A chest wall restrictor to study effects on pulmonary function and exercise - 2. The energetics of restrictive breathing

Chest wall restriction, whether caused by disease or mechanical constraints such as protective outerwear, can cause decrements in pulmonary function a nd exercise capacity. However, the study of the oxygen cost associated with mechanical chest restriction has so far been purely qualitative. The previ ous paper in this series described a device to impose external chest wall r estriction, its effects on forced spirometric volumes, and its test-retest reliability. The purpose of this experiment was to measure the oxygen cost associated with varied levels of external chest wall restriction. Oxygen up take and electromyogram (EMG) of the external intercostals were recorded du ring chest restriction in 10 healthy males. Subjects rested for 9 min befor e undergoing volitional isocapnic hyperpnea for 6 min. Subjects breathed at minute ventilations ((V)over dot(I)) of 30, 60, and 90 liters/min with che st wall loads of 0, 25, 50 and 75 mm Hg applied. Frequency of breathing was set at 15, 30, and 45 breaths per minute with a constant tidal volume (V-T ) of 2 liters. Oxygen uptake was measured continuously at rest and througho ut the hyperventilation bouts, while controlling (V)over dot(I) and V-T. In tegrated EMG (IEMG) from the 3rd intercostal space was recorded during each minute of rest and hyperventilation. Two-way ANOVA with repeated measures revealed that chest wall loading and hyperpnea significantly increased (V)o ver dot(O2) values (p < 0.01). External intercostal IEMG levels were signif icantly increased (p < 0.05) at higher restrictive load (50 and 75 mm Hg) a nd at the highest minute ventilation (90 liters/min). These data suggest th at there is a significant and quantifiable increase in the oxygen cost asso ciated with external chest wall restriction which is directly related to th e level of chest wall restriction.