CHEST WALL MECHANICS DURING EXERCISE IN PATIENTS WITH SEVERE CHRONIC AIR-FLOW OBSTRUCTION

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Abstract
Dynamic mechanical properties of the chest wall were studied in 7 patients with severe chronic airflow obstruction (CAO). Measurements were made during quiet breathing at rest and during exercise on a bicycle ergometer at work rates equivalent to 50 and 100% of their maximal work rate (.ovrhdot.Wmax). The peak inspiratory pleural pressure relative to the chest wall relaxation curve (Pmus) increased from 13.5 .+-. 1.5 cm H2O at rest to 22.4 .+-. 1.7 cm H2O at .ovrhdot.Wmax while the coincident transdiaphragmatic pressure increased from 9.7 .+-. 2.1 cm H2O at rest to 16.5 .+-. 2.3 cm H2O at .ovrhdot.Wmax. Consequently, the coincident gastric pressure relative to its value during relaxation (Pab) was negative at rest (-4.5 .+-. 1.7 cm H2O) and became even more negative (-6.3 .+-. 2.3 cm H2O) at .ovrhdot.Wmax. The increase in ventilation with increasing exercise was associated with an increase in the passive outward displacement of the abdomen (.DELTA.Vab) relative to the total volume change (.DELTA.Vab + .DELTA.Vrc), such that the ratio .DELTA.Vab/(.DELTA.Vab + .DELTA.Vrc) increased from 0.37 .+-. 0.08 at rest to 0.52 .+-. 0.05 at .ovrhdot.Wmax. There was no respiratory paradox. From the analysis of volume-pressure tracings of the chest wall compartments, it was inferred that expiratory intercostal and abdominal muscles contracted forcefully during expiration on exercise, resulting in a marked increase in pleural pressure and a change in thoraco-abdominal configuration. This represented the storage of elastic and gravitational energy, which was released during inspiration, contributing to inspiratory pleural pressures and the enhanced inspiratory flow. Although recruitment of abdominals as accessory muscles of inspiration facilitated the increase in ventilation while protecting the diaphragm, the other inspiratory muscles generated a high proportion of their maximal pressure, possibly limiting ventilation during exercise.