The most likely cause of this patient's respiratory failure is hypophosphatemia, which occurs in patients with chronic alcohol use, malnutrition, or critical illness. Symptoms rarely occur unless the serum phosphate concentration is <2.0 mg/dL (0.65 mmol/L); severe symptoms occur with a serum phosphate concentration <1.0 mg/dL (0.32 mmol/L). Symptoms include weakness, myalgia, rhabdomyolysis, arrhythmias, heart failure, respiratory failure, seizures, coma, and hemolysis.