Why are breath sounds diminished on chest auscultation in patients with emphysema?

Question

I keep reading everywhere that you get reduced breath sounds in patients with emphysema but I don't understand the reasoning behind it!

I thought with the hyperinflated alveoli there would be less attenuation of sound and therefore increased breath sounds... but clearly I am wrong and now very confused!

Can anyone please help me???

Answer 1

I like that you're trying to match up your mental model with the physical world around you.  Good work.  Keep it up.

Air is actually a poor transmitter of sound compared to solid or liquid.  This is why you can't tell the direction a sound is coming from if you're SCUBA diving and someone taps on their tank with a diving knife.  It sounds like it's coming from everywhere since the transmission speed is so much faster than in air.  

Also, if you get a chance to look at an ultrasound image, you'll see that air inside intestines totally obscures what's  behind.  The air acts like a shock absorber in the sound wave, slowing the signal.  In order to get a good image with an  ultrasound device, one has to find a "window" which is either solid tissue or liquid filled spaces to transmit the sound waves effectively.

The same physics is true of auscultated breath sounds.  If you listen to a chest that's full of swollen wet lung like a pneumonia, it will transmit sounds very effectively from far away.  The sound are described as "bronchial breath sounds" meaning that you can hear the strong flow of air through the airways even though your stethescope is listening through the patient's back, lying over the posterior, peripheral lung tissue.  

A COPD (chronic obstructive pulmonary disease, or pulmonary emphysema) patient will have dilated air sacs and large blebs of open space inside the lung tissue.  These spaces transmit sound waves poorly.  What's more, the air flow into and out of these spaces is relatively limited so there's not much sound to transmit in the first place!  

COPD-ers are also sometimes called "air trappers" because they have difficulty exhaling the inhaled air.  Sometimes you'll see that they actually tend to breathe through pursed lips.  This is a natural way to increase airway backpressure and help relieve the pressure gradient across the walls of the resistance airways which collapse when the patient exhales forcefully.  By doing this, they actually increase the efficiency of their exhalation.  

If you look at a chest xray of a COPD patient, you'll often see hyperinflated lungs with relatively flattened diaphragms.  The air in the COPD lung tends to get trapped and moves in and out less.\

I hope that helps.  Some of the physiology of breathing can get quite complex.  

Answer 2

Breath sounds are caused by air MOVING. In emphysema, air is trapped, and not moving as much. Hence decreased breath sounds.  They are also hyper inflated, so percussion is different.

Answer 3

Because the lungs have very little if no surfactant. This will make the lungs harder and more diffuclt to breathe and ausculation is less likely to be heard. With the fluid surfactant you will be able to hear the lungs more clearly. That is why patients are more likely to have CT scans or MRI for results.

Answer 4

Think about what emphysema is: a bunch of little alveoli are replaced by a single large emphysematous bleb, and the process is repeated throughout both lungs, countless times. Not only are there fewer sacs, but they're larger, and so there's less turbulent air flow.

Of course, that also means there's a heck of a lot less surface area for gas exchange!