Fluid mechanics show that high-density gases need more energy while flowing through a tube. Thus, highdensity anesthetic gases consume more energy to flow and less energy for lung inflation during general anesthesia. However, its impact has not been studied. Therefore, this study aimed to investigate the effects of high-density anesthetic gases on tidal volume in laboratory and clinical settings. In the laboratory study, a test lung was ventilated at the same pressure-controlled ventilation with 22 different gas compositions (density range, 1.22-2.27 kg/m 3 ) using an anesthesia machine. A pneumotachometer was used to record the tidal volume of the test lung and the respiratory gas composition; it showed that the tidal volume of the test lung decreased as the respiratory gas density increased. In the clinical study, the change in tidal volume per body weight, accompanied by gas composition change (2% sevoflurane in oxygen and with 0-30-60% of N 2 O), was recorded in 30 pediatric patients. The median tidal volume per body weight decreased by 10% when the respiratory gas density increased from 1.41 kg/m 3 to 1.70 kg/m 3 , indicating a significant between-group difference (P < 0.0001). In both settings, an increase in respiratory gas density decreased the tidal volume during pressure-controlled ventilation, which could be explained by the fluid dynamics theory. This study clarified the detailed mechanism of high-density anesthetic gas reduced the tidal volume during mechanical ventilation and revealed that this phenomenon occurs during pediatric anesthesia, which facilitates further understanding of the mechanics of ventilation during anesthesia practice and respiratory physiology.

Fluid mechanics shows that high-density gases need more energy while flowing through a endo-tracheal tube. This means that during general anesthesia, high-density gases left less energy for lung inflation under pressure-controlled ventilation (PCV). However, the impact have not been studied. This study investigated the effects of high-density anesthetic gases on tidal volume (TV) in laboratory and clinical settings.In laboratory study, a test lung was ventilated at fixed PCV with 22 different gas compositions (density range, 1.22-2.27 kg/m3) using an Avance® anesthesia machine. The TV of the test lung and the respiratory gas composition were recorded by a pneumotachometer. As a result, the TV of the test lung decreased as the respiratory gas density increased. In clinical study, the change in TV/body weight, accompanied by gas composition change (2% sevoflurane in oxygen and with 0-30-60% of N2O), was recorded in 30 pediatric patients. Median TV/body weight decreased by 10% when the respiratory gas density increased from 1.41 kg/m3 to 1.70 kg/m3, indicating a significant between-group difference (P<0.0001). In both laboratory and clinical settings, an increase in respiratory gas density decreases the TV during PCV, and it could be well explained by the theory of fluid dynamics.