There has been a recent upsurge in interest in the use of mechanical ventilators and frequencies well above the normal respiratory range, particularly in patients with abnormal respiratory function. It has been suggested that high frequency ventilation (HFV) may allow maintained or enhanced mass transfer of CO2 (compared with conventional techniques) while producing minimal cardiovascular compromise. It is proposed to investigate theoretical and practical aspects of HFV. Animal studies will be carried out using a ventilator of novel design which allows the quantitative collection and measurement of expired gas at ventilatory frequencies up to at least 20 Hz. The current version of the ventilator can be used only for small animals (less than 5 kg) and a larger version will be developed for use in 10-15 kg dogs. Aspects of pulmonary function to be investigated in both normal and diseased lungs will include the effect of ventilatory frequency and tidal volume on: 1) efficiency on CO2 removal and on dead space, 2) distribution of gas within the lung, 3) functional residual capacity (FRC), and 4) alveolar pressures. We will use mechanical modeling to examine the properties of various high frequency ventilators, with particular reference to the relationships between pressures and flows measured within the system and the tidal and minute volumes delivered at the airway. We believe that these studies will help to explain the apparent discrepancies between minute ventilation requirements during HFV reported by different workers in the field. We will also develop theoretical models of CO2 transport in branched airway systems, taking into account velocity profiles produced under oscillating conditions, secondary flows developed at bifurcations, and asymetrical airway branching. The results of computer simulations based on these theoretical models will be compared with empirical data from the studies using animals and mechanical airway models to allow further refinement of the theoretical models and the development of other appropriate empirical approaches.
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