*2.1. Study Population*

This is a prospective, observational study taking into account all patients admitted for acute HF with reduced ejection fraction from January 2012 to the end June 2018 in the Cardiology and Arrhythmology Clinic of the University Hospital "Ospedali Riuniti" of Ancona, Italy, and presenting with evidence of atrial or ventricular arrhythmias on admission.

The potential triggering arrhythmias considered were: Atrial fibrillation (AF), atrial flutter, supraventricular tachycardia, ventricular tachycardia and premature atrial and ventricular complexes. More specifically, this study considered significant > 20000 premature ventricular complexes per day, according to the current available literature [8].

The selection process is detailed in Figure 1 and consisted of two main phases. The first phase started with the hospitalization and aimed at detecting all potential patients with pure TCM. In order to assess the real weight of TCM in clinical practice, the patients with arrhythmia-mediated (impure) TCM were excluded, ruling out all structural or functional heart diseases. Ischemic heart disease was defined as a previous history of revascularization, or evidence of significant coronary obstruction at coronary angiography performed during hospitalization. The patients with non-significant coronary atherosclerosis and no clinical instrumental signs of ischemia were not excluded. Valvular heart disease was defined as a previous history of aortic or mitral replacement or the repair, evidence of severe aortic or mitral regurgitation, severe aortic stenosis, or moderate or severe mitral stenosis. Congenital heart diseases, cardiac amyloidosis, hypertrophic cardiomyopathy, myocarditis, non-compaction cardiomyopathy, post-partum cardiomyopathy, arrhythmogenic right ventricular dysplasia, Fabry's disease and alcoholic cardiomyopathy were defined according to the current standards. The non-invasive and invasive diagnostic procedures, such as coronary angiography or cardiac magnetic resonance, were performed according to clinical suspicion in all eligible patients. All patients were treated for HF and underwent rhythm or rate control strategies according to the current guidelines [9–12]. After discharge, the second phase of the selection process started and aimed at confirming pure TCM out of all potential patients (Figure 1). All the patients with suspected TCM were followed-up in the heart failure outpatient clinic at one month and three months after discharge, and twice a year from then on. The patients presenting an improvement of at least one New York Heart Association (NYHA) class and the recovery of at least five points of the left ventricular ejection fraction (LVEF) during the follow-up were diagnosed with arrhythmia-induced (pure) TCM and included in the analysis (Figure 1) [1,13].

**Figure 1.** Selection process.

The study was conducted according to institutional guidelines, national legal requirements, European standards and the revised Declaration of Helsinki. Being an observational study, a formal approval of the ethics committee was not sought. All patients provided prior written informed consent for anonymous collection and publication of their clinical data. The present report complies with the STROBE initiative (Table S1) [14].
