Gas compression artefacts when testing peak expiratory flow meters with mechanically-driven syringes.

Mechanically-driven syringes used to test peak expiratory flow (PEF) meters must produce the American Thoracic Society (ATS) standard waveforms with PEF accuracy of 2%. However, gas compression within the syringe could result in significant PEF inaccuracy when testing high resistance meters. The gas compression artefact was investigated in a mechanical syringe (PWG; MH Custom Design & Mfg L.C., Midvale, Ut, USA) of 13.6 L connected to a standard range mini-Wright PEF meter (Clement Clarke International, Harlow, UK). Scaled versions of the ATS standard waveform No. 24, with peak flows of 750 and 450 L x min(-1), were discharged through the PEF meter from different starting piston positions to vary syringe volume (Vsyr). The PEF recorded by the meter decreased linearly with increasing Vsyr. PEF decreased by 0.31 and 0.27% per litre for the ATS standard waveforms with PEF of 750 and 450 L x min(-1), respectively. The target PEF computed from piston displacement overread the actual PEF delivered into the PEF meter by approximately 4% when Vsyr = 13.6 L. Overreading fell to approximately 1% when Vsyr was reduced to 3.62 L. Therefore, gas compression error in commercially available large mechanical syringes can exceed the 2% inaccuracy limit when testing high resistance portable PEF meters. Measurements can be corrected for gas compression by linearly extrapolating PEF recordings to zero volume.