The superior segments of RLL, LLL and axillary subsegments of anterior and posterior segments of RUL are common sites for aspiration and will account for 85% of all Lung abscesses.
Gravitational forces determine the site of aspiration. Position of the patient at the time of aspiration determines the segment the aspiration is most likely to occur.
Basal segments of RLL used to be the most common site for aspiration during 1940 to 1960. During this period ENT surgery and Dental work was done in sitting position with Ether as the anaesthetic. The Right main bronchus is in straight line with Trachea while left main takes of at an angle. In this position gravity facilitates lodging of the aspirate to basal segments of RLL.
In supine position and with the patient on back superior segment of RLL is the most dependent segment.
In right lateral decubitus position the axillary subsegments of anterior and posterior segments of RUL is the dependant site for aspiration. Abscess is located in the middle of lateral CXR corresponding to RUL bronchus take off.
When the patient is on abdomen, aspiration does not occur, thus it is extremely unlikely for any anterior segments, middle lobe and lingula to be the site for aspiration lung abscess. When lung abscess is encountered in these sites on should suspect partial airway obstruction or trouble with deglutition as the predisposing factor for lung abscess.