The Science Journal of the American Association for Respiratory Care

2003 OPEN FORUM Abstracts

DIFFERENCES IN INTRAPULMONARY CONDITIONS BETWEEN CONVENTIONAL VENTILATION AND HIGH FREQUENCY VENTILATION

Karel Roubik, Ph.D., Martin Rozanek, M.Sc., Jan Pachl*, Ph.D., Petr Waldauf*, M.D. Czech Technical University, FEE, Dep. of Radioelectronics K337, Biomed. Lab., Technicka 2, CZ - 166 27, Prague 6, Czech Republic *Charles University, 3rd School of Medicine, Dep. of Anesthesiology and CCM, Srobarova 50, CZ - 100 34, Prague 10, Czech Republic

Introduction: When conventional mechanical ventilation (CMV) or high frequency oscillatory ventilation (HFOV) is used, different and sometimes unpredictable effects are observed. The aim of this study is to examine differences in intrapulmonary conditions during CMV and HFOV using mathematical modeling of the respiratory system. Simulations using the model are used to describe unequal effects of both the ventilation modes upon various parameters affecting intrapulmonary conditions.

Method: A mathematical model of the respiratory system has been developed. It is an electro-acoustic analogy of the respiratory system respecting its exact anatomical structure. All individual airways are represented by short acoustic wave-guides with parameters computed using the common acoustic principles and morphometry data (Shields TW et al.: General Thoracic Surgery. Philadelphia, Williams & Wilkins, 2000). Alveoli are represented by acoustic compliances computed from their dimensions and overall lung compliance. The final model comprises 67 108 859 individual components. A special method has been developed so that such complicated model could be used for simulations of the real situations. Frequencies around 0.25 Hz are considered as CMV frequencies and around 5 Hz as adult HFOV frequencies. Distribution of tidal volume VT and pressure amplitude .P among generations of bronchial tree, total lung impedance and other variables were studied under various conditions by the modeling.

Results:Changes of alveolar compliance have a significant effect on total lung impedance (TLI) during CMV (see Figure) while TLI changes during HFOV are not essential (effect of the airway inertances). Contribution of airway resistance changes is significant mainly during HFOV. TLI is essential parameter for pressure controlled ventilation modes. Therefore, HFOV ventilator is able to deliver a sufficient VT with a minimal pressure load of ARDS patients with decreased lung compliance, contrary to CMV. Distribution of VT among individual generations is more or less independent on ventilatory frequency and changes of the lung mechanical properties. These results describe differences between CMV and HFOV. Supported by MSMT J04/98:210000012, GA CR 305/00/0651.

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