Figure 3. Case 2. Ambulance (pre-hospital) and selected hospital observations and blood gas analyses. ABG analysis on arrival in the ED, demonstrated type II respiratory failure (ABG1). The patient was started on BPAP. At an FiO2 of 70%, acidosis persisted on VBG analysis (VBG1). Acidosis and hypercapnia resolved within hours after starting controlled oxygen therapy (ABG2). Recovery of the GCS lagged the resolution of hypercapnia by several hours. With the patient off BPAP, the alveolar gas equation could be applied to both ABG1 and ABG3. The alveolar partial pressure of oxygen (PAO2) was markedly elevated at the time when hypercapnia was present, while the PaO2 at that time was low (ABG1). Airway pressures (cmH20) delivered on BPAP and CPAP are in parentheses. RA – room air; BE – base excess. Reference ranges: ABG, pH (7.35–7.45), PaCO2 (35–45 mmHg), PaO2 (70–100 mmHg), HCO3– (21.0–28.0 mmol/l), BE (−2 to 3 mEq/l), Lactate (0.4–0.8 mmol/l). VBG, pH (7.35–7.45), HCO3–, Lactate. PCO2, PO2, BE and SO2 measured on a VBG have no defined reference range.