Gas inertia and ventilatory measurements under pressure: methodological considerations

A bag-in-box apparatus with a spirometer was used to measure the ventilatory minute volume in subjects exercising at air pressures up to 6.8 atm. During rest there was good agreement between minute volumes derived from the expired gas in the bag and the sum of tidal volumes from the spirometer, whereas during exercise the bag volume exceeded the spirometer volume by up to 20%. This was found to be due to the inertia of high density gas in the breathing hoses. Given sufficient flow rate the gas would continue to flow from the box to the bag following end expiration and end inspiration. The spirometer would not record this because it only responds to changes in the sum of box and bag volumes, whereas emptying the bag through a gas meter records the volume of gas actually moved. A model was constructed to investigate the phenomenon. It was concluded that many different conventional setups for respiratory measurements may be subject to this type of error. Solutions to the problem include a collapsible tube section downstream from the subject, pneumotachometers, chest-mounted magnetometers, or inductive plethysmographs.