Location of the QT Interval on the ECG
The QT interval extends from the beginning of the QRS complex (the earliest indication of ventricular depolarization) to the end of the T wave (the latest indication of ventricular repolarization). 1
Anatomical Boundaries
The QT interval represents the complete duration of ventricular electrical systole, encompassing both depolarization and repolarization phases. 2
Starting Point:
- The onset of the QRS complex marks the beginning of the QT interval 1
- This represents the earliest indication of ventricular depolarization 1
- The QRS onset may occur up to 20 ms earlier in leads V2 and V3 compared to limb leads 1
Ending Point:
- The end of the T wave marks the termination of the QT interval 1
- This represents the latest indication of ventricular repolarization 1
- Identifying the end of the T wave is often the most challenging aspect of QT measurement 1, 3
Optimal Leads for Measurement
When measuring the QT interval, use the lead showing the longest QT interval, which is typically V2 or V3. 1
Lead Selection Algorithm:
- Measure QT in leads II, V5, and V6, using the longest value obtained 1, 4
- If the longest measurement differs by more than 40 ms from other leads, the measurement may be in error and adjacent leads should be considered 1
- Avoid leads with prominent U waves; if unavoidable, use leads aVR or aVL where U waves are typically absent 1, 4
- Lead II is conventionally chosen when measuring manually, as U waves are less prominent and the end of the T wave corresponds to the end of significant repolarization 5
Common Measurement Pitfalls
Critical Caveats:
- When T and U waves are superimposed or cannot be separated, measure QT in leads not showing U waves (often aVR and aVL), or draw a tangent to the steepest portion of the T wave downslope until it crosses the baseline 1
- In neonates and infants with fast heart rates, the P wave may be superimposed on the T wave; extrapolate the end of the T wave by drawing a tangent to the downslope 1
- Normal variation of up to 50-65 ms in QT intervals measured across different leads is acceptable 1
- Always visually validate computer-generated QT measurements, as automated systems may over- or underestimate the interval 1, 3
Modern Digital ECG Considerations
Contemporary automated digital machines record all leads simultaneously, permitting temporal alignment and superimposition. 1 This facilitates more accurate assessment of the QRS onset and T wave end, but the automatically measured QT interval is often longer than measurements from individual leads. 1