Respiratory sounds in healthy people: a systematic review

There is a lack of systematised information on respiratory sounds of healthy people. This impairs health professionals from differentiating respiratory sounds of healthy people from people with respiratory diseases, which may affect patients' diagnosis and treatment. Therefore, this systematic review aimed to characterise respiratory sounds of healthy people.

The Web of knowledge, MEDLINE, EMBASE and SCOPUS databases were searched and studies using computerised analyses to detect/characterise respiratory sounds in healthy people were included. Data were extracted using a structured table-format.

Sixteen cross-sectional studies assessing respiratory sounds in 964 subjects (aged 1 day-70 yrs) were included: 13 investigated normal respiratory sounds (frequency, intensity and amplitude) and 3 adventitious respiratory sounds (crackles and wheezes). The highest sound frequencies were observed at the trachea (inspiration: 447-1323 Hz; expiration: 206-540 Hz). Women (444-999 Hz) and infants (250-400 Hz) presented the highest frequencies at maximum power. Inspiratory sounds were more intense at the left posterior lower lobe (5.7-76.6 dB) and expiratory sounds at the trachea (45.4-85.1 dB). Nevertheless, studies establishing direct comparisons between inspiratory and expiratory sounds showed that inspiratory sounds presented the highest intensities (p < 0.001). Amplitude was higher at the left upper anterior chest (1.7 ± 0.8 V) and lower at the right posterior lower lobe (1.2 ± 0.7 V). Crackles were the adventitious respiratory sound most frequently reported.

Respiratory sounds show different acoustic properties depending on subjects' characteristics, subjects' position, respiratory flow and place of recording. Further research with robust study designs, different populations and following the guidelines for computerised respiratory sound analysis are urgently needed to build evidence-base.