Comparison of formulae for calculating the corrected QT (QTc) interval in an adult population attending a diabetes clinic at a rural hospital in South Africa.

Background: A prolonged corrected QT (QTc) interval on the electrocardiograph is an important marker of cardiac autonomic neuropathy and increased risk of developing arrhythmias. Various formulae exist for the calculation of QTc, the most common being Bazett’s, which is also the default formula utilised by the Edan SE® ECG machine to automatically calculate QTc. Little or no literature exists on the comparisons of the various formulae in patients living with diabetes, more especially in those diabetes patients with HIV infection. Methods: Retrospective (n = 631) electrocardiographs were collected and analysed. QT and RR were measured for QTc calculation. QTc was calculated using three formulae, namely Bazett (QTcB), Fridericia (QTcFri) and Framingham (QTcFram). Additionally, the automated QTc (QTcM), which used Bazett’s formula, was recorded for comparison purposes. To determine the optimal formula for QTc calculation, slopes and r 2 using a QTc/RR regression analysis were calculated. The formula with the r 2 closest to zero was deemed superior when compared with its counterparts. Results: The QTc Bazett was the worst-performing formula for QTc calculation, with the QTcFri performing best across both type 2 and type 1 diabetes patients with or without HIV infection. To validate which formula was employed in the automated QTc result, a mean difference comparison was performed, which indicated a non-significant difference between the machine-calculated QTcM and QTcB (p = 0.572, 0.384, 0.980) in all groups except for the type 1 diabetic group without HIV (p = 0.009). These findings indicated that the automated QTc employed Bazett’s formula. Conclusion: Evidence from this study has shown that the best formula to calculate QTc in patients with DM, with and without HIV infection, is the Fridericia formula. The authors advise that careful consideration should be taken when selecting a formula for QTc calculation. This will improve precision diagnosis and patient care.