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

Abstract

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 (V̇) and tidal volume (VT) 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 V̇ during ventilation of a spring-loaded mechanical test lung, using both the PNT and HWA placed in series at the airway opening. Delivered VT was estimated from the numerically-integrated V̇. 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 VT 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 VT between the PNT and HWA. Such discrepancies may be compensated by accounting for alterations in gas density.

Influence of Respiratory Gas Density on Tidal Volume During Pressure-Controlled Ventilation: A Laboratory Investigation and Observational Study in Children.

“…However, the degree of the impact of this phenomenon in clinical settings remained unclear. The results of our study were generally consistent with those of previous studies [5,6]. Additionally, our results indicate two important findings: First, the true TV of the lung decreased with an increase in respiratory gas density during PCV as expected, in both laboratory and clinical settings.…”

“…Moreover, some studies have reported that changes in the respiratory gas composition affect the accuracy of respiratory flow measurements [2,5,6], In this regard, with the progress of anesthesia machines, some anesthesia workstations have integrated gas analyzers and use the information of gas compositions to improve flow and volume monitoring so that TV measured by a built-in pneumotachometer is a closer approximation of the delivered TV. External pneumotachometers with gas 4 analyzers have also been widely used.…”

Quantitative measurement of air leak in patients with chest drains

“…In some studies in the literature, different types of anemometers have been used for medical measurement, and results have been compared to flow-meters. [21] Compared to digital drainage systems, the anemometer used in our study has lower sensitivity. We attempted to overcome this technical limitation to a certain extent, by using PALS-PALF values instead of continuous measurements.…”

Influence of Respiratory Gas Density on Tidal Volume during Mechanical Ventilation: A Laboratory Investigation and Observational Study in Children

“…To date, some laboratory studies have shown that the gas density in the breathing system influences airway resistance (Mondoñedo et al 2017). Recent clinical studies showed that anesthetic gases, which have higher densities than air and oxygen, increase airway resistance (Habre et al 2001;Nyktari et al 2011).…”