title: Optimizing fresh gas flow and circuit design for the delivery of continuous positive airway pressure.
abstract: 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.
  
  
  METHODS
  Circuit work was measured during simulated inspiration (500 mL) with a lung model at inspiratory flow rates (V) 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 V; 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.m/L at a fresh gas flow rate and V of 80 L/min to 0.035 kg.m/L at a fresh gas flow rate of 250 L/min and a V 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 V minimizes circuit work, and hence represents an optimal CPAP circuit. The increases in circuit work at fresh gas flow rates above V that were found with the modified Mapleson-D circuit are not due to inertial differences, and are likely due to turbulent gas flow.
authors.affiliation: Department of Anaesthesia and Intensive Care, Flinders Medical Centre, Adelaide, South Australia.
authors.forename: A D
authors.initials: AD
authors.lastname: Bersten
authors.forename: A J
authors.initials: AJ
authors.lastname: Rutten
authors.forename: A E
authors.initials: AE
authors.lastname: Vedig
mesh_terms: D006801:Humans; D008961:Models, Structural; D011175:Positive-Pressure Respiration / Q000295:instrumentation* / Q000379:methods; D012123:Pulmonary Ventilation