UCSF

Background. Little is known about the prevalence and prognostic significance of long QT interval (QTi) among patients with chest pain during the acute phase of suspected cardiovascular injury. Previous attempts to research similar questions resulted in conflicting conclusions. This is primarily because of the inappropriate expression of the QT/RR relation when calculating the QTi corrected for heart rate (HR).

Aims. The author aims to (1) determine the prevalence and time duration of long QTi , (2) investigate whether a long QTi is associated with mortality and other adverse cardiovascular events at 1 year follow-up, and (3) determine whether the QT/RR slope is different among different HR zones (bradycardia, normal, and tachycardia).

Methods. This is a secondary analysis of data obtained from the IMMEDIATE AIM trial. Data included 24-hour 12-lead Holter electrocardiographic recordings. The sample of the present analysis consisted of patients who presented to the emergency department with chest pain (N, 145). The QTi was measured automatically and rate corrected using seven QTc formulas including subject specific correction. The formula with the closer to zero absolute mean QTc/RR correlation was considered the most accurate. The QT/RR slope was compared among different HR zones applying steady (with minimal fluctuation) beat selection technique.

Results. Subject specific QT rate correction outperformed other QTc formulas and resulted in the closest to zero absolute mean QTc/RR correlation. Using linear subject specific correction, the prevalence of long QTc interval was 14.5%. The QTc interval did not predict mortality or hospital admission at short and long term follow-up. The QT/RR slope was significantly less under the bradycardia zone (0.074 ± 0.07) compared to the normal (0.114 ± 0.1, p < 0.01) or the tachycardia zones (0.147 ± 0.1, p < 0.001). There was no difference between the normal and the tachycardia zone.

Conclusion. Adequate QT rate correction can only be performed using subject specific correction. Long QTi is not uncommon among patients presenting to the emergency department with chest pain. The QT/RR slope is different among different HR zones. These differences can be used in resetting the limits defining different HR zones boundaries.