Stabilising function of the human diaphragm in response to involuntary augmented breaths induced with or without lower-limb movements

Abstract

New findings: What is the central question of this study? Is the stabilising function of the diaphragm altered differentially in response to involuntary augmented breaths induced with or without lower-limb movements? What is the main finding and its importance? At equivalent levels of ventilation, the diaphragm generated higher passive pressure but moved significantly less during incremental cycle ergometry compared with progressive hypercapnia. Diaphragm excursion velocity and power output did not differ between the two tasks. These findings imply that the power output of the diaphragm during stabilising tasks involving the lower limbs may be preserved via coordinated changes in contractile shortening.

Abstract: Activity of key respiratory muscles, such as the diaphragm, must balance the demands of ventilation with the maintenance of stable posture. Our aim was to test whether the stabilising function of the diaphragm would be altered differentially in response to involuntary augmented breaths induced with or without lower-limb movements. Ten healthy volunteers (age 21 (2) years; mean (SD)) performed progressive CO₂ -rebreathe (5% CO₂ , 95% O₂ ) followed 20 min later by incremental cycle exercise (15-30 W/min), both in a semi-recumbent position. Ventilatory indices, intrathoracic pressures and ultrasonographic measures of diaphragm shortening were assessed before, during and after each task. From rest to iso-time, inspiratory tidal volume and minute ventilation increased two- to threefold. At equivalent levels of tidal volume and minute ventilation, mean inspiratory transdiaphragmatic pressure ( ${\overline{P}}_{\mathrm{di}}$ ) was consistently higher during exercise compared with CO₂ -rebreathe due to larger increases in gastric pressure and the passive component of ${\overline{P}}_{\mathrm{di}}$ (i.e., mechanical output due to static contractions), and yet diaphragm excursion was consistently lower. This lower excursion during exercise was accompanied by a reduction in excursion time with no difference in the active component of ${\overline{P}}_{\mathrm{di}}$ . Consequently, the rates of increase in excursion velocity (excursion/time) and power output (active ${\overline{P}}_{\mathrm{di}}$ × velocity) did not differ between the two tasks. In conclusion, the power output of the human diaphragm during dynamic lower-limb exercise appears to be preserved via coordinated changes in contractile shortening. The findings may have significance in settings where the ventilatory and stabilising functions of the diaphragm must be balanced (e.g., rehabilitation).

Keywords: CO2-rebreathe; cycle ergometry; diaphragm; exercise; hypercapnia; respiratory mechanics; respiratory muscle; ultrasound.