work of breathing
Work of breathing is the physical effort your respiratory muscles spend just to move air in and out. Every breath is a small act of labour — pushing against the lung's springiness and the friction of the airways — and that labour has an energy cost.
That cost has two main parts. One is elastic work, the effort to stretch the lungs and chest wall against their recoil; this dominates when the lungs are stiff. The other is resistive work, the effort to drive air through the airways against friction; this dominates when the airways are narrowed.
In healthy people at rest, breathing is remarkably cheap, using only a few percent of the body's total oxygen consumption — so little you never notice it. The body even tends to settle on a breathing rate and depth that minimise the combined elastic and resistive work.
Disease can make breathing expensive. Stiff lungs (fibrosis, oedema) raise elastic work; narrowed airways (asthma, COPD) raise resistive work. When the work climbs high enough, you feel breathless, recruit accessory muscles, and the breathing muscles themselves consume so much oxygen that they can tire and fail — a path toward respiratory failure.
Non-invasive ventilation helps a tiring COPD patient by sharing the work of breathing: the machine supplies pressure to move air, letting the exhausted respiratory muscles rest and recover.
Ventilatory support offloads the work of breathing.
Visible signs of high work of breathing — using neck and shoulder muscles, flaring nostrils, retractions between the ribs, an inability to speak full sentences — are key bedside clues that a patient is struggling and tiring.