A new piston ventilator for use in respiratory studies.

Abstract

SP~C~L SQUIrM~r is needed to study the effect of long-term positive pressure ventilation on functional residual capacity (FRC), oxygen consumption ('r and other respiratory parameters. The apparatus must be small, portable and able to ventilate the patient in closed circuit during FRC measurement, and it must provide various tidal volumes and minute ventilation for patients of different sizes. The operator of the equipment must be able to stop ventilation at any chosen point on the respiratory cycle and restart it from the same point to allow the switching of the patient in and out of the closed circuit without changing thc pattern of ventilation. The equipment must also allow the collection of pure expired gases for analysis and must be safe and reliable. We have designed a ventilator which makes possible the measurement of changes in FRC, VO2, and pulmonary mechanics of anaesthetized children, Simplified meohanieal details appear in Figure 1, The piston (P) of a 1500 ee syringe moves back and forth under the influence of compressed air and vacuum, alternately supplied to the chamber behind the piston through solenoid valves SA and SV respectively. The volume in the syringe at any instant is conver~ed to a voltage signal by means of a multiturn potentiometer (POT) mechanically coupled to the shaft of the piston. Thus each volume ahead of the piston from Vo to V ..... is represented by a unique voltage signal from E0 to E ..... The reciprocating action of the piston is achieved by using this voltage signal to trigger the solenoid valves SA and SV at the appropriate times. For example, when solenoid SV is open, the piston moves backwards under the influence of the vacuum, until the volume in front of it reaches V1. At this point the voltage signal El ~riggers an electronic switch to close solenoid SV and the piston stops. After a predetermined delay solenoid SA opens and the compressed air pushes the piston forward until V~ is reached. At this point the voltage Eo triggers solenoid SA to close and the piston stops. After another predetermined delay solenoid SV opens again, the piston moves back, and the cycle is repeated. The volume delivered by the syringe per cycle is VI-V0 but since Vo is usually set to correspond to zero volume, the tidal volume i~ simply V1. Figure 2 shows the main mechanical details of the ventilator. The patient is ventilated through the patient port (PP) with the help of solenoid valves IS and ES, which are activated in synchronism with the ventilator. The rate of motion o{