A model lung with direct representation of respiratory muscle activity

We describe the theory, construction and testing of an active model lung. This is designed so that when it is connected to a ventilator, the interaction between spontaneous ventilatory activity in the patient and the pressures and flows generated by the ventilator are reproduced faithfully. A waveform of negative pressure, pmus, equivalent to the action of the respiratory muscles, is applied indirectly to the compliance unit of a conventional model lung. It is shown that, when the compliance and resistance of a subject have been measured, the waveform of pmus can be determined. This was undertaken in three volunteers breathing under various conditions, including connection to a ventilator operating in different modes. When these conditions were reproduced with the lung model replacing, but matched to, each subject, the original waveforms of flow and pressure at the mouth were reproduced.