A Portable, Adjustable Forced Vital Capacity Simulator for Routine Spirometer Calibration1

Abstract

A new, portable calibrating device for verification of spirometer accuracy was tested. The instrument produces a volume-time spirogram similar to an exponential curve. In addition, a micrometer-adjustable orifice is available that produces a variety of timed volumes and flow rates. Four “calibrators” with the same orifice dimensions each produced 10 spirograms. The maximal variation for forced expiratory volume in 0.5 second (FEV_0.5) and one second (FEV₁), forced vital capacity (FVC), and FEV₁/FVC (expressed as a percentage of the mean) was less than ± 1.8 and ± 3.4 %, within and between the instruments, respectively. For forced expiratory flow during the middle half of forced vital capacity (FEF₂₅₋₇₅), variation within and between instruments was less than ± 2.3 and ± 5.1 %, respectively. The variation within and between instruments for instantaneous flows at 75, 50, and 25 % of FVC were within ± 6.5 and ± 7.3 %, respectively. The mean values obtained from these calibrators for timed volumes and FEF₂₅₋₇₅ were within 0.5 % of a pure exponential spirogram with a time constant of 0.45 s. In addition, the mean instantaneous flows previously mentioned were within 3 % of the exponential spirogram. A large syringe is still the most accurate ( ± 1.0 %) tool for calibrating static volumes; however, incorrect timed volumes and flows can be produced from spirometers that are accurately calibrated for static volume. Therefore, calibration of timed volumes and flow rates must be conducted to validate these parameters. The calibrator presented here is capable of testing a wide range of timed volumes and flow rates similar to the physiologic output of the FVC maneuver.