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 (Xrs) 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 Xrs. 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 Xrs 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 Xrs 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.

Assessment of Respiratory System Resistance during High-Frequency Oscillatory Ventilation Based on In Vitro Experiment

“…Measurements 1-3 correspond to no added resistor and measurements 4-6 correspond to the phase of the experiment with the added resistor Rp. Three demijohns representing different compliances (37, 24, and 17 mL/cmH2O) were used in both phases of the The respiratory system resistance R rs measured at a pressure oscillation frequency of f = 5 Hz was calculated from the respiratory system impedance Z rs following the spectral density method described in [24]. R rs was obtained from Z rs by converting from polar to Cartesian coordinates according to Equation (1):…”

“…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.…”

“…The configuration of the experiment is shown in Figure 1 [24]. The high-frequency oscillatory ventilator 3100B with standard accessories was used for the experiment.…”

“…The Rp5 simulated the increased resistance of the respiratory system and the glass demijohn simulated the compliance of the lungs. Measurements performed without and with Rp5 were repeated for three glass demijohns of 54, 35, and 25 L. Values of corresponding compliances were 37, 24, and 17 mL/cmH 2 O, respectively [24]. The following ventilation parameters were used in the experiment: bias flow = 30 L/min, ventilatory frequency f = 5 Hz, CDP = 12 cmH 2 O, and pressure oscillation amplitude ∆P = 20 cmH 2 O. Inspiration to expiration time was set as I:E = 1:1.…”

“…Both the signals paw and qaw were recorded at a sampling frequency f = 1000 Hz. The respiratory system resistance Rrs measured at a pressure oscillation frequency of f = 5 Hz was calculated from the respiratory system impedance Zrs following the spectral density method described in [24]. Rrs was obtained from Zrs by converting from polar to Cartesian coordinates according to Equation ( 1):…”