The intervention most likely to improve this patient's chance of survival is maintaining the tidal volume at 360 mL. This patient has acute respiratory distress syndrome (ARDS); he meets the criteria for ARDS with bilateral lung opacities and very poor oxygenation with an arterial PO₂/FIO₂ ratio of 101 mm Hg (13.4 kPa). Very few interventions have evidence supporting a survival benefit in ARDS, but the mainstay of management is a lung-protective ventilator strategy, with low tidal volume (6 mL/kg of ideal body weight [IBW]) and low plateau pressure (<30 cm H₂O). Other interventions with supporting evidence include a conservative fluid strategy to keep the lungs as dry as possible, prone positioning, and early therapeutic paralysis (for the first 48 hours) if severely hypoxemic.

The physiologic result of reducing the tidal volume would be a lower minute ventilation, and the PCO₂ would likely rise further above its current elevated level, resulting in an additional decrease in arterial pH. Allowing hypercapnia to achieve lung-protective ventilation goals in ARDS (termed permissive hypercapnia) is acceptable and even desirable if it means ventilating at lower tidal volumes and plateau pressures despite causing a reduction in arterial pH. In general, a pH as low as 7.25 is considered acceptable in this situation; however, decreasing the minute ventilation would likely lower the pH below this level. Some evidence exists that hypercapnia may also be protective in ARDS independent of the low tidal volume strategy that causes it, but this is controversial.

Raising the tidal volume would improve the abnormal PCO₂, but might cause greater injury to the lung tissue, leading to higher risk of mortality.