![]() ![]() CAD was defined as either an ICD-9 code consistent with either acute myocardial infarction or chronic coronary heart disease or having evidence of prior myocardial infarction on the index ECG. ICD-9 codes were assessed for the following conditions: heart failure hypertension coronary artery disease valvular heart disease diabetes and chronic kidney disease (see Supplementary Materials for full listing of all ICD-9 codes). In particular, we accessed Sunrise Clinical Manager (Eclipsys Corporation, Atlanta, GA), Epic (Verona, WI), Cerner (North Kansas City, MO), EmTrac (University of Pennsylvania, Philadelphia, PA), and Medview (University of Pennsylvania, Philadelphia, PA) (see Supplementary Materials). These sources also provided data related to laboratory investigations and medications. Clinical risk factors and diagnoses were assessed utilizing the International Classification of Diseases, version 9 (ICD-9) codes that were generated from emergency room visits, inpatient hospitalizations, outpatient encounters, and telephone encounters. Information about patient demographics, cardiovascular risk factors, laboratory values, and medications were extracted from electronic medical records and included a comprehensive set of clinical encounters. This study was approved by the Institutional Review Board of the University of Pennsylvania. 24– 26 Our final analysis included the 6,723 subjects with sinus tachycardia on index ECG. We also excluded individuals with baseline AF, atrial flutter, or a history of heart failure given the known alterations in repolarization currents that occur among patients with cardiac dysfunction. To ensure adequate longitudinal data, subjects with fewer than 30 days of clinical follow-up within the UPHS were excluded. Of the 91,274 patients that had an index electrocardiogram (ECG) between Januand December 31, 2009, 67,265 were eligible for the study ( Figure 1). The index visit was defined as having both a clinical encounter with a practitioner within the University of Pennsylvania Health System and a standard twelve-lead electrocardiogram (ECG). The Penn Atrial Fibrillation Free (PAFF) study is a large, multi-hospital cohort of patients from the University of Pennsylvania Health System (UPHS) that were free of atrial fibrillation (AF) or atrial flutter at the index visit. 21– 23 In a large cohort of patients with resting sinus tachycardia, we compared four different QT-correcting formulas and their association with incident cardiovascular events and all-cause mortality. 18 Further, resting sinus tachycardia results from a multitude of disease processes and is independently associated with both heart failure and all-cause mortality. 17– 20 We sought to compare the prognostic significance of the various correction methods in states of tachycardia and hypothesized those formulas utilizing a linear term such as Hodges or Framingham will provide a more accurate assessment of cardiac repolarization and be a better marker of adverse risk than the traditional Bazett formula. ![]() Since the Bazett formula utilizes an exponential term, it is believed to overestimate the QT interval during states of tachycardia. 10– 16 The Bazett method, which multiplies the measured QT interval by the inverse square root of the RR interval, 10 is most commonly used to derive the corrected QT interval in clinical practice. Multiple correction formulas have been empirically developed in an attempt to provide a standard index for the duration of myocardial repolarization. Assessment of the QT interval, therefore, requires correction for the heart rate in order to enable comparisons with standard, reference values. ![]() This response ensures that the myocardium remains completely excitable and avoids zones of heterogeneous conduction that could result in the induction of re-entrant arrhythmias. In addition, repolarization is also a function of the underlying heart rate and decreases with a progressive rise in the ventricular rate. 1– 9 The repolarization time reflects the electrophysiologic properties of ion channels that are responsible for maintaining equilibrium in myocardial cells. Prolongation of the QT interval reflects abnormalities in cardiac repolarization and is a risk marker for ventricular arrhythmias, other adverse cardiovascular events including heart failure (HF) and coronary artery disease (CAD), and death in various populations. ![]()
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