percussion

Percussion is the technique of tapping on the chest wall and judging the underlying tissue by the note it produces — much as you might knock on a wall to tell where it is hollow and where there is a beam behind it. Air, fluid and solid each return a distinctly different sound.

The usual method is to lay one finger flat against the chest and strike its middle joint sharply with a finger of the other hand, comparing matching spots on both sides. Air-filled normal lung gives a resonant note. A solid or fluid-filled area — as in pneumonia, collapse, or a pleural effusion — gives a dull, muffled note, like tapping a full barrel. An over-inflated or air-pocketed chest, as in severe emphysema or a large pneumothorax, gives a hyper-resonant, boomy note.

By percussing systematically down and across the chest, the examiner maps the boundaries between resonant and dull zones, helping locate fluid levels or areas of consolidation before any imaging. Percussion is most informative for changes near the chest surface; deep or small abnormalities may produce no detectable change, so it complements rather than replaces a chest radiograph or scan.