A 68-year-old man with a history of severe chronic obstructive pulmonary disease (COPD) and known secondary pulmonary hypertension presents with increasing dyspnoea and peripheral oedema. His arterial blood gas on room air shows pH 7.31, PaCO2 68 mmHg, PaO2 48 mmHg, and SaO2 80%. His chest X-ray shows hyperinflation and prominent pulmonary arteries. Which of the following interventions is most likely to acutely decrease his pulmonary vascular resistance?