mrcem2-1172

A 72-year-old man with a known history of heart disease presents due to a general decline in his health. He complains of feeling extremely tired and has been experiencing occasional heart palpitations over the past few days. A complete set of blood tests, including a venous gas, have been sent to the laboratory, revealing a potassium level of 7.3 mmol/l. An ECG is performed, which shows abnormal, wide QRS complexes.
What is the initial treatment that should be administered first?