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.

Excessive work of breathing during intermittent mandatory ventilation.

Difficulties were experienced in weaning a patient from a ventilator by means of intermittent mandatory ventilation (IMV). The difficulty was overcome by installing an alternative IMV system (Hudson "disposable IMV valve") through which the patient drew her spontaneous breaths. Laboratory measurements showed that the resistance of the ventilator breathing system was much higher than that of the alternative system, mainly as a result of the resistance of the humidifier. It was calculated from measurements with a preset pattern of simulated breathing that the extra, external, work of breathing through the ventilator breathing system was approximately 1.5 times the normal internal mechanical work of breathing for a normal patient; with the alternative system, the extra work was only 0.5 times the normal. It is stressed that the breathing systems of IMV ventilators should be judged with the same rigour as other systems through which the patient is required to breathe spontaneously. It is recommended that manufacturers should provide the necessary information.