## 4 Discussion  
The presented results show that changes in \(R_{\rm aw}\) can be monitored during HFOV by measuring the resistance \(R_{\rm rs}\) at an oscillation frequency of 5 Hz, when the basic mechanical properties of the respiratory system are consistent with larger animals or pediatric and adult patients. A physical model of the respiratory system was designed and an in vitro lab experiment was performed using different combinations of resistance and airway compliance values. It was shown that \(R_{\rm rs}\) increases when \(R_{\rm aw}\) increases. The results of this in vitro study also suggest that it is possible to follow the trend of \(R_{\rm aw}\) under conditions of changing lung compliance. The low standard deviations of \(R_{\rm rs}\) summarized in Table 1 indicate sufficient robustness of the algorithm used in signal processing. In Figures 3 and 4, small oscillations of the calculated \(R_{\rm rs}\) values can be seen. The oscillations are due to the processing of the noisy pressure and flow signals. The addition of a resistor to the ventilated system increased the standard deviation of \(R_{\rm rs}\). The flow was more turbulent with the added resistor and this resulted in an increase in the noise in the flow signal [24]. However, the increased turbulence did not degrade the evaluation of \(R_{\rm rs}\). Our in vitro study has some limitations. First, a single ventilation frequency of 5 Hz was investigated. The choice of ventilation frequency as the most appropriate was based on previous studies [18; 19; 24; 27; 28]. Second, we did not vary the CDP during the test, as this would be of little importance in a physical model with rigid walls. Animal studies [20; 29; 30], which mimicked immature patients and used the same FOT measurement method, reported a significant increase in \(R_{\rm rs}\) during lung derecruitment because of the low CDP applied during HFOV or the low positive end-expiratory pressure (PEEP) applied during CMV. The results are in agreement with the findings presented in [15], where the decrease in airway diameter was explained by a decrease in mean airway pressure. In contrast, only small changes in \(R_{\rm rs}\) are observed at CDP or PEEP values that are sufficient to maintain lung inflation. This is consistent with our simulation performed on a rigid model. Third, the results show that when the Rp5 resistor was added to the model, the measured \(R_{\rm rs}\) increased by more than 100 cmH\({}_{2}\)O-s/L on average, but the physical properties of the Rp5  
Figure 4: The course of computed \(R_{\rm rs}\) during ventilation of the respiratory system model with added resistor Rp5 (three 40 s long measurements correspond to measurement sections 4, 5, and 6 in Figure 2). Three glass demijohns with different compliance (37, 24, and 17 mL/cmH\({}_{2}\)O) were ventilated to investigate the effect of compliance on the measured resistance of the respiratory system. resistor may have contributed to such a large increase in \(R_{\rm rs}\). Resistor Rp5 is designed as parabolic, which means that the actual resistance value depends on gas flow rate. Moreover, the resistance of Rp5 is determined by its sudden and short decrease in airway diameter, a mechanism that is not present in vivo. The choke point created by the addition of Rp5 may cause the part of pressure-flow oscillation to be reflected into the glass demijohn and not return to the measuring system, resulting in an apparently more pronounced increase in resistance. It should also be taken into account that the physical properties of the glass demijohns used differ from the actual lungs. Pressure and flow oscillations could be deflected on the wall of the glass demijohn such that the oscillations could not return to the measuring system and would instead be damped within the glass demijohn. Such deflection does not occur in the airway tree in the lungs and could explain the difference between the actual resistance and the measured \(R_{\rm rs}\).