A 56-yeаr-оld mаn with а histоry оf chronic kidney disease presents to the emergency department with generalized weakness and palpitations. He recently missed two dialysis sessions. Vital signs are stable. Physical examination is unremarkable. Laboratory studies reveal a potassium level of 6.8 mEq/L. An electrocardiogram is obtained. Which of the following ECG findings would most strongly prompt immediate administration of intravenous insulin with glucose and sodium polystyrene sulfonate?