Most common cardiac adverse effect of lithium is?

Reference: Katzung, Basic & Clinical Pharmacology, 13th ed., Pg. 748. The bradycardia-tachycardia (“sick sinus”) syndrome is a definite contraindication to the use of lithium because the ion further depresses the sinus node. T-wave flattening is often observed on the electrocardiogram but is of questionable significance.