Effects of hypercapnia on mouth pressure during airway occlusion in conscious man

Abstract

The effects of hypercapnia and inspiratory flow-resistive loading on mouth pressure during periods of arrested airflow were studied in conscious human subjects to determine the usefulness of inspiratory muscle force in the assessment of respiratory neural efferent activity. Hypercapnia increased the peak end-inspiratory mouth pressure (Ppeak) during complete airway occlusion and the pressures at 100, 200, and 300 ms after the onset of inspiration (P100, P200, P300). During rebreathing without added mechanical loads, P100 and Ppeak increased linearly with the electrical activity of the diaphragm and changes in P100 and Ppeak during hypercapnia correlated well with ventilatory responses to PCO2 (DELTA V/DELTA PCO2) suggesting that occluded mouth pressures are reliable measures of respiratory activity. In individuals with the greatest reduction in delta V/DELTA PCO2 during inspiratory flow-resistive loading, changes in P100 and Ppeak with PCO2 increased only minimally. In contrast, there was a much greater increase in occluded mouth pressures with hypercapnia in the presence of mechanical loading when inspiratory flow-resistive loading failed to depress delta V/DELTA PCO2. In all subjects, occluded mouth pressures were greater at any given PCO2 during mechanical loading than during free breathing. Mechanical loading resulted in augmented respiratory neural efferent activity unexplained by alterations in chemical stimulation.

Effects of hypercapnia and inspiratory flow-resistive loading on respiratory activity in chronic airways obstruction.

“…An increase in occlusion pressure dturing mechanical loading has previously been described in normal awake individuals (7)(8)(9) and has been demonstrated in unanesthetized goats (19). The observation that changes in occlusion pressure during mechanical loading are abolished in the goat by light anesthesia suggests that the augmented respiratory activity is mediated through supraspinal mechanisms and that awareness of the added resistance is necessary for load compensation (19).…”

“…One device for loading has been the addition of an external resistance to airflow. The reactions of normal individuals to these loads have been reported previously (7)(8)(9), but the respiratory responses to ventilatory loading in patients with lung disease have not been evaluated. In the present study, both chemical and neuromechanical control mechanisms in normal individuals and in patients with chronic airways obstruction were assessed by determining the changes in respiratory efferent activity during hypercapnia and after increasing the resistance to airflow during hypercapnia.…”

“…The mouth pressure during airway occlusion 100 ms after the onset of inspiration (P1oo) has been suggested as a more reliable measure of respiratory neuron efferent activity (11). Also, changes in occlusion pressure during hypercapnia correlate with changes in the electrical activity of the diaphragm and with ventilatory responses to CO2 in normal individuals (9). Consequently, Ploo was used in the present study as an index of respiratory neural efferent activity.…”

Comparison of the respiratory responses to external resistive loading and bronchoconstriction.

“…The response to resistive loads applied at the mouth has been used as an indirect test of the respiratory reReceived for publication 9 September 1980 and itn revised form 9 February 1981. sponses to spontaneously occurring airway obstruction (1)(2)(3)(4)(5). In both conscious animals and normal humans, these external loads enhance the drive to the respiratory muscles as assessed from measurements of inspiratory muscle force (occlusion pressure) and electrical activity (diaphragm electromyogram) (1)(2)(3)(4)(5)(6)(7).…”

“…In both conscious animals and normal humans, these external loads enhance the drive to the respiratory muscles as assessed from measurements of inspiratory muscle force (occlusion pressure) and electrical activity (diaphragm electromyogram) (1)(2)(3)(4)(5)(6)(7). However, external loads may inadequately reproduce many ofthe effects of airway obstruction on lung mechanics or thoracic mechanoreceptors.…”