Objective: To examine the effect of varying circuit design and the fresh gas flow rate on the circuit work imposed by a continuous positive airway pressure (CPAP) circuit.
Design: Circuit work was measured during simulated inspiration (500 mL) with a lung model at inspiratory flow rates ([Latin capital letter V with macron above]) of 40, 60, and 80 L/min during the administration of 10 cm H2O CPAP through either a modified Mapleson-A or modified Mapleson-D circuit, both alone and when connected to a face mask (i.e., simulating an intubated and nonintubated patient). Fresh gas flow was varied from 10 to 250 L/min.
Results: The minimum circuit work occurred at a fresh gas flow rate approximating [Latin capital letter V with macron above]; however, circuit work was consistently lower for the modified Mapleson-A circuit compared with the modified Mapleson-D circuit. As the fresh gas flow rate was increased sequentially to 250 L/min, circuit work remained close to the minimum value for the modified Mapleson-A, but increased gradually with the modified Mapleson-D, e.g., from 0.017 kg[middle dot]m/L at a fresh gas flow rate and [Latin capital letter V with macron above] of 80 L/min to 0.035 kg[middle dot]m/L at a fresh gas flow rate of 250 L/min and a [Latin capital letter V with macron above] of 80 L/min. Rotation of the fresh gas flow inlet did not change the circuit work vs. fresh gas flow rate relationship. Addition of a face mask resulted in a smaller increase in circuit work for the modified Mapleson-D with increasing fresh gas flow rate. However, unlike the modified Mapleson-A circuit alone, the addition of a mask caused circuit work to increase with increasing fresh gas flow rate.
Conclusions: The modified Mapleson-A circuit at a fresh gas flow rate equal to [Latin capital letter V with macron above] minimizes circuit work, and hence represents an optimal CPAP circuit. The increases in circuit work at fresh gas flow rates above [Latin capital letter V with macron above] that were found with the modified Mapleson-D circuit are not due to inertial differences, and are likely due to turbulent gas flow. (Crit Care Med 1991; 19:266)
(C) Williams & Wilkins 1991. All Rights Reserved.