Peak expiratory flow profiles delivered by pump systems - Limitations due to wave action

Pump systems are currently used to test the performance of both spirometers and peak expiratory flow (PEF) meters, but for certain flow profiles the i nput signal (i.e., requested profile) and the output profile can differ. We developed a mathematical model of wave action within a pump and compared t he recorded flow profiles with both the input profiles and the output predi cted by the model. Three American Thoracic Society (ATS) flow profiles and four artificial flow-versus-time profiles were delivered by a pump, first t o a pneumotachograph (PT) on its own, then to the PT with a 32-cm upstream extension tube (which would favor wave action), and lastly with the PT in s eries with and immediately downstream to a mini-Wright peak flow meter. Wit h the PT on its own, recorded flow for the seven profiles was 2.4 +/- 1.9% (mean +/- SD) higher than the pump's input flow, and similarly was 2.3 +/- 2.3% higher than the pump's output flow as predicted by the model. With the extension tube in place, the recorded flow was 6.6 +/-: 6.4% higher than t he input flow (range: 0.1 to 18.4%), but was only 1.2 +/- 2.5% higher than the output flow predicted by the model (range: -0.8 to 5.2%). With the mini -Wright meter in series, the flow recorded by the PT was on average 6.1 +/- 9.1% below the input flow (range: -23.8 to 2.5%), but was only 0.6 +/- 3.3 % above the pump's output flow predicted by the model (range: -5.5 to 3.9%) . The mini-Wright meter's reading (corrected for its nonlinearity) was on a verage 1.3 +/- 3.6% below the model's predicted output flow (range: -9.0 to 1.5%). The mini-Wright meter would be deemed outside ATS limits for accura cy for three of the seven profiles when compared with the pump's input PEF, but this would be true for only one profile when compared with the pump's output PEF as predicted by the model. Our study shows that the output flow from pump systems can differ from the input waveform depending on the opera ting configuration. This effect can be predicted with reasonable accuracy u sing a model based on nonsteady flow analysis that takes account of pressur e wave reflections within pump systems.