What formulas are used to calculate the corrected QT interval (QTc) from the QT interval and the RR interval?

QT Interval Correction Formulas

The two most commonly used formulas for calculating corrected QT interval (QTc) are Bazett's formula (QTc = QT/√RR) and Fridericia's formula (QTc = QT/∛RR), though current guidelines from the American Heart Association, American College of Cardiology, and Heart Rhythm Society recommend linear regression-based correction functions over Bazett's formula for more accurate QT-rate correction. 1

Primary Correction Formulas

Bazett's Formula

• QTcB = QT/√RR (where RR is measured in seconds) 2, 1
• This is the simplest and most widely used formula in clinical practice, despite its significant limitations 1, 3
• Bazett's formula was derived from only 39 young subjects, limiting its generalizability 1

Fridericia's Formula

• QTcF = QT/∛RR (QT divided by the cube root of RR interval in seconds) 2, 3
• This formula performs better than Bazett's at extreme heart rates and is specifically recommended for atrial fibrillation 1, 4
• Fridericia's formula most closely approximates QTc during atrial fibrillation to QTc during sinus rhythm 5, 4

Additional Linear Correction Formulas

Framingham Formula

• QTLC = QT + 0.154(1-RR) for a reference RR interval of 1 second 6
• This linear regression model corrects QT more reliably than Bazett's formula 6
• Demonstrates markedly less heart-rate dependence than Bazett's 1

Other Alternative Formulas

• Hodges, Karjalainen-Nomogram, Sagie-Framingham, and Rautaharju formulas all show less heart-rate dependence than Bazett's 1
• Linear regression functions are endorsed as the gold-standard method for QT correction 1

Critical Limitations of Bazett's Formula

Systematic Errors at Extreme Heart Rates

• Overcorrects at heart rates >90 bpm (can be substantially erroneous at high heart rates) 2, 1, 7
• Undercorrects at heart rates <50 bpm 2, 1, 7
• Retains a strong positive residual correlation with heart rate (r = 0.32) 1

Clinical Impact

• Using a 440 ms threshold with Bazett's formula would incorrectly flag 30% of normal ECGs as abnormal, compared to <2% with other formulas 8
• The upper normal limit for Bazett's QTc is 483 ms, compared to 457-460 ms for other formulas 8

Guideline Recommendations

Class I Recommendation

• The American Heart Association, American College of Cardiology Foundation, and Heart Rhythm Society issue a Class I guideline recommendation that linear regression-based correction functions—not Bazett's formula—should be used for QT-rate correction 1
• The correction method must be explicitly reported in ECG analysis 1

Practical Clinical Use

• Despite guideline recommendations, Bazett's formula remains the standard in most clinical settings due to simplicity 1
• When using Bazett's formula, avoid application at heart rates >90 bpm or <50 bpm 7
• For heart rates <50 bpm, perform mild aerobic activity to achieve a heart rate closer to 60 bpm before measuring 7
• For heart rates >90 bpm, allow additional resting time to achieve a lower heart rate 7

Special Situation: Atrial Fibrillation

Recommended Approach

• Use Fridericia's formula specifically for atrial fibrillation 5, 4
• Identify the shortest and longest RR intervals on the rhythm strip 5
• Calculate QTc for each using Fridericia's formula 5
• Average the two QTc values 5

Quick Clinical Assessment

• Print a long rhythm strip and determine whether the interval from R wave to the peak (or nadir) of the T wave is more than 50% of the RR interval 5
• If >50%, QTc would exceed the critical threshold of 500 ms 5

Normal Values and Critical Thresholds

Normal Upper Limits

• Men: ≤450 ms 2, 1, 7
• Women: ≤460 ms 2, 1, 7
• Children: <440 ms 2
• Values from 440-470 ms are considered "grey zone" due to overlap between affected and controls 2

High-Risk Threshold

• QTc ≥500 ms indicates high risk for torsades de pointes and requires dose reduction or drug discontinuation 2, 1, 5
• An increase of >60 ms from baseline warrants cessation or dose reduction of the offending drug 2

Critical Pitfalls to Avoid

Measurement Consistency

• Always use the same lead for serial QT measurements in the same patient (QT length varies across the 12 leads) 1, 7, 5
• Measure QT in leads II, V5, and V6, using the longest value 2

Automated Measurements

• Never rely solely on automated measurements without visual validation 1, 7, 5
• Computer algorithms can misidentify T-wave termination or erroneously include U-waves 1

Formula Mixing

• Never compare Bazett-corrected QT intervals with QT intervals corrected by other formulas (e.g., Fridericia, Framingham) because mixing correction methods introduces systematic errors 1

Bundle Branch Block

• Subtract the increased QRS duration from the QT interval if new bundle branch block develops, to avoid falsely interpreting QRS widening as QT prolongation 5
• Alternatively, measure the JT interval (from end of QRS to end of T wave) to eliminate the confounding widened QRS 5

When to Avoid Correction

• Do not attempt QT correction when RR interval variability is large (e.g., atrial fibrillation with highly variable ventricular response) or when T-wave termination is ambiguous 1, 7