The QT interval in atrial fibrillation

Area of Science:

• Cardiology
• Electrophysiology
• Medical Statistics

Background:

• The QT interval in electrocardiograms (ECG) reflects ventricular depolarization and repolarization.
• Heart rate significantly influences QT interval duration, necessitating rate correction for accurate interpretation.
• Atrial fibrillation presents unique challenges for QT interval assessment due to irregular ventricular rates.

Purpose of the Study:

• To investigate the relationship between QT intervals and ventricular rates in patients with atrial fibrillation.
• To evaluate the adequacy of standard heart rate correction formulas in atrial fibrillation.
• To compare QT intervals in atrial fibrillation with those in sinus rhythm.

Main Methods:

• ECG recordings of 100-second duration were obtained from 50 patients diagnosed with atrial fibrillation.
• Analysis focused on the correlation between QT intervals and both mean and instantaneous ventricular rates.
• QT intervals were corrected for heart rate using Bazett's formula on a beat-by-beat basis.

Main Results:

• The mean ventricular rate was 94 bpm, with a mean QT interval of 357 ms and a mean corrected QT (QTc) of 444 ms.
• A significant linear relationship was observed between mean QT interval and mean RR interval (slope +21%) between subjects.
• A significant correlation existed between QT intervals and preceding RR intervals within recordings (average slope +7%), which was steeper at faster heart rates.

Conclusions:

• QT interval in atrial fibrillation is dependent on both mean and instantaneous ventricular rates, with mean rate being the primary determinant.
• Standard beat-by-beat QT interval correction formulas are inadequate for atrial fibrillation.
• Atrial fibrillation appears to be associated with a prolonged mean QT interval compared to sinus rhythm.