34 arrangemets of bag(B), pop-off valve(P) and fresh gas inlet(F) location in the semi-closed circle system were tested to determine which arrangement best conserved fresh gas using N_2O as fresh gas at a given inflow rate under constnt ventilatory condition and the influence of the relative loation of B, P and F on the efficieney of fresh gas conservation was evaluated.The influences of inflow rate, tidal volume and respiratory rate under constnt minute volume wer also examined in the semi-closed system which has been most widely used in the present day practice.As the results of investigating data, the following facts were revealed;(1) The high efficiency of all the arrangements both during spontaneous and controlled respiration (S.R. and C.R.) was obtained when P was located in Seg. 1 (from the expiratory valve to the inspiratory valve close to the inspiratory valve) or Seg. 2 (from the expiratory valve to the inspiratory valve close to the expiratory valve) and F was located in Seg. 1 or immediately downstream from the inspiratory valve.(2) P was prefered to be located in Seg. 1 during S.R. and in Seg. 2 during C.R.(3) When P and F were located in Seg. 1 or Seg. 2 during S.R., if P, F and B were closely located each other, the efficiency was decreased greatly.(4) The low efficiency occurred when P and F were located in immediately downstream from the inspiratory valve with S.R. and closely located each other near the expiratory valve with C.R. (5) If the minute volume was constant, the change in tidal volume and respiratory rate had no effect on the efficiency.(6) The efficiency was directly related to inflow rate and increased in proprtion to inflow rate.From all the above results, the system represented by B_2B_2F_1(B and P were in Seg.2,F was in Seg. 1) was the best arrangement tested and this was consistent with the statements in the view point of CO_2 elimination efficiency.
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