For the Love of Lung Sounds


Howard Rodenberg, MD, MPH, Dip(FM) | | Monday, December 3, 2007

One of the beauties of a career in emergency care is that, in comparison with other medical specialties, things are often very easy. While the job is often emotionally and physically challenging, the limitations of the ED setting mean that the clinical work is actually quite basic. For example, I was taught all I ever needed to know about diagnostic psychiatry by a social worker in Maine:

"If the patient depresses you, they have a depressive disorder.

If they annoy you, they have a personality disorder.

If they confuse you, they're schizophrenic."

We do a very good job in medicine of trying to confuse people. Learning about breath sounds in medical school drove me crazy. There were vesicular sounds, tubular breath sounds, and adventitious sounds (which might be continuous or discontinuous). This is not to neglect bronchophany, pectoriloquy, and egophany, or the delightful "musical" breath sounds (which probably varies in pitch and tone depending on whether the examiner listens to Bach, George Strait, or AC/DC). To this day, even as I sit and read the descriptions in the text, I still have no idea what most of them are.

(I think my confusion about breath sounds has also led to my convoluted theory about why I never use an expensive stethoscope. The ostensible reason I give people for carrying around the ten-dollar version from the uniform store is that I'm afraid a good one will be stolen. The real reason, I fear, is that with a good stethoscope I might hear more things and be forced to describe them. Either that, or if I hear something abnormal with the cheap tubes, it must really be there.)

After years of fretting about fremitus (vocal and tactile I'll let you look it up), it finally occurred to me that like psychiatry, breath sounds might also be made ridiculously easy. This is because the only two kinds of breath sounds are wheezes and rales. Each is linked to a simple principle of physics.

Let's start with the basic sciences. Physics is the study of matter, and of the changes that matter undergoes in response to various forces. Since air is made up of matter (there are 45 billion billion molecules that's 45,000,000,000,000,000,000 within every cc of air), it makes sense to think that physical principles would govern the production of breath sounds.

The first concept of note is that the flow of air or fluid through a tube is proportional to the diameter of the tube by a power of four (Poiselle's Law). As the diameter of a tube grows larger, the flow through the tube increases exponentially. Speaking hypothetically, if a tube is 1 mm in diameter, then flow through this tube may be expressed as a factor of 1 to the fourth power, or 1 "flow unit;" if it's 2 mm wide, it can carry up to 16 "flow units." Poiselle's Law explains why flow through an 18-gauge IV catheter is more than double the flow rate of a 22 gauge catheter. This dictum has important implications for the production of breath sounds.

When things move through air, they set up a vibration as the molecules of static air are displaced by the moving object. Moving air within the bronchial tree displaces static air, and produces a vibration. When the vibration occurs at certain frequencies discernable to our ears, we perceive it as a sound. The pitch of the sound produced by the airflow is dependent upon the diameter of the tube through which the sound flows. As the radius of the tube falls, the pitch rises (that's why the smallest tubes on a pipe organ produce the highest notes); as the width of the tube rises, the pitch necessarily falls.

The second crucial concept is that of surface tension. Surface tension is a result of the mutual attraction between water molecules. It's what makes water hold together as puddles on the sidewalk or droplets on your car. Surface tension is powerful, but it can be broken with a concentrated and sustained force, like dropping a rock into a pool. In his recent book "A Short History of Everything," Bill Bryson sums up surface tension as "what gives the sting to a belly flop."

(By the way, this book is highly recommended reading for the thinking person of any stripe. It's the best one-volume look at our world I've ever seen the kind of book you keep off your shelf because you're constantly re-reading it. Simply amazing stuff.)

These laws of physics produce the only two kinds of breath sounds there can be. The first category includes those noises produced by airflow. Normal breath sounds result from airflow through the bronchi. As the normal bronchial tubes are open and unobstructed, these "normal" noises are of relatively low pitch (not quite as deep as the late, lamented Barry White my first, my last, my everything in dating mood music). As airways narrow during bronchospasm, airflow falls, and we begin to hear wheezes. Wheezes are nothing more than the vibrations of higher pitch produced by airflow through smaller tubes. They are another point along the spectrum from normal breath sounds, and not a phenomena unto themselves (for that matter, you could consider normal breath sounds as simply very low pitched wheezes). As airways continue to narrow, the pitch of the wheezes continue to rise. Eventually airflow becomes so restricted, and the pitch of the wheezes so high, that our ears can no longer perceive them (presumably a dog could, if a dog cared to do so). That's why the silent chest in an asthmatic is so dangerous it suggests extremely poor airflow and imminent respiratory collapse.

Rhonchi are nothing more than wheezes of constantly changing pitch. Clinically, rhonchi occur when airflow within a bronchus is partially blocked (most often caused by a buildup of mucous or other inflammatory material). As air moves over these irregularly shaped and flexible obstructions, the degree of the obstruction keeps changing as the material moves, buffeted by changes in body position, coughing and the airflow itself. The near constant motion of the material means that the diameter of the airway is continually in flux, and so an uneven, irregular pitch results. It is of interest to note that rhonchi are most often a lower-pitched sound of the larger airways, as mucoid material or foreign bodies in narrower airways effectively prohibit the significant airflow. The diminution in airway patency results in a higher pitch to the rhonchi, which again places the sound beyond our perception.

Rales result from the disruption of surface tension of intra-alveolar fluid. Think of fluid in the alveoli like water in a balloon. As the balloon expands, water molecules cling to both the side of the balloon and to each other. Eventually, as the balloon continues to expand, the attraction of the water molecules to the wall is more than their attraction to each other, and the bonds between the water molecules begin to break. The "snap" of a rale is the auditory manifestation of untold billions of molecular bonds breaking at once.

During inspiration, the increase in negative pressure within the chest pulls lung tissue apart in a centrifugal fashion (the pull radiates away from the center of the chest, the bronchial tube and the center of the alveoli). As the alveolar walls expand, the surface tension of intra-alveolar fluid (found in heart failure and volume overload) breaks. The concurrent rupture of surface tension in millions of alveoli produces the snapping sound we hear as a rale.

Everything else is a variant of the sounds produced by airflow (wheezes) or surface tension (rales). Looking back at my book of bedside diagnosis, continuous adventitious sounds are wheezes, and discontinuous ones are rales. Tubular, vesicular and musical sounds are all related to airflow, and are therefore classified as wheezes. You can do the same exercise yourself with your diagnostic manuals. Just remember that under this new definition, not all airflow sounds are abnormal. "Normal" breath sounds are a result of airflow, and are (as such) a type of wheeze.

What is the absence of breath sounds called? It's not called anything that I know of except bad, because breath sounds are truly absent only when something else present at the listening site has replaced normal lung function. This may be blood (hemothorax), free air (pneumothorax), fluid (pleural effusion), or consolidated and inflamed pulmonary tissue (pneumonia). Even Martha Stewart would note (from her cell) that none of these are good things. (If we wanted to get really philosophical, we could argue that by definition the absence of a breath sound is not actually heard, as one cannot hear something that doesn't exist like the theological argument that evil is not real, but only the absence of good).

I hope these concepts help you sort out the myriad of pulmonary squeaks and squawks. I'll leave you with one final simplification, one that sums up the entire discipline of internal medicine:

"Health is only the absence of a sufficient work-up."

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