It has been reported that insulin-induced hypoglycemia affects repolarization of the cardiac cells in both healthy subjects [3, 4] and people with diabetes [5–7]. The altered repolarization is notable on the electrocardiogram (ECG) as a flattened T wave [4, 6] and a prolonged heart rate-corrected QT interval (QTc) [3, 5]. A prolonged QTc is associated with an increased risk of sudden cardiac death [8, 9], and QTc has thus been the primary variable investigated in studies of the proarrhythmic effect of hypoglycemia. The degree of QTc prolongation during clamped hypoglycemia ranges from 5ms [6] to 60ms [3] with one study reporting a prolongation of 156ms [5]. Thus, in some studies the prolongation of QTc is seen to be insignificant while other studies show a significant and potentially dangerous prolongation. We hypothesize that differences in methodology when measuring the QT interval and heart rate correcting the QT interval may partly explain the discrepancies between the reported prolongations of QTc. It is known that Bazett's formula for heart rate correction is associated with both over- and undercorrection of QTc at heart rates outside a narrow range [10]. Nevertheless, Bazett's formula is still the most often used heart rate correction, when investigating QTc during hypoglycemia. The methodology when measuring the T wave offset also has an impact on the QT interval as for example the often used tangent method is more sensitive to the flattening of the T wave seen during hypoglycemia [11]. In addition, modern insulin analogues may have a different effect on the QTc than human insulin, although this has previously been shown not to be the case [12].

Using the MA method significant prolongation of QTcB from t_baseline to t_hypo was observed (ΔQTcB: 7(2); P < .05) but no prolongation of QTcF was seen (ΔQTcF: 1(2); P > .05). With the SI method both QTcB and QTcF prolonged significantly (ΔQTcB: 42(6); P < .001, ΔQTcF: 35(6); P < .001) (Table 2). Heart rate and T/R ratio decreased significantly with both the MA and SI methods (P < .001). The changes in variables from t_baseline to t₁₅ were similar to the change from t_baseline to t_hypo (Table 2). At t_post, QTcB and QTcF had returned to t_baseline levels while RR interval and T/R ratio remained decreased for both MA and SI methods (Table 2).

The use of the SI method for measuring the end of the T wave is known to be sensitive to changes in T wave amplitude, although the method was originally meant for cases with partial T-U fusion [11, 16]. In particular a flattened T wave will cause an overestimation of the QT interval with the SI method when compared to the MA method (Figure 2). Since there exists no gold standard of measuring the QT interval, neither of the measurement techniques can from this study be judged more correctly than the other. However, the discrepancy between the two methods illustrates that comparing studies of hypoglycemia using different QT measurement methods may be problematic. Also, it is apparent from the results that the SI method produces significantly longer QT intervals than the MA method which could indicate a higher probability of false positives with the SI method. An approach which could eliminate the bias associated with measuring the QT interval is to use alternative T wave morphology variables. Xue and Reddy [17] used principal component analysis of the T wave and showed that this approach had superior reproducibility than several QT measurement methods. Alternative T wave morphology parameters might therefore be better at characterizing changes in repolarization during hypoglycemia.

A pathologically prolonged QTc interval is usually defined as >450ms for men and >470ms for women [18]. In the present study, mean QTc did not exceed these thresholds with any of the methods. This could indicate that prolongation of the QTc cannot in itself explain the mechanism implicated in the dead-in-bed syndrome.

The differences between QTcB and QTcF in this study were larger at hypoglycemia than at baseline. It is known that Bazett's formula tends to overcorrect the QTc at higher heart rates [10]. In this study we observed a significant increase in heart rate during hypoglycemia which may have contributed to an overcorrection by Bazett's formula compared to Fridericia's formula. Similar findings of differing results using the two correction formulas have been reported [6] although they in other cases produce similar results [19].

One of the main limitations of the study is the absence of a control group. Without a control group it is less clear if the observed QT prolongation is caused by hypoglycemia per se. Indeed, insulin could act as a confounding variable as it has been shown to cause moderate QT interval prolongation [20]. To account for the effect of insulin, we measured the ECG variables of interest 15 minutes after insulin injection where the subjects were still normoglycemic. We anticipated that this measurement would quantify the effect of hyperinsulinemia alone. The results show that the change in ECG variables 15 minutes after insulin injection is comparable to the change at hypoglycemia. This could indicate that the observed changes during hypoglycemia may not be caused by hypoglycemia per se but rather by hyperinsulinaemia. We acknowledge that the small number of subjects in the study limits its generalisability to the general population, although measurements on each subject were repeated to reduce intrasubject variation. Additionally, the use of only one ECG lead for QT measurement may have introduced some variation in the measurements, which could have been mitigated by the use of several leads.