How to easily measure the QT (QT interval) interval in an electrocardiogram (ECG)?

How to Measure QT Interval in ECG Easily

The easiest and most accurate way to measure the QT interval is to select the lead with a T-wave amplitude of at least 2 mm and a well-defined T-wave end (typically leads II, V5, or V6), measure from the beginning of the QRS complex to the end of the T wave, and use the tangent method to determine the T-wave endpoint when unclear. 1

Lead Selection

• Choose the lead with the longest QT interval that has a well-defined T-wave end 1
• Typically leads II, V5, or V6 provide the clearest measurements 1
• Lead V3 correlates best with the mean QT interval across all leads, followed by lead II 2
• Avoid leads with U waves if possible (often aVR and aVL are free of U waves) 1
• Document which lead is being used for measurement, especially for serial measurements 1

Measurement Technique

Manual Measurement:

1. Identify the QRS onset: Find the beginning of the QRS complex (earliest indication of ventricular depolarization) 1

2. Identify the T-wave end: This is often the most challenging part 1

   • For clear T waves: Mark where the T wave returns to the isoelectric baseline
   • For difficult T waves (biphasic, notched, or with superimposed U waves): Use the tangent method - draw a line along the steepest portion of the descending limb of the T wave and mark where it intersects the isoelectric baseline 1, 3

3. Measure the interval: Count the distance from QRS onset to T-wave end in milliseconds (each small box = 40 ms on standard ECG paper) 1

4. Correct for heart rate: Apply a correction formula to account for heart rate effects 1, 4

   • Bazett's formula (QTc = QT/√RR) is most commonly used but may overestimate at fast heart rates 1
   • Fridericia's formula (QTc = QT/∛RR) is more accurate at heart rates outside 60-80 bpm 4

Electronic Calipers Method:

1. Increase ECG size (standardization from 1 to 2,3, or 4) and recording speed (from 25 to 50 mm/s) to enhance visualization 1
2. Place first caliper at beginning of QRS complex
3. Place second caliper at end of T wave
4. Record the measured interval
5. Use built-in QTc calculator if available 1

Special Situations

Wide QRS Complex:

• If a bundle branch block develops, subtract the difference in QRS widths before and after the block from the QT measurement
• Alternatively, measure the JT interval (from end of QRS to end of T wave) 1

Atrial Fibrillation:

• Identify the shortest and longest R-R intervals
• Calculate QTc for each and average the two values
• Alternatively, determine if the interval from R wave to T-wave peak is >50% of the R-R interval 1

U Waves:

• If U wave is separate from T wave, do not include it in measurement
• If U wave is fused with T wave, consider the entire complex as the T wave 1

Interpretation Guidelines

• Normal QTc values:
  • Men: ≤450 ms
  • Women: ≤460 ms 4
• Prolonged QTc:
  • Men: >450 ms
  • Women: >460 ms 4
• High-risk prolongation: >500 ms 1, 4

Common Pitfalls to Avoid

• Measuring in leads with unclear T-wave endpoints
• Including U waves in the measurement when they are separate from T waves
• Using inconsistent leads for serial measurements
• Relying solely on automated measurements without visual validation 1
• Failing to correct for heart rate 1
• Not adjusting for wide QRS complexes 1

Modern Approaches

• Smartphone heart monitors have shown reasonable accuracy for QTc measurement when compared to 12-lead ECGs, especially for QTc intervals <500 ms 5
• Automated algorithms are improving but should be visually validated, particularly when reporting QT prolongation 1, 6

The tangent method has been shown to be particularly effective, with studies demonstrating that even inexperienced ECG readers can achieve higher accuracy (71-77%) than many cardiologists when using this technique 3.