A 62-year-old man with COPD and home oxygen therapy is intubated for an acute exacerbation. Pre-intubation ABG showed pH 7.25, PaCO2 10.5 kPa, HCO3 34 mmol/L. After intubation, the registrar sets RR 18, TV 500 mL (8 mL/kg), PEEP 5, FiO2 0.3. One hour later, ABG: pH 7.52, PaCO2 4.8 kPa, HCO3 30 mmol/L. The patient develops new cardiac arrhythmias. What has happened?