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

How to Calculate the QT Interval

Measure the QT interval from the beginning of the QRS complex to the end of the T wave, using the tangent method to identify the T-wave endpoint, then correct for heart rate using Bazett's formula (QTc = QT/√RR), though linear regression formulas are preferred when available. 1, 2

Manual Measurement Technique

Identifying the Start and End Points

• Start point: Measure from the beginning of the QRS complex, which is usually readily apparent 1

• End point: The end of the T wave is determined by drawing a tangent line from the peak of the T wave following the steepest downslope of the T wave; the intersection of this tangent line with the isoelectric baseline defines the end of the T wave 1, 2, 3

• This "teach-the-tangent" or "avoid-the-tail" method improves accuracy compared to eyeballing the endpoint 2, 4

Lead Selection

• Choose a lead with a T-wave amplitude of at least 2 mm and a well-defined T-wave end 1

• The recommended leads for measurement are lead II, V5, or V6, with the longest value being used 1, 3

• Critical pitfall: Always use the same lead for serial measurements in the same patient, as QT intervals vary significantly across the 12 leads 1, 2, 3

• Document which lead you are using for future reference 1

Heart Rate Correction

Bazett's Formula (Standard Clinical Use)

• QTc = QT interval (in seconds) / √RR interval (in seconds) 1, 3

• Despite limitations, Bazett's formula remains the standard for clinical use 1, 3

Important Limitations of Bazett's Formula

• Overcorrects at heart rates >90 bpm (overestimates QTc) 2, 3

• Undercorrects at heart rates <50 bpm (underestimates QTc) 2, 3

• Do not use Bazett's formula for very fast or slow heart rates 2

Practical Solutions for Extreme Heart Rates

• For heart rates <50 bpm: Have the patient perform mild aerobic activity to achieve a heart rate closer to 60 bpm before measuring 2

• For heart rates >90 bpm: Allow additional resting time to achieve a lower heart rate before measuring 2

• Alternative: Linear regression functions are recommended over Bazett's formula for more accurate QT-rate correction when available 2, 3

Normal Values and Clinical Thresholds

Adult Values

• Normal QTc: ≤450 ms in men, ≤460 ms in women 2, 3

• Prolonged QTc: >470 ms in men, >480 ms in women (99th percentile) 1

• Highly abnormal: QTc >500 ms in either sex, which correlates with higher risk for torsades de pointes 1, 3

Neonatal Values

• Mean QTc on day 4 of life is 400±20 ms 1

• Upper normal limit is 440 ms (no gender differences in neonates) 1

Special Situations

Bundle Branch Block or Wide QRS

• Do not interpret QT prolongation at face value when a new bundle branch block develops 1

• Method 1: Subtract the difference in QRS widths before and after the block from the total QT interval 1, 3

• Method 2: Measure the JT interval (from the end of the QRS complex to the end of the T wave), which eliminates the QRS duration entirely 1, 3

• Whichever adjustment method you choose, apply it consistently for serial monitoring 1

Atrial Fibrillation

• QT interval varies beat-to-beat due to varying RR intervals 1

• Method 1: Identify the shortest and longest RR intervals, calculate QTc for each, then average the two QTc values 1

• Method 2: Print a long rhythm strip and determine if the interval from R wave to T-wave peak is >50% of the RR interval on average, which indicates QTc would exceed 500 ms 1

Difficult T-Wave Morphologies

• Biphasic or notched T waves: If the T wave is notched, consider the end of the T wave as the end of the entire complex 3

• T-U wave fusion: When T waves have superimposed U waves, use the tangent method from the T-wave peak 1

• Discrete U waves: Do NOT include discrete U waves that arise after the T wave has returned to baseline 3

Measurement Tools

Electronic Calipers

• Increase waveform size (standardization from 1 to 2,3, or 4) and recording speed (from 25 to 50 mm/s) to enhance visualization 1

• Place electronic calipers at the beginning of the QRS complex and end of the T wave 1

• Many systems have built-in QTc calculators that automatically compute QTc when QT and RR intervals are entered 1

Automated Measurements

• Critical pitfall: Never rely solely on automated measurements without visual validation 2, 3

• Computer algorithms can over- or underestimate the QT interval, particularly with difficult T-wave morphologies or technically suboptimal ECGs 1

• Always visually confirm QT-interval prolongation reported by automated systems 3

Common Pitfalls to Avoid

• Failing to use the same lead for serial measurements in the same patient 2, 3

• Including discrete U waves in the QT measurement when they occur after the T wave returns to baseline 3

• Not accounting for QRS prolongation in bundle branch block 3

• Using Bazett's formula at extreme heart rates (<50 or >90 bpm) 2

• Relying on automated measurements without visual confirmation 2, 3

• Measuring during large RR interval variability or when T-wave end identification is unreliable 2