How do you calculate and what is the normal range for the corrected QT interval for a male or female with rate controlled atrial fibrillation and left bundle branch block?
First, the QTintervall has to be corrected with the preceeding cycle length. The intervall should not be corrected with the mean heart rate.
Second, bundle branch is a problem for QT correction, because depolarisation is slower due to myocardial conduction, hence repolarisation is also longer. Therefore it is not reasonable to just correct for the QRS-width. There was a nice paper on this topic recently: Chiladakis J Optimal QT/JT interval assessment in patients with complete bundle branch block.Ann Noninvasive Electrocardiol. 2012 Jul;17(3):268-76. I give you the conclusion from this paper here, with which agree: The Hodges, Nomogram and Framingham correction methods provide best assessment of QT/JT intervals in BBB, whereas Bazett's formula exaggerates heart rate-dependency of ventricular repolarization intervals.
I agree with Dr. Bänsch answer.
Only one comment to add.. Any corrected QT formula has its benefits and drawbacks and the differences obtained comparing all of them (I will ad Fredericia's approach too) are minimal.
Saul Drajer, MD. Maimonides University - Clinica de la Esperanza - Buenos Aires, Argentina
As far as I know, there is no agreed, evidence-based method of correcting QT for heart rate in AF.
I agree with Dr. Bänsch's comments.
The dynamic coupling between heart rate and ventricular repolarisation duration has raised discussions regarding generalizations. Studies dealing with QT-to-RR interval adaptation dynamics have been systematically carried out during regular sinus rhythm. Therefore, extrapolation of QT interval to heart rate relationship must be made with caution in subjects with heart rhythm other than regular sinus one.
Malik et al. (Heart. 2002;87:220-8; Pacing Clin Electrophysiol. 2004;27:791-800) have showed that the QT/RR coupling is highly variable among healthy men and women, in some instances moving away from classical Bazett's model. We have observed (Computer in Cardiology 2000, p:159-62), using spectral coherence technique, a phase delay between RR interval variation and ventricular repolarisation duration response, which accounted for the effect of not only the most immediate one but of several preceding cycle lengths on current ventricular repolarisation duration. Similar findings have been observed by Pueyo et al. using more sophisticated approaches (Biomedical Signal Processing and Control. 2008; 3: 29-43).
Recently, Baumert et al. (Physiol Meas. 2013;34:1435-48) reported that for equivalent baseline heart rates, QT interval variability decouples from RR interval variability in older subjects.
Mechanistically, understanding RR-to-QT interval coupling dynamics in atrial fibrillation may be challenging and troublesome. In the presence of left bundle branch block, ashman phenomenon observed in atrial fib rhythm may lead to transient AV conduction block, which poses additional difficulties in interpreting RR-to-QT interval coupling in this setting.
Those findings indicate that existing models of QT/RR interval adaptation although clinically relevant must be employed with caution in different clinical settings.
These limited observations do not solve the issue raised by Dr. Brawn, but hopefully shed some light on future research lines addressing this exciting theme.
Peace and blessings.
I recommend this paper ( free full-text available online):
G R Pai and J M Rawles. The QT interval in atrial fibrillation.
Br Heart J. 1989 June; 61(6): 510–513.
Dear Martin,
I just remembered your question when I've seen this recently published paper:
http://www.sciencedirect.com/science/article/pii/S0022073613005669
Hope it helps.
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