Evaluation of the Humphrey A.D.E. breathing system.

A new breathing circuit (the Humphrey A.D.E., double lever model) was evaluated in adults to determine (1) the fresh gas flow (FGF) needed to achieve normocapnia during controlled ventilation and to just induce rebreathing during spontaneous ventilation, (2) end-expired CO2 (PECO2) at those FGF values, (3) the standard deviation of FGF requirements for controlled and spontaneous breathing (reliability of recommended FGF settings) and (4) the magnitude of change in PECO2 produced by varying FGF from the recommended values (sensitivity of the system). The FGFs that provided normocapnia with controlled ventilation and just induced rebreathing with spontaneous ventilation were 67 +/- 10 and 52 +/- 7 ml . kg-1 . min-1 (mean +/- SD), respectively. PECO2 values were 36.0 +/- 0.3 and 41.6 +/- 3.9 mmHg respectively. During controlled ventilation low reliability was offset by low sensitivity so that PECO2 changed little when FGF was raised or lowered from recommended values (0.2 mmHg/ml . kg-1 . min-1). In contrast, during spontaneous ventilation low reliability was additive with high sensitivity when using FGFs lower than the mean value that just induced rebreathing. A threshold was reached where lowering FGF from recommended values caused large changes in PECO2 (1.1 mmHg/ml . kg-1 . min-1). It is concluded that the FGF recommended by Humphrey for controlled ventilation is satisfactory. However, the FGF recommended by Humphrey for spontaneous ventilation may result in hypercapnia in some patients. This can be prevented either by using a higher FGF of 66 ml . kg-1 . min-1 routinely in all patients or by using lower flows with CO2 monitoring.