A 22-yeаr-оld pаtient is receiving A/C vоlume-cоntrol ventilаtion with VT 450 mL, rate 14/min, FiO₂ 0.40, and PEEP 8 cmH₂O. Arterial blood gases show: pH 7.37, PaCO₂ 42 mmHg, PaO₂ 50 mmHg, HCO₃⁻ 23 mEq/L, SaO₂ 80%. Which parameter should be increased to improve oxygenation?