Damping of the dynamic pressure amplitude in the ventilatory circuit during high-frequency oscillatory ventilation

Abstract

The study deals with experimental measuring of attenuation of dynamic pressure during high-frequency oscillatory ventilation. The experimental circuit consists of high-frequency oscillatory ventilator Sensormedics 3100 B, patient circuit and lung model 5600i. Different values of the flow resistance and alveolar compliance were modelled during the experimental measurement. The ventilatory parameters of the oscillatory ventilator were constant during whole measurement. We confirm a decrease of the amplitude of the pressure swings in the alveolar space during high-frequency oscillatory ventilation. Dependence on the mechanical properties of the respiratory system was found. IntroductionArtificial lung ventilation (ALV) is used in many patients when spontaneous breathing is insufficient or intentionally inhibited. ALV is the most efficient method for treatment of acute respiratory failure. However, there are still strong adverse effects of ALV upon patient's respiratory system [1]. The adverse effects caused by ALV are known as ventilator induced lung injury. Current trend is to minimize the adverse effects of ALV in the respiratory care and new ventilatory regimens are introduced in order to ensure protective ventilation. One of these new techniques is high-frequency oscillatory ventilation (HFOV). Frequency of HFOV is higher compared with conventional artificial lung ventilation (CV) and the frequency rate is in range 3-25 Hz. Higher ventilatory frequency allows use of significantly smaller tidal volumes, which are comparable to anatomical dead space or even smaller. Also the pressure swings (ΔP) are decreased in the distal parts of the ventilatory circuit during HFOV in comparison with CV. The pressure swings are superimposed on the mean airway pressure (MAP) that inflates the lung. Use of MAP and smaller pressure swings during HFOV prevent the lungs from overdistension, barotrauma and volutrauma [2]. The difference in the frequency, pressure and tidal volume represent the most significant difference between HFOV and CV. It was shown in animal experiments that amplitude of the pressure swings is damped during HFOV. The alveolar pressure was measured by a capsule that was placed into the lung. The amplitude of the pressure swings was rapidly decreasing in the airways and only about 10 % of the input pressure was present in the alveolar space. The results were affected by the weight of the capsule and the placement of the capsule in the alveolar space [3,4]. It was showed that MAP is stable with different ventilator parameters and remains on the same level in all parts of the respiratory system [5]. The attenuation of the pressure swings was also studied by the mathematical model [6]. The simulations conducted on the model confirm the significant decrease of the amplitude of the pressure swings during HFOV. Less than 10 % of the input pressure amplitude is present in the alveolar space. The result of simulation conducted on the mathematical model can be seen in Image 1. Ratio of amplitude of the pressure swings for ...

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