Schematic diagram of breathing system that allows remote administrations of inspiratory resistance. Medical air is supplied to the subject, with a reservoir of 2 L. Excess flow and expiration escapes through the one-way expiratory valve, close to the mouth to minimise rebreathing (inspiratory and expiratory valves: Hans Rudolf, Kansas City, MO, USA). Resistive loading is induced by discontinuing the delivery of medical air, forcing the subject to draw air through the resistor (porous glass disc). A diving mouthpiece (Scubapro UK Ltd, Mitcham, UK) connects to a bacterial and viral filter (GVS, Lancashire, UK), sampling lines (Vygon SA, Ecouen, France), connect to a pressure transducer (MP 45, ± 50 cmH2O, Validyne Corp., Northridge, CA, USA) and amplifier (Pressure transducer indicator, PK Morgan Ltd, Kent, UK) for inspiratory pressure readings, and to a gas analyser (Gas Analyser; ADInstruments Ltd, Oxford, United Kingdom) for respiratory gases. A mildly hyperoxic state was achieved through a constant administration of oxygen at a rate of 0.5 L/min. Periodically throughout scanning carbon dioxide challenges were administered to raise PETCO2 to match the PETCO2 rise during inspiratory loading periods.