High-frequency ventilation: oscillatory dynamics.

Abstract

OBJECTIVES: To determine the influence of the dynamic properties of the oscillator on the oscillatory volume delivered through the endotracheal tube to the lung or lung surrogate (delivered volume) under conditions of high-frequency ventilation. In particular, the relation between the tidal volume of the pump (oscillator) and the delivered volume was analyzed. PaCO2 was measured further as a function of the delivered volume in a number of experiments performed with healthy dogs. DESIGN: Laboratory study. SETTING: Engineering and animal laboratory. SUBJECTS: Lung surrogates and healthy dogs. INTERVENTIONS: An experimental oscillatory system was connected to various lung surrogates. In addition, six beagle dogs received high-frequency ventilation with different delivered volumes during the study. Control of the mean airway pressure was achieved by a peripheral pressure chamber located at the exhaust port of the bias flow tube. RESULTS: The delivered volume, which is the quantity of interest from a physiologic point of view, can deviate considerably from the tidal volume of the pump due to dynamic (particularly resonance) effects. Because the delivered volume and the mean airway pressure have to be controlled independently, two independent quantities are necessary for control purposes (e.g., the tidal volume of the pump and the mean pressure at the exhaust port). Furthermore, it was found that a minimal condition for adequate gas exchange is a delivered volume that exceeds the machine-related deadspace. For this reason, and in order to maximize the CO2 gradient, the exhaust tube must be as short as possible. CONCLUSIONS: a) The delivered volume has to be monitored under clinical conditions; b) however, because the impedance of the endotracheal tube in general considerably exceeds the impedance of the lung, the influence of the impedance of the lung on the delivered volume is generally small, and thus an in vitro calibration may serve as a useful approximation; c) at least two independent quantities are needed for an adequate oscillatory control; d) a necessary (not necessarily sufficient) condition for adequate CO2 removal is that the delivered volume must exceed the machine-related deadspace; e) in a clinical environment involving extremely pathologic lung conditions, e.g., adult respiratory distress syndrome, mechanical lung characteristics may deviate substantially from those characteristics used in this study (i.e., the results obtained may not necessarily be applicable under all clinical situations).