The stethoscope is an extremely useful instrument for evaluating lung disease. This is the main exception to our basic principle, that the vast majority of medical diagnoses are made by careful history-taking, not by physical examination. That may surprise patients, but it’s absolutely true.
Normal lung sounds are simply a rush of air. For certain common lung diseases, we have to listen, specifically, for the following abnormal sounds (“adventitious” in medical-speak):
** Rales — These are “crackles,” heard during inspiration (when a person inhales, breathes in). To imitate them, we can listen as we rub a stethoscope along our hair. If a person has a fever, rales in just one part of the lung suggests Pneumonia. Rales at the bottoms of both lungs suggests Heart Failure. At the very tops of the lungs, they may suggest Tuberculosis (though most TB patients have normal-sounding lungs). If we hear crackles on expiration, we have to specify “expiratory rales,” because anyone hearing the term “rales” will assume inspiratory.
** Wheezes — These are long, high-pitched sounds heard during expiration (when the person breathes out). We hear them throughout the lung, not simply in one place (if so, it’s a rhonchus, see below). Wheezes are due to tightening or obstruction of the bronchi, called “bronchospasm” (see Lower Respiratory Anatomy, with diagrams). We’ll wind up diagnosing either Asthma, Acute Bronchitis, or COPD (either Chronic Bronchitis or Emphysema).
** Rhonchi — Squeaks and gurgles, high or low pitched, long or short, heard during inspiration or expiration or both. They occur in the same conditions as wheezes. Many clinicians think they’re different, and so diagnose wheezes as “asthma,” and rhonchi as “bronchitis,” but that’s erroneous.
** Stridor — a rare finding, it’s a longish high-pitched sound heard only on inspiration. It’s always best heard right over the trachea, and means something happening in the Upper Airway (trachea or throat), which is usually bad. If the sound is heard best elsewhere in the lungs, it’s probably a rhonchus.
** Prolonged Expiratory Phase — A very important finding, one missed by many clinicians. Think of how we sound when we breathe normally: a brief breath in, then exhale, and pause before the next breath. This “exhale + pause” looks longer than the breath in, when simply watching someone from the outside.
But listening to the inside of the lungs with a stethoscope, it’s the opposite: inspiration normally sounds longer than expiration, because the pause sounds silent. However, with bronchospasm, we hear the high-pitched wheeze of struggled expiration all through the pause. If the pitch is too high, we can’t hear the sound itself, just the time it occupies (unless we’re a dog).
If you ever get seen for a cough or shortness of breath, and your provider tells you, “The lungs are clear,” be sure to ask, “Does inspiration sound longer than expiration?” If not, and the normal ratio of inspiration-longer-than-expiration is reversed, then it’s bronchospasm.
** Diminished or Absent Lung Sounds — With some diseases, we can hardly hear breath sounds at all. Usually that occurs in just one lung from something in the pleural space, either fluid from a Pleural Effusion, or air from a Pneumothorax (punctured lung). If there’s a pleural effusion on both sides, the bottoms of both lungs lose their normal sounds. Also, patients with Emphysema (COPD) may have large chests from empty space inside the worn-out lung alveoli, so breath sounds are very quiet. And some patients just don’t breathe deeply enough, unless we ask them to.
Do Note that all of the above serves as a guide for clinicians, very useful in most cases, but not absolute. Many patients with Pneumonia have normal-sounding lungs (I’ve seen clinicians miss the diagnosis because they charted “lungs clear”). Then, there are rare patients with Heart Failure whose lungs sound wheezy, as if they have Asthma. Conversely, I had one patient with shortness of breath who had rales at the bottoms of both lungs. I did a number of tests for Heart Failure; all were normal. Finally, I gave him asthma treatment, & the rales disappeared!