Effects of flow-resistive loading on mouth occlusion pressure during CO2 rebreathing.

To evaluate mouth occlusion pressure as an index of neural drive to the respiratory muscles that is independent of lung mechanics, the occlusion pressure response to rebreathing was studied in 7 normal subjects under control conditions and during flow-resistive loading. Inspiratory, expiratory, and combined inspiratory-expiratory flow resistances of 5 and 17 cm H2O per liter per sec were studied in 7 normal subjects. Pressure at the mouth was measured 150 msec after the onset of inspiration against the occluded airway. In all subjects, the ventilatory response to CO2 rebreathing was consistenly decreased by the 3 types of resistive loads, and this decrease was often greater with the higher load. In contrast, the occlusion pressure response was usually increased with inspiratory and inspiratory-expiratory resistance, indicating increased inspiratory muscle output due to these loads. With expiratory resistance, the occlusion pressure response was decreased in most of the subjects, suggesting a decrease in muscular output with expiratory loading. The inspiratory muscle pressure response to resistive loading could have been mediated by neural reflex and/or intrinsic muscle mechanisms. Occlusion pressure, therefore, appears to reflect over-all inspiratory neuromuscular output of the respiratory system during CO2 rebreathing, even with the addition of flow-resistive loads. Measures of mouth occlusion pressure provide a distinct advantage over ventilatory parameters in studies of respiratory control mechanisms during mechanical loading.