CHANGE IN THE WORK OF BREATHING IMPOSED BY FIVE ANAESTHETIC BREATHING SYSTEMS

Measurements of pressure and flow in five anaesthetic breathing systems were studied with various fresh gas flows (FGF) using volunteers, and mechanically simulated breathing. Pressure measurements were made at the patient connection and in the neck of the reservoir bag. A pneumotachograph was used to measure flow in the patient connection. The change in the work of breathing imposed by the circuit was derived from the pressure-volume graph and apportioned to stages of the breathing cycle. All the systems studied increased the work of breathing. Expiratory work was always increased more than inspiratory work. The additional work imposed on expiration and the total extra work imposed on breathing were further increased at FGF greater than the minimum clinical requirement. Although various anaesthetic breathing systems have been examined to determine whether or not they permit rebreathing, the extra amount of work imposed by an anaesthetic system, on a patient breathing spontaneously, has not been studied in depth. Where the effect of the anaesthetic system on the work demanded from the patient has been studied, methods used have centred on the measure-ment of the aerodynamic resistance of the compo-nents of the system (Orkin, Siegel and Rovenstine