Testing:
(1) standard 12 lead ECG.
(2) Ideally the QT interval from 3 complexes from each lead are averaged (I would think that the same 3 complexes should be used for all 12 leads).
(3) Measurement of the QT interval may be problematic in the presence of U waves.
(4) If the end of the T wave cannot be identified in a particular lead, then that lead is excluded from the analysis.
(5) The longest QT intervals are usually seen in leads V2 to V5 (Highham and Campbell).
(6) The process can be calculated using the QT-Guard program.
Parameters:
(1) minimum (shortest) QT interval in milliseconds
(2) maximum (longest) QT interval in milliseconds
QT dispersion =
= (maximum QT interval) - (minimum QT interval)
An alternative measure is the QTc dispersion using the corrected QT interval. For a patient with regular rate and constant R-R intervals:
QTc dispersion =
= (maximum QTc interval) - (minimum QTc interval) =
= (QT dispersion) / (SQRT(R-R interval in milliseconds))
Interpretation:
• Pye et al reported the dispersion in normal patients was 38 milliseconds (+/- 8)
• Pye reported that patients with heart disease and sustained ventricular arrhythmias had a mean dispersion of 77 milliseconds.
• Patients with the long QT syndrome may have a markedly increased dispersion (> 150 milliseconds). A markedly increased QT dispersion may identify patients at a high risk of developing ventricular arrhythmias.
Limitations:
• Kautzner et al found that measurements of the QT dispersion were variable with low reproducibility. This may limit the use of QT dispersion for risk stratification.
