A patient with diabetes mellitus is admitted unresponsive to the emergency room. Initial laboratory findings are serum potassium 2.8 mEq/L, serum sodium 138 mEq/L, serum chloride 90 mEq/L, and blood glucose 628 mg/dl. Cardiac monitoring shows multifocal premature ventricular contractions (PVCs). The nurse understands that the patient's PVCs are most likely caused by