How do you identify a prolonged QTc (corrected QT) interval?

Identifying a Prolonged QTc Interval

A prolonged QTc interval is identified by measuring the QT interval from the beginning of the QRS complex to the end of the T wave, and then applying an appropriate heart rate correction formula, with values >450 ms in men and >460 ms in women considered prolonged. 1

Proper QT Interval Measurement Technique

Manual Measurement

• Start at the beginning of the QRS complex (first deflection from the isoelectric line after the P-wave) 2
• End at the point where the T wave returns to the isoelectric line 2
• When the end of the T wave is difficult to define, use the tangent method: identify the intercept between the steepest tangent at the descending part of the T-wave and the isoelectric line 2, 1
• Measure in leads with well-defined T-wave ends, typically leads II, V5, or V6 2, 1
• Use the longest value obtained from these leads 2

T-Wave Morphology Challenges

• When the T wave is flattened or has an unclear endpoint, measurement becomes more difficult 2
• If T and U waves are superimposed, measure in leads without U waves (often aVR and aVL) or extend the downslope of the T wave using the tangent method 2
• When the P wave may be superimposed on the T wave (particularly with prolonged QT), extrapolate the end of the T wave by drawing a tangent to the downslope 2

QT Correction for Heart Rate

The QT interval must be corrected for heart rate to obtain the QTc value. Several formulas are available:

Correction Formulas

1. Bazett's formula: QTc = QT/√RR

   • Simplest to use but overcorrects at heart rates >80 bpm and undercorrects at lower rates 2, 1
   • Most widely used despite limitations 3

2. Fridericia's formula: QTc = QT/∛RR

   • Recommended for heart rates <60 or >80 bpm 1
   • More accurate than Bazett's at higher heart rates 4, 5

3. Other formulas: Framingham, Hodges

   • Linear regression functions that effectively remove rate dependence 2, 6

Important Considerations

• Use Fridericia's formula when heart rate is outside the 60-80 bpm range 1
• Do not attempt rate correction when RR interval variability is large (e.g., atrial fibrillation) 2
• For consistent serial measurements, use the same lead and correction formula 1

Normal and Abnormal QTc Values

Normal QTc Values

• Men: ≤450 ms 2, 1
• Women: ≤460 ms 2, 1
• Children: <440 ms 2

Abnormal QTc Values

• Borderline prolonged: 451-480 ms (men), 461-480 ms (women) 1
• Prolonged: >480 ms (both sexes) 1
• High-risk prolongation: >500 ms (both sexes) 1
• QTc values from 440 to 470 ms are considered a "grey zone" due to overlap between affected and normal individuals 2
• Abnormally short QTc: ≤390 ms 1

Automated vs. Manual Measurement

Automated Measurement

• Only rely on automated QT interval measurements if the ECG is otherwise normal 2
• Automated measurements often yield longer QT intervals than manual measurements in individual leads 2, 4
• Visually validate any QT prolongation reported by a computer algorithm 2

Manual Verification

• All physicians should be able to measure the QT interval manually 2
• When automated QTc-Bazett is >430 ms, manual verification is recommended 5
• Machine-measured QTc using Bazett's formula tends to overestimate the QTc interval length 4

Practical Approach to QTc Assessment

1. Obtain a standard 12-lead ECG
2. Identify leads with clear T wave endpoints (typically II, V5, V6)
3. Measure QT interval from beginning of QRS to end of T wave
4. Note the heart rate
5. Apply appropriate correction formula based on heart rate:
   • For HR 60-80 bpm: Bazett's formula
   • For HR <60 or >80 bpm: Fridericia's formula
6. Compare result with normal values based on gender
7. For values in the "grey zone" (440-470 ms), consider clinical context and risk factors

Special Considerations

• Women typically have longer baseline QTc intervals than men 1
• Athletes often have longer QT intervals due to lower resting heart rates, but QTc is usually normal 1
• Many medications can prolong the QT interval, requiring monitoring during treatment 1
• QTc prolongation >500 ms significantly increases the risk of torsades de pointes and sudden cardiac death 1