LQT16 Long QT syndrome-16 (LQT16) is characterized by a markedly prolonged corrected QT (QTc) interval and 2:1 atrioventricular (AV) block, with onset in the perinatal period. Patients experience bradycardia or ventricular tachyarrhythmias that may result in syncope, cardiac arrest, and/or sudden death. Patients with LQT14, LQT15, or LQT16, resulting from mutation in calmodulin genes CALM1, CALM2, or CALM3, respectively, typically have a more severe phenotype, with earlier onset, profound QT prolongation, and a high predilection for cardiac arrest and sudden death, than patients with mutations in other genes.