# In Vitro Estimation of Relative Compliance during High-Frequency Oscillatory Ventilation
###### Abstract  
High-frequency oscillatory ventilation (HFOV), which uses a small tidal volume and a high respiratory rate, is considered a type of protective lung ventilation that can be beneficial for certain patients. A disadvantage of HFOV is its limited monitoring of lung mechanics, which complicates its settings and optimal adjustment. Recent studies have shown that respiratory system reactance (\(X_{\text{rs}}\)) could be a promising parameter in the evaluation of respiratory system mechanics in HFOV. The aim of this study was to verify in vitro that a change in respiratory system mechanics during HFOV can be monitored by evaluating \(X_{\text{rs}}\). We built an experimental system consisting of a 3100B high-frequency oscillatory ventilator, a physical model of the respiratory system with constant compliance, and a system for pressure and flow measurements. During the experiment, models of different constant compliance were connected to HFOV, and \(X_{\text{rs}}\) was derived from the impedance of the physical model that was calculated from the spectral density of airway opening pressure and spectral cross-power density of gas flow and airway opening pressure. The calculated \(X_{\text{rs}}\) changed with the change of compliance of the physical model of the respiratory system. This method enabled monitoring of the trend in the respiratory system compliance during HFOV, and has the potential to optimize the mean pressure setting in HFOV in clinical practice. high-frequency oscillatory ventilation; continuous distending pressure; lung compliance; respiratory system reactance; rigid respiratory system model +
Footnote †: journal: Medical  
monitoring methods should be involved to individualize HFOV settings, including electrical impedance tomography (EIT) [15], optoelectronic plethysmography [16], or respiratory system impedance analysis [17]. A lack of an individual approach to HFOV settings could have affected the outcome of earlier studies [18,19]. Currently, it is not clear how to set _CDP_ properly with respect to an individual patient's airway condition. _CDP_ should be set to reach the maximal recruitment of the lungs while minimizing their overstrain [20]. An improperly high level of _CDP_ can lead to ventilator-induced lung injury. Many studies have been performed with an aim to obtain an optimal setting for _CDP_[21, 22, 23, 24, 25, 26, 27, 28]. However, none of the suggested approaches have been broadly implemented in clinical practice, and _CDP_ is commonly being set as a value of the mean airway pressure from the preceding CMV increased by several cmH\({}_{2}\)O [25]. Nevertheless, this way of setting _CDP_ is quite questionable [25]. As shown in [29], the _CDP_ directly influences the compliance of the lungs, which is also tied to the recruitment (volume) of the lungs [29, 30, 31]. However, monitoring of the ventilation parameters is very limited in HFOV. For example, the 3100B ventilator (Vyaire Medical, Mettawa, IL, USA), which is commonly used in clinical practice for HFOV, and which was used in this study as well, does not enable monitoring of tidal volume or parameters of lung mechanics, such as lung compliance. This lack of monitoring ability also exists in the 3100A ventilator (Vyaire Medical, Mettawa, IL, USA), which is suitable for the ventilation of neonatal and pediatric patients. Recent studies have shown that reactance of the respiratory system (\(X_{\text{rs}}\)) measured using the forced oscillation technique (FOT) is a promising tool for evaluation of the mechanics of the respiratory system in HFOV [20,28]. During FOT, reactance is found using high-frequency pressure oscillations applied at the entrance of the respiratory system, and using the induced flow.