Percussion of the thorax has two aims: determining the lung (and heart) borders (topographic percussion), and obtaining information on the status of the underlying lung tissue (comparative percussion).
Topographic percussion
The lung borders are determined posteriorly and (if indicated) the mobility upon breathing is checked. Anteriorly, the relative and absolute lung-liver border is determined. Then, if indicated, the position of the heart (e.g. when suspecting a tension pneumothorax) and Traube’s space can be percussed.
If an abnormality, causing a dull percussion sound (such as a lung infiltrate or pleural fluid), is present in the lower part of the left or right lung then percussion will produce an incorrect border location. The discrepancy in relation to the reference points and the contralateral lung, as well as the findings during history-taking, inspection, palpation and/or auscultation will usually lead to the correct diagnosis.
Comparative percussion
The aim is to detect abnormalities in the thorax and to gain an impression of the nature and size of these abnormalities. Differences between left and right can be very small. Therefore, it is of great importance to percuss at symmetrical positions on both sides whilst applying an exactly identical percussion technique. For a precise comparison it is necessary to immediately compare the corresponding areas on the left- and right-hand side. By always comparing left and right in this manner, areas with abnormal percussion tones can be detected.
In practice the topographic and comparative percussion methods will normally be carried out alternately numerous times to avoid requiring the patient to change position unnecessarily. Hence, the reason for discussing these two methods consecutively for the different thorax sides.