A 72-year-old woman with a history of acute myocardial infarction 3 days ago presents with acute dyspnea and orthopnea. Physical examination reveals bibasilar crackles and elevated jugular venous pressure. Chest X-ray shows bilateral interstitial and alveolar infiltrates. Arterial blood gas on 40% oxygen: pH 7.32, PaCO₂ 52 mmHg, PaO₂ 58 mmHg, HCO₃⁻ 24 mEq/L. Which ventilation-perfusion abnormality is the primary mechanism of hypoxemia in this patient?