Percussion is the tapping of the body which results in vibration of the underlying tissue at its own natural frequency and at a number of secondary frequencies. The produced tones provide information on the tissue up to a depth of approximately 5 cm.
Percussion sounds
A percussion sound can be resonant, dull or tympanic.
A resonant percussion tone is the tone heard above sponge-like air-containing tissue. It is a relatively low-frequency, long-lasting sound with many subtones.
Besides resonance, the terms hyperresonance and tympanic are also used. Both are varieties of resonance. These percussion tones distinguish themselves from resonance through a lower frequency and other subtones. Hyperresonance percussion tones are observed with an increase in air content in the lung tissue (e.g. with pneumothorax and with pulmonary emphysema).
A dull percussion tone is heard above an organ that does not contain air. The clearest example is an organ completely filled with fluid (e.g. a full bladder); other examples are the liver and muscles. These tones are called ‘dull’ because there is hardly any resonance. They have a weak, high-frequency sound that fades away quickly. There are hardly any subtones. Terms such as ‘shortened duration’, ‘dampening’ and ‘flat percussion tone’ are also used. Percussion of the thorax causes a dull percussion tone to be heard at the transition from the lungs to the liver, above the heart and above the shoulder blades. A dull tone anywhere else on the thorax always indicates an abnormality. Such dull percussion tones can be observed in places where the lungs contain less air, irrespective of the nature of the substance that has replaced the air [e.g. with fibrosis (connective tissue), atelectasis (stricture of a bronchial tree because of which part of the lung does not contain air), infiltrate (lung inflammation causing an influx of inflammatory cells, and thereby forcing a number of alveoli to close and reduce air capacity) or with a large (pathological) amount of pleural fluid (causing an insulating layer of fluid between both pleural membranes right around the lungs).
Tympanic tones are ones that are heard above the air-filled hollow abdominal organs. The resonance pattern is high-frequency, long-lasting and musical because of the nature of the subtones.
This sound is heard low at the anterior sid of the thorax, at the position of the fundus of the stomach (Traube’s space). There are no known pulmonary or pleural disorders that produce a tympanic percussion tone. Therefore, abdominal organs are always the source of tympanic percussion tones.
