A new short review of classic lung sounds includes both audio clips and interpretations made in the light of modern pulmonary acoustics.
Modern computer-assisted techniques have also allowed precise recording and analysis of lung sounds, prompting the correlation of acoustic indexes with measures of lung mechanics. This innovative, though still little used, approach has improved our knowledge of acoustic mechanisms and increased the clinical usefulness of auscultation.
• What are characteristic features of stridor?
Stridor is a high-pitched, musical sound produced as turbulent flow passes through a narrowed segment of the upper respiratory tract. It is often intense, being clearly heard without the aid of a stethoscope. Evaluating stridor is especially useful in patients in the intensive care unit who have undergone extubation, when its appearance can be a sign of extrathoracic airway obstruction requiring prompt intervention. In cases of such obstruction, stridor can be distinguished from wheeze because it is more clearly heard on inspiration than on expiration and is more prominent over the neck than over the chest. Although stridor is usually inspiratory, it can also be expiratory or biphasic. Other causes of stridor in adults include acute epiglottitis, airway edema after device removal, anaphylaxis, vocal-cord dysfunction, inhalation of a foreign body, laryngeal tumors, thyroiditis, and tracheal carcinoma. The stridorous sound of vocal-cord dysfunction deserves special mention because it is often confused with asthma and is responsible for numerous visits to the emergency department and hospitalizations.
• What causes wheezing and what is the difference between a wheeze and a rhonchus?
The wheeze is probably the most easily recognized adventitious sound. Although wheezes are always associated with airflow limitation, airflow can be limited in the absence of wheezes, and the pitch of an individual wheeze is determined not by the diameter of the airway but by the thickness of the airway wall, bending stiffness, and longitudinal tension. Wheezes can be inspiratory, expiratory, or biphasic. Although typically present in obstructive airway diseases, especially asthma, they are not pathognomonic of any particular disease. In asthma and COPD [chronic obstructive pulmonary disease], wheezes can be heard all over the chest, making their number difficult to estimate. Localized wheeze is often related to a local phenomenon, usually an obstruction by a foreign body, mucous plug, or tumor. Wheezes may be absent in patients with severe airway obstruction. In fact, the model cited above predicts that the more severe the obstruction, the lower the likelihood of wheeze.
The rhonchus is considered to be a variant of the wheeze, differing from the wheeze in its lower pitch — typically near 150 Hz — which is responsible for its resemblance to the sound of snoring on auscultation. The rhonchus and the wheeze probably share the same mechanism of generation, but the rhonchus, unlike the wheeze, may disappear after coughing, which suggests that secretions play a role.
Morning Report Questions
Q: What are the differences between fine and coarse crackles?
A: Two categories of crackles have been described: fine crackles and coarse crackles. On auscultation, fine crackles are usually heard during mid-to-late inspiration, are well perceived in dependent lung regions, and are not transmitted to the mouth. Uninfluenced by cough, fine crackles are altered by gravity, changing or disappearing with changes in body position (e.g., bending forward). Coarse crackles tend to appear early during inspiration and throughout expiration and have a “popping” quality. They may be heard over any lung region, are usually transmitted to the mouth, can change or disappear with coughing, and are not influenced by changes in body position. Typically, fine crackles are prominent in idiopathic pulmonary fibrosis, appearing first in the basal areas of the lungs and progressing to the upper zones with disease progression. However, fine crackles are not pathognomonic of idiopathic pulmonary fibrosis; they are also found in other interstitial diseases. Coarse crackles are commonly heard in patients with obstructive lung diseases, including COPD, bronchiectasis, and asthma, usually in association with wheezes. They are also often heard in patients with pneumonia and congestive heart failure.
Q: What causes a pleural friction rub?
A: In persons with various lung diseases, the visceral pleura can become rough enough that its passage over the parietal pleura produces crackling sounds heard as a friction rub. This sound is more prominent on auscultation of the basal and axillary regions than on auscultation of the upper regions. One explanation for this difference is the fact that the basal regions lie on the steep portion of the static pressure-volume curve, whereas the upper regions lie on the flat portion of the curve. Thus, for a given change in transpulmonary pressure, the basal regions undergo greater expansion. Typically, the pleural friction rub is biphasic, with the expiratory sequence of sounds mirroring the inspiratory sequence. Typically, pleural friction rubs are heard in inflammatory diseases (e.g., pleuritis) or malignant pleural diseases (e.g., mesothelioma).