A 28-year-old male is pulled out of a river after being submerged for approximately 8 minutes. On arrival at the emergency department, he is unresponsive, with a core temperature of 34.2°C. His pupils are fixed and dilated. Arterial blood gas shows severe metabolic acidosis (pH 6.8, HCO₃⁻ 8 mEq/L, base excess −18). Chest X-ray reveals bilateral pulmonary edema with frothy fluid in the airways. What is the primary mechanism of death in this drowning victim?