Is the QT interval on an electrocardiogram (EKG) measured from the beginning of the S wave to the beginning or end of the T wave?

QT Interval Measurement on EKG

The QT interval is measured from the beginning of the QRS complex (not the S wave) to the end of the T wave. 1

Correct Measurement Technique

Starting Point

• The QT interval begins at the onset of the QRS complex, which represents the earliest indication of ventricular depolarization 1
• This is the beginning of the entire QRS complex (Q wave if present, or R wave if no Q wave exists), not specifically the S wave 1

Ending Point

• The QT interval ends at the termination of the T wave, representing the latest indication of ventricular repolarization 1
• The end of the T wave is determined by drawing a tangent to the steepest downslope of the T wave and identifying its intersection with the isoelectric baseline 1

Common Pitfalls and How to Avoid Them

Identifying the End of the T Wave

• The end of the T wave is often the most challenging aspect of QT measurement, particularly when T waves have low amplitude or when T-U wave fusion is present 1
• For notched or biphasic T waves, the end of the entire T wave complex should be considered the endpoint 1
• Discrete U waves that arise after the T wave returns to baseline should NOT be included in the QT interval measurement 1

Lead Selection

• Measure the QT interval in the lead showing the longest QT, which is typically V2 or V3 1
• However, if this measurement differs by more than 40 ms from adjacent leads, the measurement may be in error and should be rechecked 1
• Use the same lead consistently when monitoring a patient over time to ensure valid comparisons 1
• The lead should have a T-wave amplitude of at least 2 mm with a well-defined T-wave end 1

Special Circumstances

Bundle Branch Block or Wide QRS:

• If the QRS becomes prolonged (e.g., new bundle branch block), this will artificially increase the total QT interval 1
• Subtract the difference in QRS widths before and after the block, or measure the JT interval (from end of QRS to end of T wave) instead 1
• This adjustment must be applied consistently when monitoring over time 1

Superimposed U Waves:

• When T and U waves cannot be separated, measure the QT in leads not showing U waves, often aVR and aVL 1
• Alternatively, extend the downslope of the T wave by drawing a tangent to the steepest portion until it crosses the baseline 1

Clinical Significance

• The QT interval represents the time required for both ventricular depolarization and repolarization 1, 2
• Accurate measurement is critical because QT prolongation (especially QTc >500 ms) significantly increases the risk of torsades de pointes and sudden cardiac death 1, 3
• Always visually validate computer-generated QT measurements, as automated systems may over- or underestimate the interval 1