J. Clin. Med. 2019, 8, 1527 2 of 17HDL levels and ischemia-reperfusion related ventricular arrhythmias (VA), with high plasma HDLlevels exerting a protective role [1,2]. Furthermore, low HDL cholesterol levels are linked with increasedrisk of sudden cardiac death [3–5], and several reports have postulated a correlation between plasmaHDL concentrations and the onset of fatal arrhythmias [6,7]. Last but not least, lower circulating HDLlevels in patients with paroxysmal atrial fibrillation were found [8–10].Ventricular tachyarrhythmias often precede cardiovascular death and the prolongation of theventricular cardiac repolarization time, which often underlies these rhythm disturbances, is one of thepredictors of sudden cardiac death [11–13].A previous experimental study revealed that increasing HDL cholesterol shortens cardiacventricular repolarization in isolated cardiomyocytes and the QT interval corrected for heart rate (QTc)in healthy humans [14].The electrocardiographic QT interval mainly reflects cardiac ventricular repolarization, andprolongation of the heart rate-corrected QT interval (QTc) has long been recognized as a marker ofsudden cardiac death, cardiovascular death, and all-cause mortality [15,16] both in patients withprevalent coronary heart disease and in middle-aged and older adults without prior cardiovasculardisease (CVD) [17].Despite the experimental evidence indicating that increasing levels of HDL could exert a directeect on the cardiac repolarization, specifically shortening the QT interval, little is known aboutthe epidemiological relationship between HDL and QTc in humans, and whether HDL cholesterolconcentrations are associated with the risk of QTc prolongation.Indeed, the association between HDL concentrations and QTc interval has been investigated onlyin one study on 440 primary hypercholesterolemic patients, which failed to show any associationbetween HDL cholesterol levels and the QTc interval [18].To date, several questions about the association between QTc and HDL remain open: Does HDLexhibit the QT-shortening effect also in vivo and in non-ischemic myocardium? What is the magnitudeof this effect in the general population? Which type of relationship between HDL levels and prolongationof ventricular repolarization exist (e.g., linear, dose-response)?In this study we sought to explore the possible association between HDL concentrations and theQTc interval in the general population, hypothesizing a non-linear association between HDL levelsand the risk of QTc interval prolongation.This investigation could lead to a better understanding of underrecognized pleiotropic eects ofHDL cholesterol on the heart.2. Methods2.1. Study PopulationThe Ticino epidemiological stiness (TEST) study was a population-based, observational study of