses 500 ms and another does not.
• 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.
• EHR comparison: checks whether the printed ECG QTc conflicts with the preferred method. If the EHR method is unknown, the tool treats it as an unknown comparator rather than assuming it is Bazett.
• 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.
• Repolarization cross-check: uses JT/JTc logic to focus on repolarization after subtracting QRS duration; it is a supporting signal, not a universal replacement for QTc.
• Modified-QT estimate: when bundle-branch block, bifascicular block, or ventricular pacing is documented with QRS 120 ms or wider, Bogossian-style modified QT is displayed as a specialized comparator.
• Unspecified ECG context: if QRS is wide but the ECG context still says regular narrow-QRS rhythm or uncertain, the tool shows wide-QRS comparison numbers but avoids promoting a specialized method as definitive.

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 can overcorrect at higher heart rates, while Fridericia is commonly supported for drug-QT assessment because it is less rate distorted.
• 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.
• 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.
• Magnesium should be normal in at-risk patients; intravenous magnesium is a treatment for torsades regardless of measured serum magnesium. Clinically relevant hypocalcemia means calcium below the lab reference range, especially if severe, symptomatic, persistent, or paired with other torsades risks.

Evidence Links

• FDA / ICH E14-S7B QT/QTc guidance Q&A
• AHA/ACCF/HRS ECG standardization recommendations, Part IV
• AHA/ACCF/HRS ECG standardization recommendations, Part III
• AHA/ACCF torsades prevention statement, JACC 2010
• AHA/ACCF torsades prevention statement, free full text
• Tooley et al., atrial fibrillation QT assessment
• Computer ECG interpretation errors in atrial fibrillation
• Bagliani et al., atrial flutter ECG approach
• Marcus, premature ventricular complex evaluation
• Bogossian et al., bundle branch block QTc evaluation
• Luo et al., commonly used QT correction formula comparison
• Vandenberk et al., QT correction formula monitoring comparison
• CredibleMeds QTdrugs resources