Читать книгу Respiratory Medicine - Stephen J. Bourke - Страница 67

The source of breath sounds in the lungs is turbulent airflow in the larynx and central airways. The quality of the breath sound (as well as the sound of the voice) heard at the chest wall will vary depending on the medium it has to travel through (Fig. 2.4). A normally aerated lung will conduct low‐pitched sounds modestly but high‐pitched sounds very poorly. Normal breath sounds are therefore rather low‐pitched and faint, and are slightly longer in inspiration than expiration. The rate of airflow at the periphery of the lung is so slow it generates no audible sound at all. Therefore, terms such as ‘good air entry’ and ‘vesicular breath sounds’ are clearly wrong and should be avoided. Normal breath sounds should be called normal breath sounds – it’s that easy.

Figure 2.4 Summary of sound transmission in the lung. Sound is generated either by turbulence in the larynx and large airways or by the voice. Both sources are a mixture of high (H)‐ and low (L)‐pitched components. Normal aerated lung filters off the high‐pitched component but transmits the low‐pitched component quite well. This results in soft, low‐pitched breath sounds and low‐pitched vocal resonance. Consolidated lung transmits high‐pitched sound particularly well. This results in loud, high‐pitched breath sounds (bronchial breathing), high‐pitched bleating vocal resonance (aegophony) and easy transmission of whispered (high‐pitched) speech (whispering pectoriloquy). Pleural effusion causes reduction in the transmission of all sound, probably because of reflection of sound waves at the air–fluid interface. Breath sounds are absent and vocal resonance is much reduced or absent.

A solid medium (consolidated lung) conducts sound better, particularly high‐pitched sound. Breath sounds heard over a consolidated lung are therefore similar to those heard with the stethoscope held over the larynx and are referred to as bronchial breathing. The sound is louder and harsher, has a higher frequency ‘hiss’ and tends to be similar in inspiration and expiration.

Vocal resonance is assessed by listening over the chest with the stethoscope as the patient says ‘ninety‐nine’. Normal aerated lung transmits the ‘booming’ low‐pitched components of speech and attenuates the high frequencies. Consolidated lung, however, transmits the higher frequencies better, so that speech takes on a bleating quality known as aegophony. Whispering ‘ninety‐nine’ produces only high‐pitched sounds. This can barely be heard over normally aerated lung but is transmitted surprising well over consolidated lung and is referred to as whispering pectoriloquy.

Figure 2.5 Signs of localised lung disease.

A reduction in the intensity of breath sounds (diminished breath sounds) over an area of lung may indicate obstruction of a large bronchus and collapse of a lobe of the lung. A pleural effusion produces an air–fluid interface, which sound just bounces off. Breath and voice sounds are usually absent over an effusion.