hyperinflation

If air trapping is air that keeps getting left behind, hyperinflation is the consequence: lungs that sit over-expanded, like a barrel that is always at least partly full. The chest can become rounded ('barrel chest'), and the dome-shaped diaphragm is pushed down and flattened.

Physiologically, hyperinflation is over-expansion of the lungs from retained air, raising the resting lung volume (functional residual capacity). A flattened diaphragm is a poor pump: it has lost its curved shape and pulls less efficiently, so the breathing muscles work harder for each breath. During exertion, dynamic hyperinflation adds even more trapped air, intensifying breathlessness.

On a chest radiograph it appears as a flat diaphragm, widened spaces between ribs, and a long, narrow heart shadow; lung function shows a raised total lung capacity and residual volume. Treatments that open the airways and allow fuller emptying — bronchodilators — and techniques like pursed-lip breathing can reduce hyperinflation and the sensation of breathlessness.