Fragmented QRS is a marker of mortality in patients with severe COVID-19: A retrospective observational study - İbrahim Halil Özdemir; Bülent Özlek; Mehmet Burak Özen; Ramazan Gündüz; Nurullah Çetin; Eda Özlek; Bekir Sedat Yıldız; Hakan Tıkız (Anatolian Journal of Cardiology, 2021)