Reading the sounds of breathing

“In 1816, I was consulted by a young woman presenting with general symptoms of disease of the heart. Owing to her stoutness, little information could be gathered by application of the hand and percussion. The patient’s age and gender did not permit me to resort to… placing my ear to her chest. I recalled a well-known acoustic phenomenon: if you place your ear against one end of a wood beam, the scratch of a pin at the other end is distinctly audible. It occurred to me that this physical property might serve a useful purpose in the case I was dealing with. I then tightly rolled a sheet of paper, one end of which I placed over the precordium (chest) and my ear to the other. I was surprised and elated to be able to hear the beating of her heart with far greater clearness than I ever had with direct application of my ear. I immediately saw that this might become an indispensable method for studying, not only the beating of the heart, but all movements able of producing sound in the chest cavity.”

This passage, translated from the French, was written by physician Rene Laennec, the inventor of the stethoscope. Up until 1816, doctors listened to a patient’s heart and lungs by pressing an ear directly to the person’s chest. Perhaps veterinarians in those days did the same, although this likely presented considerable difficulties with patients ranging from agitated horses to heavily furred dogs to tiny kittens. Dr. Laennec strove to improve his creation, trying different materials and designs, finally settling on a wooden cylinder. He listened to many patients, comparing what he heard with autopsy results (in those cases that ended unhappily.) In 1819, he published the first work on the use of the stethoscope to listen to body sounds. The stethoscope is still one of the quintessential instruments in modern medicine.

So you bring me Huff, the hound, or Puff, the Persian, for breathing problems. Before picking up our stethoscope, let’s review what constitutes “normal.” The purpose of respiration is to deliver oxygen-rich air to the lungs. Oxygen moves into the blood. Carbon dioxide moves out. Normal respiratory rates vary greatly, depending on breed, exercise, and environment. In dogs, panting is an adaptation for regulating body temperature and has little to do with gas exchange. Here’s a cool fact. When Huff gets hot, blood flow to his tongue increases six-fold. The engorged tongue hangs out, providing increased surface area for heat dissipation. Huff also drools, his wet tongue cooling him further through evaporative heat loss. Small dogs usually pant faster than large. Cats rely more on modifying their environment and behavior to cool down (in other words they refuse to chase Frisbees at noon in August) but Puff may occasionally pant when hot or excited. “It’s not normal panting,” you report. “Huff’s just not right.”

Dr. Laennec said, “The most important part of an art is to be able to observe properly.” Before grabbing our stethoscope, let’s do just that. Breathing rate, depth, and pattern may all alter depending on metabolic needs and respiratory system mechanics. Huff may take unusually long, deep breaths, or make long, forceful expirations. Are there audible sounds like a cough or wheeze? Are the abdominal muscles heaving with each breath? If Huff is working really hard to breathe, that’s called “labored respiration.”

Now let’s assess the color of non-pigmented areas of his gums and tongue. Everything nice and pink? That’s good. If he is blue, or pale grey, that’s not so good, indicating inadequate oxygenation of the blood. What about general demeanor? An animal can have labored respiration but not appear distressed. Conversely, Huff may look anxious and unsettled, lips pulled back in a grimace, eyes dilated with fear. If our patient is experiencing an anxious emotional state associated with difficulty breathing, this is called dyspnea — an emergency requiring immediate attention. Dyspneic cats may appear agitated or they may be strangely non-responsive to external stimuli, acting almost dazed, as though all their attention is focused inward on the struggle to move air. Paying attention to all these things help localize the problem and establish a diagnosis.

Now it’s stethoscope time. Listening to a normal animal’s lungs doesn’t actually tell us a helluva lot. What it does do is train the clinician’s ear. People engaged in professions requiring specific, repetitive listening, like musicians or stethoscope-wielding doctors, actually develop enhanced sensitivity to the frequency ranges of interest. Over time, our auditory and neurological systems literally alter and adapt as we tune our ears. Listening to normal patients, day after day, year in, year out, we build a mental library of lung sounds and their significance. What may indicate in a young cat may be perfectly normal in an old dog. A pug sounds different than a greyhound. And so on. With experience, we also cultivate the art of “selective hearing,” i.e., the ability to tune out irrelevant noise, like the rub of fur against stethoscope… or the gregarious owner who keeps chatting at us despite the stethoscope plugged into our ears.

The respiratory system extends from mouth to chest and we must evaluate all areas. We listen at the nose and mouth, along the windpipe, then lungs, both sides, high and low. Disease can manifest as decreased or increased sounds. We listen for changes in frequency, pitch, duration, and location, then correlate findings with the clinical picture. No audible breath sounds in a healthy, relaxed cat? Normal. Same thing in a dypneic cat? Could be pleural effusion — fluid around the lungs that dampens breath sounds. Harsh tracheal sounds from a big dog? Probably laryngeal paralysis, or kennel cough infection. Musical wheezes in a cat? Feline asthma. Lung crackles? Maybe pneumonia, heart failure, even cancer. Although radiographs and other diagnostics will help further evaluate respiratory disease, we can learn an enormous amount just by astute observation and the use of an instrument that began as a humble rolled-up piece of paper.