The Interaction of FRC and Ventilation on Occlusion Pressure in Conscious Man

Analytical and experimental results relating the interaction of functional residual capacity (FRC) and ventilation (V) on occlusion pressure in conscious man are presented. An analytical model was developed relating the airway pressure measured 100 ms after occlusion (P100) with FRC and V just before occlusion. By relating the change in diaphragmatic force with time during breathing to that during occlusion, it was found that P100 and IFRC could be linearly related. In order to test this model, 16 normal adult subjects with different FRC values (2.0 to 5.6 1) were studied during periods of increasing ventilation. Subjects increased V by rebreathing a mixture of 7% CO2 and 93% 02. Neither AP100/APC02 nor AV/APCO2 could be correlated to FRC since the differences in motoneuron activity in response to hypercapnia were probably greater than the variations in FRC. Linear regression analysis on all 16 subjects demonstrated a significant effect of FRC on the relationships P100 versus V, and AP100/APCO2 versus A V/APCO2. These experimental findings were in close agreement with analytical predictions and suggest that subjects with larger FRC values have a greater ventilation and ventilatory response for the same occlusion pressure and occlusion pressure response, respectively.