THE WORK OF BREATHING

The respiratory muscles, in ventilating the lungs, do mechanical work of two kinds. The first kind is done against elastic forces in the lungs and chest wall, which together tend always to bring the chest to position it occupies at the end of a normal expiration. Expiration or inspiration from this position of equilibrium involves the performance of ‘elastic’ work and the storing of potential energy which is then available to do ‘viscous’ work during the return to the position of rest. Thus, during quiet breathing, inspiration is ‘active’ and expiration ‘passive’. The other kind of work is frictional or ‘viscous’ work and is expended in forcing air through the air-passages and in displacing soft and inelastic tissues. Work of this kind is not stored as mechanical energy but is converted into heat and motion, and it must be done during both inspiration and expiration.
Elastic work is increased when the lungs or chest wall are made more rigid by pulmonary congestion, infiltration or fibrosis, or by ankylosing spondylitis or scleroderma. Viscous work is increased by rapid breathing (with or without change in depth), by obstruction of the airway, as in tumour, asthma or emphysema, and by obesity and deformity of the chest. The work done by the respiratory muscles may be expressed in terms of their oxygen consumption; each rise of 1l./min. in ventilation normally requires an extra O₂ consumption of 0.5 to 1 ml./min. or more. A person’s ability to increase ventilation is reduced by old age, general unfitness (an important but neglected factor in patients who get little exercise) and by various kinds of paralysis of the respiratory muscles, as well as by the factors which increase the work of breathing.

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