Comparison of pneumotachography and anemometery for flow measurement during mechanical ventilation with volatile anesthetics

Volatile anesthetics alter the physical properties of inhaled gases, such as density and viscosity. We hypothesized that the use of these agents during mechanical ventilation would yield systematic biases in estimates of flow (V) and tidal volume (V-T) for two commonly used flowmeters: the pneumotachograph (PNT), which measures a differential pressure across a calibrated resistive element, and the hot-wire anemometer (HWA), which operates based on convective heat transfer from a current-carrying wire to a flowing gas. We measured during ventilation of a spring-loaded mechanical test lung, using both the PNT and HWA placed in series at the airway opening. Delivered V-T was estimated from the numerically-integrated . Measurements were acquired under baseline conditions with room air, and during ventilation with increasing concentrations of isoflurane, sevoflurane, and desflurane. We also evaluated a simple compensation technique for HWA flow, which accounted for changes in gas mixture density. We found that discrepancies in estimated V-T between the PNT and HWA occurred during ventilation with isoflurane (6.3 +/- 3.0%), sevoflurane (10.0 +/- 7.3%), and desflurane (25.8 +/- 17.2%) compared to baseline conditions. The magnitude of these discrepancies increased with anesthetic concentration. A simple compensation factor based on density reduced observed differences between the flowmeters, regardless of the anesthetic or concentration. These data indicate that the choice and concentration of anesthetic agents are primary factors for differences in estimated V-T between the PNT and HWA. Such discrepancies may be compensated by accounting for alterations in gas density.