How to assess QTc interval prolongation?

Assessment of QTc Interval Prolongation

The most accurate assessment of QTc interval prolongation requires measuring the QT interval from the onset of the QRS complex to the end of the T wave, using the lead with the longest T wave, and applying appropriate heart rate correction formulas based on heart rate. 1

Proper QTc Measurement Technique

Lead Selection

• Select the electrocardiographic lead with the longest, most well-defined T wave 1
• Avoid leads with U waves when possible 1
• Use the same lead for serial measurements in the same patient 1
• If lead change is necessary, clearly document this with the QT measurement 1

Measurement Method

• Measure from the onset of QRS complex to the end of the T wave 1
• For notched or biphasic T waves, use the end of the entire T wave complex 1
• If a U wave is discrete (occurring after T wave returns to baseline), do not include it 1
• If T wave and U wave are superimposed or cannot be separated:
  • Measure QT in leads without U waves (often aVR and aVL) 1
  • Or extend the downslope of the T wave by drawing a tangent to the steepest portion until it crosses the TP segment 1

QRS Prolongation Considerations

• If QRS becomes prolonged (e.g., new bundle branch block), either:
  • Subtract the increased QRS length from the QT interval, or
  • Measure the JT interval (from end of QRS to end of T wave) 1

Heart Rate Correction Methods

• Bazett's formula (QTc = QT/√RR):

  • Acceptable only for heart rates between 60-80 bpm 2
  • Overcorrects at fast heart rates and undercorrects at slow heart rates 1, 2

• Fridericia's formula (QTc = QT/∛RR):

  • Preferred for heart rates outside normal range (>80 bpm or <60 bpm) 2
  • More accurate for drug studies and monitoring medications 2

• Linear regression functions:

  • Recommended over Bazett's formula by AHA/ACC/HRS 1
  • More effectively remove rate dependence of the adjusted QT 1

Normal and Abnormal QTc Values

Category	Men	Women
Normal	≤450 ms	≤460 ms
Borderline prolonged	451-480 ms	461-480 ms
Prolonged	>480 ms	>480 ms
High-risk prolongation	>500 ms	>500 ms
Abnormally short	≤390 ms	≤390 ms

2, 1

Risk Factors for QTc Prolongation and Torsades de Pointes

• Patient-specific factors:

  • Female sex
  • Older age
  • Heart disease (LVH, low LVEF, ischemia)
  • Genetic predisposition (family history of syncope, sudden death, LQTS)
  • Electrolyte abnormalities (hypokalemia, hypomagnesemia, hypocalcemia)
  • Metabolic impairment (renal or hepatic failure)
  • Bradyarrhythmias 1

• Medication-related factors:

  • QT-prolonging drugs (especially Class I and III antiarrhythmics)
  • Rapid IV administration of QT-prolonging drugs
  • Multiple QT-prolonging drugs used concurrently 1, 3

ECG Warning Signs of Impending Torsades de Pointes

• Sudden bradycardia or long pauses (e.g., compensatory pauses after ventricular ectopy)
• Enhanced U waves
• T wave alternans
• Nonsustained polymorphic ventricular tachycardia 1

Monitoring Recommendations

• Obtain baseline ECG before starting QT-prolonging medications 3
• For Class III antiarrhythmic drugs, notify prescriber if QTc increases ≥25% from baseline 1
• For drugs like sotalol, discontinue or reduce dose if QTc ≥520 ms 3
• Document QTc in patient's medical record before drug initiation and at least every 8 hours thereafter 1
• Document QTc before and after increases in drug dosage 1
• Use the same recording system and standardized acquisition procedures for serial comparisons 1

Common Pitfalls to Avoid

• Overreliance on Bazett's formula at heart rates outside 60-80 bpm can lead to significant over/under-correction 2
• Inconsistent lead selection introduces unnecessary variability 2
• Difficulty identifying T-wave end, particularly with U waves or bifid T waves 2
• Failing to perform baseline ECG before starting QT-prolonging medications (compliance with recommendations is extremely low, <2% in some studies) 4
• Overlooking QT prolongation in high-risk patients - about 25% of acutely admitted psychiatric patients may have borderline or prolonged QTc intervals 5
• Overinterpreting a single borderline QT measurement without follow-up 2

By following these standardized measurement techniques and monitoring protocols, clinicians can more accurately assess QTc interval prolongation and identify patients at risk for potentially life-threatening arrhythmias.