QTc Calculator

The QTc Calculator corrects the QT interval for heart rate, producing the corrected QT interval (QTc) essential for assessing risk of ventricular arrhythmias, particularly Torsades de Pointes (TdP). The QT interval represents the total duration of ventricular depolarization and repolarization on ECG, and naturally shortens as heart rate increases, necessitating correction formulas to standardize measurement to a heart rate of 60 bpm.

Three correction formulas are offered. Bazett (QTc = QT/√RR) is the FDA-required standard but overcorrects at high heart rates and undercorrects at low rates. Fridericia (QTc = QT/RR^(1/3)) provides more accurate correction across extreme heart rates and is increasingly preferred in research per ICH E14 guidelines. Framingham (QTc = QT + 154×(1-RR)) is a linear correction avoiding nonlinear distortions at any heart rate.

QT prolongation increases TdP risk, a potentially fatal polymorphic ventricular tachycardia. QTc above 500 ms carries substantially increased risk. QT prolongation can be congenital (Long QT Syndrome, 1 in 2,000 people) or acquired through medications (antiarrhythmics, antibiotics, antipsychotics, methadone), electrolyte imbalances (hypokalemia, hypomagnesemia, hypocalcemia), and structural heart disease.

Accurate QT measurement requires careful T wave termination identification. The tangent method is most reproducible. Automated ECG algorithms should be verified clinically as they can be inaccurate with artifact, abnormal morphology, or bundle branch block. QTc monitoring is recommended for patients on QT-prolonging medications, especially during initiation or dose adjustment.

Risk factors for TdP include concomitant QT-prolonging drugs, female sex (2-3x higher risk), advanced age, bradycardia, heart failure, hepatic/renal impairment, and electrolyte disturbances. When QTc exceeds 500 ms or increases by more than 60 ms from baseline, drug discontinuation or dose reduction should be strongly considered and electrolytes optimized.

Pharmacogenomic testing can identify patients with genetic ion channel variants at increased risk for drug-induced QT prolongation, enabling personalized medication selection in the era of precision medicine.