Diaphragmatic EMG and occlusion pressure response to elastic loading during CO2 rebreathing in humans

Abstract

The effects of external elastic loading (EL) (19 cmH2O/l), applied continuously (C) and intermittently (I) during CO2 rebreathing, on diaphragmatic electromyogram (EMGdi), mouth occlusion pressure (P0.15), and ventilation (VI) were studied in normal subjects. EMGdi was analyzed as moving time average and quantitated in terms of peak (mean p) and average rate of rise of inspiratory activity (mean p/TI). CEL resulted in an increased mean p/TI response to CO2 in all subjects with P0.15 increasing in proportion to EMGdi. Tidal volume (VT) during rebreathing was decreased in all cases with VI being preserved in four of six runs due to increased breathing frequency (f). Although mean p was increased for a given end-tidal CO2 (PACO2) level during CEL, for a given rate of rise of inspiratory activity mean p was decreased in three of five subjects, indicating a diminished threshold for inspiratory "off-switch." CEL results in an augmented inspiratory drive that serves to increase muscle output and stabilize VT; the increased drive and decrease inspiratory off-switch threshold shorten TI mediating the compensatory increase in f. The first breath IEL resulted in decreased VT and mean p without change in mean p/TI, and all increased with subsequent loaded breaths independent of changes in PCO2. Load compensation for externally applied EL is mediated by neural mechanisms independent of chemical drive.

Inspiratory conductance: an alternative index of the relationship between inspiratory effort and airflow with potential for clinical use

“…Mouth occlusion pressure (MOP) was mainly explored as an indirect measure of central respiratory drive (15, 16). It is directly dependent on the rate of contraction of inspiratory muscles (17). MOP is usually measured by taking a peak pressure measurement with a transducer placed close to the mouth while the subject breathes through an apparatus incorporating an occluder that remains closed for the first 100 ms of inspiration (18); such an index is usually referred to a P 0.1 .…”

“…MOPs are therefore measured while no flow is occurring and inspiratory muscle contraction is close to isometric; the pressure fall is caused by rarefaction of gas in the airways and by transmission of the pressure wave across lung parenchyma and other compliant intrathoracic structures, and will, of course, take place at the speed of sound. Therefore, the pressure change is not mechanically influenced by airway resistance (15, 16, 20). It is reasonable to regard P 0.1 and dP/dtmax as measures of inspiratory muscle contraction (15–18).…”

“…Therefore, the pressure change is not mechanically influenced by airway resistance (15, 16, 20). It is reasonable to regard P 0.1 and dP/dtmax as measures of inspiratory muscle contraction (15–18). Both indices rise in response to an increase in airflow resistance.…”

“…Both indices rise in response to an increase in airflow resistance. However, these indices are subject to diaphragmatic performance and would be expected to fall when the diaphragm becomes fatigued even when airflow resistance continues to rise (16, 21).…”

“…This contention is based on the reasonable and evidence‐based assumption that mouth occlusion pressure fall is a proxy measure of the mechanical effort of breathing (15–18, 20), and that the immediate post‐release airflow is dependent to a substantial extent on the resistance to airflow in the airways.…”