What are the normal ranges for QT (QT interval) and PR (PR interval) intervals on an electrocardiogram (ECG)?

Normal ECG Intervals: QT and PR

PR Interval Normal Range

The normal PR interval is 120-200 milliseconds (0.12-0.20 seconds). This represents the time from atrial depolarization to the beginning of ventricular depolarization and is measured from the start of the P wave to the beginning of the QRS complex.

QT Interval Normal Range

The QT interval must be corrected for heart rate (QTc) to provide clinically meaningful values, and normal ranges differ by gender: QTc <450 ms in men and <460 ms in women. 1, 2

Gender-Specific Normal Values

• Men: Normal QTc is less than 450 ms; prolonged QTc is ≥450 ms 1, 2, 3
• Women: Normal QTc is less than 460 ms; prolonged QTc is ≥460 ms 1, 2, 3
• Short QT interval: ≤390 ms in both men and women 1, 2, 3

Age-Related Considerations

• Children <12 years: The 98th percentile limit is approximately 450 ms without gender differentiation 1, 2
• Adolescents (12-16 years): Gender differences emerge, with approximately 8-10 ms difference appearing during this age range 2
• Adults >40 years: Gender differences become smaller and practically disappear in older adults 1, 3

QT Correction Methods: Critical for Accurate Interpretation

Linear regression functions or Fridericia's formula should be used instead of Bazett's formula for QT-rate correction. 1, 3

Why Bazett's Formula Is Problematic

• Overcorrects at high heart rates (>80-85 bpm), falsely prolonging QTc 2, 4, 5
• Undercorrects at low heart rates (<60 bpm), falsely shortening QTc 2, 4, 6
• Results in 30% of normal ECGs being misclassified as abnormal when using a 440 ms threshold 4
• Despite these limitations, Bazett's formula (QTc = QT/√RR) remains the most commonly auto-populated value on ECG machines 5

Recommended Correction Methods

• Fridericia's formula (QTc = QT/∛RR): More accurate, especially at higher heart rates, and recommended by the FDA 2, 3, 7
• Framingham formula: QTLC = QT + 0.154(1-RR), provides reliable correction across heart rate ranges 6
• Hodges formula: Shows the least correlation with heart rate (correlation coefficient 0.11 vs 0.33 for Bazett) 4

Practical Approximation for Fridericia Correction

Add or subtract 5% to the uncorrected QT for each 10 bpm increase or decrease in heart rate from 60 bpm. 8 This provides a quick bedside estimate without calculation.

Clinical Risk Stratification

FDA Severity Levels for QTc Prolongation

The FDA recommends reporting three severity levels: 1, 2, 3

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

High-Risk Thresholds

• QTc >500 ms: Associated with significantly increased risk of Torsades de Pointes 3, 9
• Increase >60 ms from baseline: Clinically significant and warrants intervention 3, 9
• Increase >30 ms from baseline: Requires monitoring 9

Measurement Best Practices

Lead Selection and Technique

• Measure in leads II, V3, or V5, using the longest measured value 2
• Select leads with T-wave amplitude ≥2 mm and well-defined T-wave end 3
• Measure from the beginning of the QRS complex to the end of the T wave 3

When NOT to Correct QT Interval

Do not attempt QT correction when: 1, 3

• RR interval variability is large (e.g., atrial fibrillation)
• End of T wave is unreliable or poorly defined
• Heart rate is <40 bpm or >120 bpm 2

Adjustments for Conduction Abnormalities

QT interval prolongs in ventricular conduction defects and requires adjustment for QRS duration. 1, 3, 9 Use the JT interval (QT duration minus QRS duration) with JT-specific normal standards, or incorporate QRS duration as a covariate in the correction formula. 1

Common Pitfalls to Avoid

• Ignoring gender differences: Women have 12-15 ms longer QTc in younger adults and 6-10 ms longer in older age groups 1, 2
• Using Bazett's formula at abnormal heart rates: This creates false positives at high heart rates and false negatives at low heart rates 4, 5, 8
• Misinterpreting athlete ECGs: Athletes often have longer uncorrected QT due to lower resting heart rates, but QTc should be normal 2, 3
• Not accounting for electrolyte abnormalities: Hypokalemia, hypomagnesemia, and hypocalcemia affect QTc values 2
• Relying solely on automatic measurements: Automatic QTc-Bazett has only 54% sensitivity for detecting prolonged QTc; manual verification is needed when automatic QTc-Bazett >430 ms 8

Practical Algorithm for Automatic ECG Interpretation

When reviewing an automatic ECG report: 8

1. If automatic QTc-Bazett <430 ms → prolonged QTc is ruled out
2. If automatic QTc-Bazett ≥430 ms → convert to Fridericia correction
3. If QTc-Fridericia >430 ms → perform manual measurement to confirm

This approach has 0% negative predictive error (95% CI: 0-7%) for identifying truly prolonged QTc intervals. 8