A 70-year-old woman vomits during a generalised seizure and becomes cyanosed. One hour later her temperature is 38.4°C, oxygen saturation is 88% on room air and chest radiography shows patchy opacities in dependent lung segments. She has no dental disease or swallowing disorder. Six hours later she is alert, afebrile and maintaining an oxygen saturation of 95% on 2 L/min; repeat examination shows improving breath sounds and no purulent sputum. Which one of the following is the most appropriate next step?