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The Jackson-Rees modification of Ayre's T-piece was examined experimentally and theoretically to see what effects the theoretical restrictions of Onchi, Hayashi and Ueyama (1957) (1:1 I:E ratio, zero deadspace volume) and the square-wave approximation of Mapleson (1958) may have had on the calculated critical fresh-gas flow rate necessary to prevent rebreathing of exhaled alveolar gases. In the experimental examination six conscious alert volunteers breathed through the system as the fresh-gas flow rate was varied in steps, and their ventilation was recorded at each value. Assuming that under rebreathing conditions a normal arterial PCO2 is maintained by hyperventilation, this experimental situation is identical with one of the two considered theoretically by assuming sine-wave inspiratory and expiratory waveforms, zero longitudinal mixing in deadspace volumes and perfect mixing in the alveoli. Agreement between experimental and theoretical results justifies the assumptions made in the theoretical analysis and the results indicate that a fresh-gas flow rate of about twice the normal ventilation is necessary to eliminate rebreathing completely from the Jackson-Rees modification of Ayre's T-piece. The exact requirement depends on the deadspace/tidal volume (VD/VT) and I:E ratios. The onset of hyperventilation at lower fresh-gas flow rates is gradual and, in spontaneous breathing with VD/VT approximately equal to 40% and I:E ratio = 1:1.2, is only 10-20% at a fresh-gas flow rate of 1.5 times the normal ventilation.

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