How do you calculate the QT interval on an electrocardiogram (ECG)?

How to Calculate the QT Interval on ECG

Measure the QT interval from the beginning of the QRS complex to the end of the T wave, identifying the T wave's end by drawing a tangent to its steepest downslope and marking where this line intersects the baseline. 1, 2

Manual Measurement Technique

Identifying the Start Point

• Begin measurement at the onset of the QRS complex, which is usually readily apparent on the ECG 1

Identifying the End Point (Critical Step)

• Draw a tangent line along the steepest downslope of the T wave and mark where it intersects the isoelectric baseline—this intersection defines the end of the T wave 1, 2
• This "teach-the-tangent" or "avoid-the-tail" method provides the most accurate and reproducible measurements 3
• If the T wave is notched or bifid, measure to the end of the entire T-wave complex 1

Lead Selection

• Choose a lead with T-wave amplitude of at least 2 mm and a well-defined T-wave end 1
• Leads II, V5, and V6 typically provide the longest and most reliable QT measurements 2
• Always use the same lead for serial measurements in the same patient, as QT intervals vary significantly across the 12 leads 1, 2

Handling U Waves (Common Pitfall)

• Do NOT include discrete U waves that arise after the T wave returns to baseline 1, 2
• If U waves are superimposed on T waves and cannot be separated, measure in leads aVR or aVL which often lack U waves 1
• Alternatively, use the tangent method to the T-wave downslope to avoid including the U wave 1

Heart Rate Correction (QTc Calculation)

Formula Selection

• While Bazett's formula (QTc = QT/√RR) remains the most widely used clinically, linear regression functions are recommended over Bazett's for more accurate rate correction 1, 2, 4
• Bazett's formula overcorrects at heart rates >90 bpm and undercorrects at heart rates <50 bpm 4, 5
• The Fridericia formula (QTc = QT/∛RR) provides better accuracy across varying heart rates 1, 5

When to Avoid Rate Correction

• Do not attempt QT correction when RR interval variability is large (e.g., atrial fibrillation) or when the T-wave end is unreliable 1, 4

Special Circumstances

Bundle Branch Block or Wide QRS

• Subtract the difference in QRS widths before and after the block from the measured QT interval 1
• Alternatively, measure the JT interval (from end of QRS to end of T wave) to eliminate QRS duration from the measurement entirely 1
• Do not interpret QT prolongation due to new conduction block as acquired long QT syndrome 1

Atrial Fibrillation

• Identify the shortest and longest RR intervals, calculate QTc for each, and average the two values 1
• Alternatively, assess whether the interval from R wave to T-wave peak exceeds 50% of the RR interval on average, indicating QTc >500 ms 1

Extreme Heart Rates

• For heart rates <50 bpm, have the patient perform mild aerobic activity to achieve a rate closer to 60 bpm before measuring 4
• For heart rates >90 bpm, allow additional resting time to achieve a lower heart rate 4

Normal Values and Clinical Thresholds

• Normal QTc: ≤460 ms in women, ≤450 ms in men 1, 2, 4
• QTc ≥500 ms in either sex correlates with higher risk for torsades de pointes 1, 2
• Women have 6-15 ms longer QTc than men in younger age groups; this difference diminishes after age 40 1

Electronic and Automated Measurement

Electronic Calipers

• Increase waveform size (standardization 1 to 2-4) and recording speed (25 to 50 mm/s) to enhance visualization 1
• Many systems include built-in QTc calculators that compute corrected values automatically 1

Automated Computer Measurements

• Always visually validate QT-interval prolongation reported by computer algorithms—this is essential 1, 2, 4
• Automated measurements using simultaneous lead recording often produce longer QT intervals than single-lead sequential recordings 1
• Computer algorithms may over- or underestimate the QT interval, particularly with difficult T-wave morphologies 6

Critical Pitfalls to Avoid

• Failing to use the same lead for serial measurements in the same patient 2, 4
• Including discrete U waves in the QT measurement 2, 4
• Not accounting for QRS prolongation in bundle branch block 2, 4
• Relying solely on automated measurements without visual validation 2, 4
• Using Bazett's formula at extreme heart rates without recognizing its limitations 4, 5