# In Vitro Estimation of Relative Compliance during High-Frequency Oscillatory Ventilation
## 4 Discussion
This was caused by an increased noise amplitude in the flow signal after the addition of the resistor, as shown in Figure 3, in which the flow signals without and with the added resistor during one breathing cycle are compared. The presented method for measurement of \(X_{\text{rs}}\) during HFOV is simple enough to be used bedside. The single component added into the patient circuit is an orifice for pressure and flow measurement. In our study, we used a system that consists of the orifice, sensors, digitizing hardware, and a laptop with evaluation software [38]; in a clinical scenario, any standard monitoring device capable of proximal pressure and flow measurement and real-time data streaming would be feasible. A possible disadvantage of this method is an increase of the flow resistance, together with the dead space when the orifice (used for pressure and flow measurement) is added into the patient circuit. A limitation of our study is that the test used a single ventilator model with a frequency \(f\) = 5 Hz. This choice was based on previous studies [33, 34, 35, 36], and we did not investigate the possible use of other ventilatory frequencies. As the described method allows determination of relative compliance with respect to airway inertance and resistance, its practical application is predetermined to maneuvers, during which the change in lung compliance is the dominant parameter affecting changes in respiratory system mechanics. An example of such a maneuver, and thus a possible advantage of using the relative compliance determination, is optimizing _CDP_ in the initial phase of HFOV or its reassessment during the course of ventilation. Another advantage of relative compliance determination could be evaluating efficiency of recruitment maneuver, commonly implemented as a stepwise increase and consequent decrease of _CDP_, while evaluating influenced physiological parameters [42]. In these situations, when a  
Figure 3: Comparison of the measured flow signal prior to and after the addition of the Rp5 resistor. constant airway inertance and resistance can be assumed, observing a short-time trend of relative compliance gives a faster response than standard methods that utilize assessment of the respiratory gas exchange. Because maximal compliance and maximal carbon dioxide elimination are highly correlated [29], relative compliance change could indicate appropriateness of a conducted _CDP_-optimization step. [PERSON] et al. [43] documented that in adult patients with atelectatic lung disease, both underinflation and overinflation of the lungs impaired lung compliance. Changes in functional residual capacity during a recruitment procedure impact lung compliance as the position of ventilation changes along the inflation and deflation limbs of the pressure-volume curve [44]. As a consequence, a decrease in the relative lung compliance indicates moving ventilation outside of the linear lung-compliance region (i.e., it is able to detect either emerging lung collapse or lung hyperinflation). This method of relative lung compliance measurement could also facilitate control of modern ventilators that use mathematical modeling of the respiratory system as an integral part of their control systems, real-time systems for support of spontaneous breathing during HFOV, and in other applications with advanced control systems [45; 46; 47].