DIFFERENCE BETWEEN OCCLUDED AIRWAY PRESSURE AND GENERATED MUSCLE PRESSURE

During the measurement of occluded airway pressure (Pao), the gas in the lung expands. This volume change causes Pao to underestimate the generated inspiratory muscle pressure (Pmus), which has been recommended as an index of respiratory drive. Analysis shows that the error is proportional to the thoracic gas volume at occlusion and to the effective respiratory system elastance (Ers′) and is inversely proportional to the dry atmospheric pressure. Examples of potential error in various species are presented. In normal human subjects at sea level measurement of Pao would underestimate Pmus on the order of 5 percent depending on Ers′. In subjects with large functional residual capacity due to obstructive lung disease, the error can be as large as 20 percent. It is recommended that, in all studies of occlusion pressure, an estimation of this error be made. © 1979 Pergamon Press Ltd.