Physical Principles of Chest Auscultation

Margot R. Roach, MD

Address for correspondence: 104 Sea Shore Drive, RR1, Tatamagouche, NS B0K 1V0; E-mail: mroach@pchg.net

Conflict of interest: None declared

Can J Gen Intern Med 2006;1:XX–XX

ABSTRACT

Respiratory sounds are generated mechanisms that obey basic physical principles. Understanding these principles can improve one’s diagnostic acumen in auscultation of the chest.

SOMMAIRE

Les sonorités pulmonaires sont des mécanismes produits qui obéissent à des principes physiques de base. La compréhension de ces principes peut améliorer l’acuité diagnostique à l’auscultation du thorax.

Intelligent examination of the chest requires knowledge of how the sounds generated there are produced and transmitted in both health and disease. Sound is described by its frequency, amplitude, and quality. The way sound is transmitted and the amount of damping or attenuation and reflection it experiences depend on the impedance of the surrounding tissue.

              The airways, with tidal flow, have 22 generations of tubes excluding the trachea and the alveoli. In the first 10 generations, inspiratory flow is turbulent; slightly less so on expiration. Turbulence produces sound with a broad frequency spectrum (200 to 2000 Hz), increasing with tube size and flow rate. A sound’s overtones (compare middle C on a piano and a violin) depend on the fundamental frequency and the overtones produced by the thoracic cavity, and vary with the degree of inflation, as well as the size, shape, and structure of the chest wall. Tracheobronchial sounds come from the first three to four generations of airways, and vesicular sounds from generations four to ten. Beyond this, flow is non-turbulent, so no sounds are produced. Normal tracheobronchial sounds are heard over the neck, often the sternum, and occasionally the upper spine, whereas the vesicular sounds are heard over the mid-thorax laterally.

              Maximum sound amplitude occurs at its origin and is damped or attenuated as a function of distance from the source. The amount of damping depends on frequency of the sound (high frequencies are damped more than low ones are) and the impedance of the tissue. This means that solids transmit sound faster than liquids, and gases transmit the slowest. Because consolidated lung has an impedance between that of liquid and metal, it will dampen the sounds very little. Hence, if a lobe is consolidated, the tracheobronchial sounds will be transmitted to the chest wall through that lobe, whereas they are effectively damped out in travelling through normal lung. Similarly, vesicular sounds may have an increased intensity over a segment that is consolidated if it is fed by medium-sized bronchi. An overinflated chest increases the distance the sound must travel through air, and hence the sounds are more attenuated.

              At an interface, if the two impedances are comparable, most of the sound passes through; if they are different, most of the sound is reflected. At an air–water interface, only 0.1% of sound is transmitted, compared with 100% at a water–water interface. Thus, tracheobronchial sounds will be heard over an effusion over a region of consolidation, but there will be no audible sounds if the effusion is over normal lung.

              Wheezes are high-pitched musical expiratory sounds with a single frequency that is velocity dependent. They are produced by eddy shedding rather than by turbulence, and are particularly apt to occur if a small tube opens into a much larger one. By way of example, eddies are easy to see in water if a log sticks out into a stream; the edge of the flow divider acts the same way in the lung.

              Chests should be examined first with normal breathing and then with deep breathing. However, if the patient has irritable airways that are prone to collapse, often no expiratory sounds are heard. Wheezes can be heard in patients with minimal bronchospasm only with forced expiration, a manoeuvre that makes bronchioles collapse from the increased pressure in the thoracic cavity.

              Crackles are produced by air moving through liquid. The viscosity of the fluid determines the character and frequency of the sound. Watery fluids, as occur in pulmonary edema, have a higher pitch than do thick viscous fluids that occur in pneumonia. Take a bottle with liquids of varying viscosity and try blowing bubbles with different sizes of straws and different flow rates. By changing the size of the bottle, and by including one with a small neck, you can make the overtones vary and change the quality of the sound. Crackles are usually heard best in inspiration. They are audible without a stethoscope if the fluid is in the first few generations of bronchi. If the sputum is very viscous, it will be hard to move and crackles may be absent. In these situations (e.g., in tuberculosis), you can increase the chance of hearing crackles if you ask the patient to take a deep inspiration and then to produce two short coughs during expiration to loosen the sputum, before inspiring once more.

              The fine crackles observed occasionally with atelectasis do not come from the alveoli as the flow rate is too low. They likely originate in generations 12 to 14 of the bronchial tree; therefore, they are associated only with large areas of atelectasis. Since the adjacent lung may be overinflated, there may be more attenuation. Hence, these crackles may be harder to hear.

              Voice sounds are produced in the larynx and more proximal parts of the airway, and can be transmitted back into the open airways when the vocal cords are open. Loud sounds, or even whispered ones, may set the large bronchi vibrating and can then be heard over an area of consolidation. However, as voice sounds are carried primarily in the air, they are reflected at a pleural effusion regardless of whether it lies over a consolidated area. This difference in transmission of breath sounds and voice sounds allows one to determine whether there is an effusion over an area of consolidation.

              Pleural rubs are due to the two layers of pleura partially sticking to each other and creating a sound like that produced by rubbing wet leather. Pleural rubs, and the pain associated with them, are diminished if fluid separates the pleura or if they are stuck together.

              In summary, respiratory sounds are generated and propagated in accordance with basic physical principles. Understanding these principles can improve your diagnostic acumen in auscultation of the chest.

Dr. Roach trained in mathematics/physics in New Brunswick, medicine at McGill, and biophysics at UWO. She obtained her FRCPC in 1965 and did postdoctoral studies in Oxford before taking appointments in medicine and biophysics at UWO. She has published research on the elastic properties of arteries and the consequent changes seen in arteriosclerosis and aneurysmal disease. A pioneer in medical biophysics, she has won many prestigious teaching and research awards, and is now happily retired in Tatamagouche, Nova Scotia.

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Nath AR, Capel LH. Inspiratory crackles and mechanical events of breathing. Thorax 1974;29:695-8.