How do you calculate the QTc interval in a 12-lead electrocardiogram (ECG)?

How to Calculate QTc Interval

Measure the QT interval manually from the onset of the QRS complex to the end of the T wave in lead II, V3, or V5 (whichever shows the longest, clearest T wave), then correct for heart rate using Fridericia's formula (QTc = QT/∛RR) rather than Bazett's formula, as Bazett systematically overcorrects at heart rates above 80-85 bpm. 1, 2

Step 1: Select the Appropriate Lead

• Choose the lead with the longest T wave amplitude (at least 2 mm) and a well-defined T-wave end 2
• Lead II, V3, or V5 are recommended for QT measurement 2
• Use the same lead consistently for serial measurements in the same patient, as QT intervals vary significantly across the 12 leads 2
• Document which lead you are using for future reference 2

Step 2: Measure the QT Interval

Identifying the Start Point

• Mark the beginning of the QRS complex (where it first deviates from the isoelectric baseline) 2

Identifying the End Point (Most Critical Step)

• Use the "tangent method": draw a line tangent to the steepest downslope of the T wave and mark where it intersects the isoelectric baseline 2, 3
• This intersection point is the end of the T wave 2
• If the T wave is biphasic or notched, the end of the entire T-wave complex should be considered the endpoint 2
• Exclude discrete U waves that occur after the T wave returns to baseline 2
• If U waves are superimposed on the T wave, include them in the measurement 2

Measurement Technique

• Increase waveform size (standardization from 1 to 2,3, or 4) and recording speed (from 25 to 50 mm/s) to enhance visualization 2
• Measure the QT interval in milliseconds (ms) 2
• Measure the preceding RR interval in the same manner 2

Step 3: Correct for Heart Rate

Recommended Formula: Fridericia

• QTc = QT / ∛RR (where RR is in seconds) 1, 2
• Fridericia's formula is more accurate than Bazett's, especially at heart rates >80 bpm 1, 4
• Bazett's formula (QTc = QT/√RR) overcorrects at higher heart rates and undercorrects at lower heart rates 1, 2

When Correction May Be Inaccurate

• Heart rates <40 bpm or >120 bpm may produce inaccurate corrections with any formula 2
• In these cases, document the limitation and consider using multiple correction methods 2

Step 4: Interpret the Results

Normal Values (Gender-Specific After Puberty)

• Men: QTc <450 ms 1, 2
• Women: QTc <460 ms 1, 2
• Children <12 years: QTc <450 ms (no gender difference) 1

Abnormal Values

• Prolonged: QTc ≥450 ms (men) or ≥460 ms (women) 1, 2
• High risk: QTc >500 ms or increase >60 ms from baseline 1, 2
• Short QT: QTc ≤390 ms 1

Special Circumstances

Bundle Branch Block or Wide QRS

• Subtract the difference in QRS width before and after the block from the QT measurement 2
• Alternatively, measure the JT interval (from end of QRS to end of T wave) to eliminate the confounding widened QRS 2
• Apply the chosen method consistently over time 2

Atrial Fibrillation

• Identify the shortest and longest RR intervals, calculate QTc for each, and average the two values 2
• Alternatively, assess whether the interval from R wave to T-wave peak is >50% of the RR interval on average (suggests QTc >500 ms) 2

Athletes

• Athletes typically have longer uncorrected QT intervals due to lower resting heart rates, but their QTc is usually within normal limits 2

Common Pitfalls to Avoid

• Do not rely on automated machine measurements using Bazett's formula, as they overestimate QTc by an average of 40 ms compared to manual measurement with Fridericia's formula 4
• Do not measure QT in leads with low-amplitude T waves or unclear T-wave endpoints 2
• Do not use different leads for serial comparisons in the same patient 2
• Do not include discrete U waves in the QT measurement 2
• Do not forget to account for gender differences when interpreting QTc values after puberty 1, 2
• Do not measure QT during significant sinus arrhythmia without accounting for RR variability 2