A 28-year-old man is brought to the hospital with a 6-hour history of sudden onset severe abdominal pain, persistent vomiting, and bloody diarrhea. He works in a small jewelry workshop and admits to accidentally ingesting a solution used for metal plating. On examination, he is severely dehydrated with a blood pressure of 70/40 mmHg and a heart rate of 130/min. His mucous membranes show a characteristic garlic odor on his breath. Cardiac auscultation reveals an irregular rhythm. ECG shows prolonged QT interval and multiple ectopic beats. Serum electrolytes show severe hypokalemia (K^+^ 2.1 mEq/L) and hypomagnesemia. What is the most appropriate immediate management?