Added inspiratory work of breathing during CPAP ventilation: comparison of two demand-valve devices with a continuous flow-system.

Measurements of added inspiratory work (AIW) of breathing imposed by three different CPAP systems were performed in 10 patients. One system was a continuous flow system while the two others were demand flow systems separated from respirators (Ohmeda VD 101 and Draeger CPAP 800 devices). AIW was calculated from pressure and flow signals recorded at the mouthpiece level. The AIW calculated with the two demand-flow systems was found to be the same as the AIW calculated with the continuous flow system although the results obtained by the Draeger device were less constant. The results obtained with the Ohmeda device in our patients conflicted with data previously published using a lung model.