Concerning findings include Respiratory rate > 30 / minute Inability to maintain arterial oxygen saturation > 90% with fractional inspired oxygen (FIO2) > 0.60 pH < 7.25 Partial pressure of carbon dioxide (PaCO2) > 50 mm Hg (unless chronic and stable) The decision to initiate mechanical ventilation should be based on clinical judgment that considers the entire clinical situation and not simple numeric criteria.