Can the QTc (corrected QT interval) be falsely elevated in patients with Left Bundle Branch Block (LBBB)?

QTc Measurement in Left Bundle Branch Block

Yes, QTc can be falsely elevated in patients with Left Bundle Branch Block (LBBB) due to the widening of the QRS complex that occurs with this conduction abnormality. 1

Why QTc is Falsely Elevated in LBBB

LBBB causes significant alterations in the ECG pattern that directly impact QTc measurement:

• LBBB widens the QRS complex, which automatically extends the measured QT interval
• The ST-T waves in LBBB markedly deviate in either positive or negative direction depending on the ECG lead 1
• The steeply sloping ST segments in LBBB patients cause ST amplitude to vary with heart rate 1
• Secondary repolarization abnormalities occur as a direct result of the altered ventricular depolarization sequence 1

Understanding the Mechanism

In LBBB, the normal QRS complex-T-wave axes discordance is present, where:

• Leads with predominantly negative QRS complexes typically have elevated ST-segments and upright T-waves
• Leads with large monophasic R-waves demonstrate ST-segment depression and inverted T-waves 1

This pattern of secondary repolarization changes occurs without actual changes in the action potential of individual ventricular myocytes. Rather, they result from the altered sequence of ventricular depolarization 1.

Clinical Implications

The false elevation of QTc in LBBB has important clinical implications:

• May lead to incorrect diagnosis of QT prolongation
• Could result in unnecessary medication adjustments or restrictions
• May create confusion when evaluating patients for risk of torsades de pointes
• Complicates assessment of drug-induced QT prolongation

Formulas to Correct QTc in LBBB

Several formulas have been developed to estimate the true QTc in patients with LBBB:

1. QT-LBBB formula: QT-LBBB = QTLBBB - (0.86 × QRSLBBB - 71) 2
2. Yankelson method: Replace the QRS duration with a fixed value of 88 ms for females and 95 ms for males after deriving QTc 3
3. Bogossian formula: QTm = QTb - 48.5% × (QRSb) 4
4. QTcLBBBNEW: 0.945 × QTcRBKLBBB - 26 5

The most recent research suggests that the QTcLBBBNEW formula shows the highest correlation with intrinsic QTc (without LBBB) and has minimal heart rate dependency 5.

Practical Approach to QTc Assessment in LBBB

1. Measure the standard QTc interval using your preferred correction formula (Bazett, Fridericia, etc.)
2. Apply one of the LBBB-specific correction formulas to estimate the true QTc
3. Consider using the JT interval (QT minus QRS duration) as it remains relatively preserved before and after development of LBBB 3
4. Be aware that using uncorrected QTc with Bazett's formula in LBBB markedly increases the proportion of individuals with apparently prolonged QTc, especially with increasing heart rate 5

When to Be Particularly Cautious

Exercise special caution in these scenarios:

• Patients on QT-prolonging medications
• Patients with electrolyte abnormalities
• Patients with intermittent LBBB (comparing QTc between LBBB and normal conduction)
• Patients with heart failure or structural heart disease
• When making decisions about medication safety

Remember that patients with LBBB are also at risk for false ST-segment alarms during monitoring due to the marked deviation of ST-T waves and the variability of ST amplitude with heart rate changes 1.