Measurement of respiratory airway-resistance by flow interruption method

Abstract

The interrupter method, commonly used in lung ventilators, assumes that the sudden pressure change upon occlusion at the mouth is equal to the trachea-alveolar pressure-difference before occlusion. The objective of this study is to evaluate the accuracy of the interrupter method by comparison with invasive pressure-flow measurements. Also of interest will be the effect of the shutter closure-speed, breathing direction and breathing frequency. The investigation will be conducted on a simulated respiratory system based on a six-generation hollow-lung model. Measurements have been made of trachea and alveolar pressures, and flow during unassisted breathing, at 0.5 to 3 Hz, and ventilator assisted breathing, at 0.5 to 1 Hz. The pressure and flow are in phase, which supports the quasi-steady assumption. The instantaneous airway-resistance, during unassisted breathing, varies greatly between the inspiratory and expiratory phases. However, an average value could be obtained from the slope of the pressure-flow plot. The airway resistance decreases slightly at higher breathing frequency. The airway resistance obtained during ventilator-assisted breathing compares resonably well with that from the invasive method. However, the interrupter technique shows a dependence on the shutter speed and breathing direction.