How is the QT interval measured on an electrocardiogram (EKG)?

How to Measure the QT Interval on an EKG

The QT interval is measured from the beginning of the QRS complex to the end of the T wave, with the end of the T wave best identified by drawing a tangent line from the peak of the T wave along its steepest downslope to where it intersects the isoelectric baseline. 1, 2

Lead Selection for Measurement

• Measure the QT interval in lead V2 or V3, as these typically show the longest QT interval and have the largest T-wave amplitude, making the end of the T wave easier to identify 1, 3
• If the measurement in V2 or V3 differs by more than 40 ms from adjacent leads, verify the measurement in surrounding leads to ensure accuracy 1
• Always use the same lead for serial measurements in the same patient to ensure consistency and avoid false changes due to lead-to-lead variation 3, 4
• The T wave should have an amplitude of at least 2 mm for reliable measurement 1, 3

Identifying the Start and End Points

Beginning of QRS Complex

• The onset of the QRS complex is usually readily apparent and represents the earliest indication of ventricular depolarization 1, 5
• Note that the QRS onset may occur up to 20 ms earlier in V2 and V3 compared to limb leads 1

End of T Wave (The Challenging Part)

• Draw a tangent line from the peak of the T wave following the steepest portion of the T-wave downslope 1
• The intersection of this tangent line with the isoelectric baseline (TP segment) defines the end of the T wave 1, 2
• If the T wave is notched or bifid, consider the end of the entire complex as the end of the T wave 1, 4

Handling Difficult T-Wave Morphologies

When U Waves Are Present

• Do not include discrete U waves that arise after the T wave has returned to baseline 1, 4
• If T and U waves are superimposed and cannot be separated, measure the QT in leads that do not show U waves, typically aVR and aVL 1
• Alternatively, extend the downslope of the T wave using the tangent method, though this may underestimate the true QT interval 1

When T Waves Have Low Amplitude

• Choose a different lead with better T-wave visualization 1
• Increase waveform size and recording speed (from 25 to 50 mm/s) when using electronic calipers to enhance visualization 1

Rate Correction (QTc Calculation)

• Use linear regression formulas rather than Bazett's formula for more accurate rate correction, as Bazett's formula overcorrects at fast heart rates (>90 bpm) and undercorrects at slow rates (<50 bpm) 1, 2
• Despite its limitations, Bazett's formula (QTc = QT/√RR interval in seconds) remains the most widely used in clinical practice 1, 4
• Do not attempt rate correction when RR interval variability is large (as in atrial fibrillation) or when the end of the T wave cannot be reliably identified 1, 2

Normal Values and Critical Thresholds

• Normal QTc values are ≤460 ms in women and ≤450 ms in men 2, 4
• A QTc ≥500 ms correlates with significantly increased risk of torsades de pointes and warrants immediate action 1, 3, 4
• An increase in QTc of >60 ms from baseline is also clinically significant 1, 3

Special Circumstances

Bundle Branch Block

• Subtract the increase in QRS duration from the total QT measurement, or alternatively measure the JT interval (from end of QRS to end of T wave) to eliminate the conduction delay component 1, 3, 4

Atrial Fibrillation

• Identify the shortest and longest RR intervals, calculate QTc for each, and average the two values 1, 3
• Alternatively, assess whether the interval from R wave to T-wave peak exceeds 50% of the RR interval, which suggests QTc >500 ms 1

Critical Pitfalls to Avoid

• Never rely solely on automated computer measurements without visual validation, as algorithms frequently over- or underestimate the QT interval 1, 4
• Failing to use the same lead for serial measurements will produce spurious changes due to normal lead-to-lead variation of up to 50-65 ms 1, 4
• Including discrete U waves in the measurement artificially prolongs the QT interval 4
• Not accounting for QRS prolongation in bundle branch blocks leads to falsely elevated QT measurements 4

Practical Measurement Technique

• Use electronic calipers when available, increasing waveform size to 2-4× standard and paper speed to 50 mm/s for better visualization 1
• Measure the QT interval over an average of 5 beats for greater accuracy 6
• Document which lead was used for measurement to ensure consistency in follow-up ECGs 1, 3
• When monitoring patients on QT-prolonging drugs, measure QTc 2-4 hours after each dose during initiation and titration 1, 6