What is the definition of a prolonged QT interval on an electrocardiogram (EKG)?

Definition of Prolonged QT Interval on EKG

A prolonged QT interval is defined as a corrected QT (QTc) ≥450 ms in men and ≥460 ms in women, with values ≥500 ms considered severely prolonged and associated with significantly increased risk of life-threatening arrhythmias. 1, 2, 3

Gender-Specific Thresholds

The AHA/ACCF/HRS guidelines establish clear, practical clinical limits for abnormal QT intervals:

• Men: QTc ≥450 ms is considered prolonged 1, 2, 3
• Women: QTc ≥460 ms is considered prolonged 1, 2, 3
• Both sexes: QTc ≤390 ms is considered abnormally short 1, 2, 3

These thresholds represent the practical clinical cutoffs recommended for routine use, though they acknowledge that women naturally have 12-15 ms longer QT intervals than men in younger adults, with this difference diminishing to 6-10 ms in older age groups. 3

Alternative High-Risk Thresholds

For more conservative risk stratification, the AHA recommends using the 99th percentile values:

• Men: QTc ≥470 ms 2
• Women: QTc ≥480 ms 2

These higher thresholds may be more appropriate when considering borderline cases or when the clinical context suggests lower risk. 2

Severity Grading for Drug Safety and Risk Assessment

The FDA recommends a three-tier severity classification system, particularly important for drug safety monitoring and torsades de pointes risk stratification:

• Level 1: QTc >450 ms (or >460 ms for women) 2
• Level 2: QTc >480 ms 1, 2, 3
• Level 3: QTc >500 ms 1, 2, 3

QTc >500 ms represents the highest risk threshold for torsades de pointes and warrants immediate clinical action. 2, 3 This includes discontinuation of offending drugs, aggressive electrolyte repletion, and continuous cardiac monitoring. 2

Clinically Significant Changes from Baseline

When monitoring serial ECGs (particularly during drug therapy):

• An increase of >30 ms from baseline warrants close monitoring 1, 2, 3
• An increase of >60 ms from baseline is considered clinically significant and requires intervention 1, 2, 3

These delta values are critical for identifying drug-induced QT prolongation even when absolute values remain below traditional thresholds. 1

Pediatric Considerations

In children, the definitions differ slightly:

• Children <12 years: The 98th percentile limit is approximately 450 ms without gender differentiation 1, 2
• Children 12-16 years: Gender differences of approximately 8 ms begin to appear, similar to adults 1

Important caveat: Bazett's formula may produce false QT prolongations in children, so alternative correction methods should be considered. 1

Critical Measurement Considerations

Heart Rate Correction Formula Matters

Bazett's formula systematically overcorrects at heart rates >80-85 bpm, producing falsely prolonged QTc values. 2, 3 This is a common pitfall in clinical practice.

• Bazett's formula (QTc = QT/√RR): Most commonly used but least accurate at higher heart rates 3
• Fridericia's formula (QTc = QT/∛RR): More accurate, especially at heart rates >80-90 bpm 2, 3
• Linear regression functions: Recommended over Bazett's formula by AHA/ACC 3

Before making clinical decisions based on a prolonged QTc, recalculate using Fridericia's formula if the heart rate is elevated. 2

Bundle Branch Block Adjustment

The QT interval prolongs in ventricular conduction defects, requiring adjustment for QRS duration. 1, 3 This can be accomplished by:

• Incorporating QRS duration and RR interval as covariates into the QT-adjustment formula 1
• Using the JT interval (QT duration minus QRS duration) with JT-specific normal standards 1

Failure to adjust for bundle branch block will result in falsely prolonged QTc values, as the prolongation reflects delayed depolarization rather than abnormal repolarization. 2

Common Pitfalls to Avoid

• Do not attempt QT correction when RR interval variability is large (e.g., atrial fibrillation), as correction formulas become unreliable 3
• Recognize that time of day influences QT interval, so serial comparisons should ideally be performed at similar times 1
• Significant interreader variability exists in QT measurement, so whenever possible, a single reader should overread sequential tracings of an individual patient 1
• Select leads with T-wave amplitude ≥2 mm and well-defined T-wave end for accurate measurement 3