Figure 4. Case 3. Ambulance (pre-hospital) observations and selected hospital observations and blood gas analyses. ABG on arrival in the ED showed type II respiratory failure (ABG1). In the absence of an identifiable risk factor for oxygen-induced CO2 retention, the SpO2 target range was set at 94–98% and the patient was started on BPAP. On oxygen 5 L/min via nasal prongs (approximate FiO2: 40%), the SpO2 was below the target range (92%). Elevation of the FiO2 to 55% on BPAP was associated with a paradoxical worsening of hypoxia and a marked rise in the PaCO2 (ABG2). In response to this, the FiO2 was further increased to 60% overnight. This was associated with a decline in GCS to 11–15. Blood gas analysis was not performed at that time. Significant hypercapnia persisted following resolution of the acute event in association with a worsening metabolic alkalosis (ABG5). Airway pressures (cmH20) delivered on BPAP and CPAP are in parentheses. RA – room air; NHF – nasal high flow; 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.