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.