How do you check for QT prolongation on an electrocardiogram (ECG)?

How to Check for QT Prolongation on ECG

Measure the QT interval manually from the beginning of the QRS complex to the end of the T wave in the lead showing the longest QT (typically V2 or V3), then correct for heart rate using Bazett's formula (QTc = QT/√RR in seconds), and consider it prolonged if QTc exceeds 470 ms in males or 480 ms in females. 1

Step-by-Step Measurement Technique

1. Select the Appropriate Lead

• Choose the lead with the longest QT interval, which is usually V2 or V3 (mid-precordial leads) due to their proximity to the heart and large T-wave amplitude 1
• Alternative leads include V5, V6, or lead II if these show a well-defined T-wave with clear ending 1
• If U waves are present and cannot be separated from T waves, use leads aVR or aVL where U waves are typically absent 1
• Critical pitfall: Always measure in the same lead for serial comparisons in a given patient 1

2. Identify the QT Interval Boundaries

Start point: Beginning of the QRS complex 1

End point: Where the T wave returns to the isoelectric baseline 1

• When the T-wave end is difficult to determine (low amplitude, biphasic, or notched T waves), draw a tangent line from the peak of the T wave following the steepest downslope to where it intersects the isoelectric baseline 1
• Use the TP segment (not PQ segment) as the isoelectric reference line 1
• The T wave should have amplitude of at least 2 mm for reliable measurement 1

3. Correct for Heart Rate (Calculate QTc)

Bazett's Formula (most commonly used): QTc = QT interval (seconds) ÷ √RR interval (seconds) 1

Important limitations:

• Bazett's formula overcorrects at heart rates >85 bpm (common in hospitalized patients) and undercorrects at slow rates 1
• Alternative formulas like Fridericia (QT ÷ ∛RR) may be more accurate at faster heart rates 1
• Linear regression formulas are preferable to Bazett's when available 1

4. Interpret the QTc Value

Normal upper limits (99th percentile): 1

• Males: 470 ms
• Females: 480 ms
• Both sexes: QTc >500 ms is highly abnormal and associated with increased risk of torsades de pointes 1

Clinical thresholds: 1

• Normal: ≤450 ms (men), ≤460 ms (women)
• Borderline: 450-470 ms (men), 460-480 ms (women)
• Prolonged: >470 ms (men), >480 ms (women)
• High risk: >500 ms (either sex)

Special Circumstances

Bundle Branch Block or Wide QRS

• Subtract the difference in QRS width before and after the block from the total QT measurement 1
• Alternatively, measure the JT interval (QT minus QRS duration) to eliminate the conduction delay component 1
• A new bundle branch block increases total QT but should not be considered true acquired QT prolongation 1

Atrial Fibrillation

• Identify the shortest and longest RR intervals, calculate QTc for each, then average the two values 1
• Alternatively, assess whether the interval from R wave to T-wave peak averages >50% of the RR interval on a long rhythm strip 1
• Do not attempt rate correction when RR variability is large 1

Computer-Generated Measurements

• Always visually validate automated QTc measurements, especially when baseline ECG is abnormal 1, 2
• Automated measurements can be inaccurate and may label QTc >440 ms as "borderline" despite this being exceeded by 10-20% of the population 1
• If computer-measured uncorrected QT is confirmed manually, the corrected value can be trusted 1
• Different ECG machines use different algorithms; use the same machine for serial comparisons 1

Monitoring Frequency in Clinical Practice

For patients on QT-prolonging drugs: 1

• Document QTc before drug initiation
• Recheck every 8-12 hours after starting or increasing dose
• More frequent monitoring if QTc prolongation is observed
• Document before and after each dose increase

Discontinue or hold drug if: 1

• QRS widens by >25% (or >50% increase is concerning)
• QTc increases by >60 ms from baseline 1
• Absolute QTc exceeds 500 ms

Common Pitfalls to Avoid

• Don't rely solely on automated measurements - manual verification is essential 1, 2
• Don't switch leads between measurements - this introduces variability of up to 50-65 ms 1
• Don't use Bazett's formula at heart rates >85 bpm without recognizing its limitations 1
• Don't measure QT when U waves are superimposed on T waves - choose a different lead 1
• Don't forget that some drugs (like amiodarone) prolong QT without high arrhythmia risk 1