An assessment of the Neff circulator.

THE PROBLEMS of eliminating expired carbon dioxide from anaesthetic circuits have excited the ingenuity of designers for many years. Carbon dioxide absorption systems, initially introduced in the form of "to-and-fro" circuits, produce an apparatus dead-space which tends to increase as the absorption material becomes exhausted. Means of circulating the gases through the absorber in order to reduce this deadspace were developed and these generally employed the fresh gas flow to drive some form of Venturi injection system for motive power. The first commercial model of the Aga Spiropulsator (1.940) featured such a system. Of more recent origin, a Waters-type canister for paediatric anaesthesia designed by Takaoka incorporates an injector driven by fresh gas and which re-circulates anaesthetic gases around the absorber. Generally, however, the problem has been overcome by using a circle absorption system. This involves the operation of 1-way valves by the patient which contribute to the work of breathing. Clinical spirometers designed for the measurement of basal metabolic rate frequently employ an electrically driven fan which circulates the enclosed atmosphere in the belief that this eliminates the work of breathing through the apparatus. None of these systems resolved the problem of under-mask dead-space. Most attempts to achieve this have involved a closer fitting face-mask (e.g. the RendellBaker Soucek Mask) or partitioned T-pieces (e.g. the Rendell-Baker Partitioned T-piece and the Bird Partitioned Y-connector). Revell (1959)1 proposed a device which incorporated a small compressed-air-driven turbine for impelling anaesthetic gases around the absorption circle, which, in association with a partitioned Y-piece, was found to be effective in reducing under-mask dead-space. ~ Neff and his colleagues in 1968 '~ described a device using the fresh gas flow to drive a Venturi circulator. The authors state that the device can largely eliminate apparatus deadspace, ensure rap idmixing of the anaesthetic atmosphere, and compensate for apparatus breathing resistance. The present communication describes a detailed laboratory investigation of this device supplemented by a limited clinical evaluation.