Why do patients with emphysema have increased chest diameters?

Doctors sometimes broadly classify respiratory diseases into 'obstructive' and 'restrictive' subtypes. In layman's terms this would roughly correspond to difficulty breathing out and difficulty breathing in, respectively.

Emphysema is a chronic 'obstructive' respiratory disease (the patient struggles to breath out). This is due to loss of the elastic tissue in the walls of the lungs. Normally, exhalation is almost completely accomplished by simple elastic recoil once you've breathed in, but in emphysema, the loss of the elastic tissue means that this force is not enough on its own. In other words, you have to forcefully exhale too.

However, this increased pressure that you generate comes from outside the lungs, and so it also tends to collapse the airways. In other words, even though you are attempting to force more air out, you are also narrowing the airways at the same time. This narrowing tends to prevent the air's exit, and so at the end of each breath, more air is trapped in the lungs of an emphysemic patient than in a normal person.

Let's go through that in more detail. (If this has been enough detail already, thank you very much, then just skip to where the writing gets big again!)

Imagine your airways as a pipe. For air to flow in the pipe, the pressure has to be greater at one end than the other. The air will then flow from the high to the low pressure area. In humans, the top end of the pipe - where the airway comes out of our mouths - is always at atmospheric pressure, which is obviously out of our control. What we can control, though, is the pressure at the bottom end of the pipe - the lower airways, especially the alveoli.

This is effected by changing the pleural pressure. Since the lungs are compressible to some degree, an increase in the pleural pressure will increase the alveolar pressure. As a result, the alveolar pressure will be higher than atmospheric pressure, and air will flow outwards: we exhale. The opposite is true for inhalation: we decrease the pleural pressure, the alveolar pressure follows suit, and air flows inwards.

On the right's a picture I made of the possible relative pressures during a normal expiration - intrathoracic pressure (+10), alveolar pressure (+5) and atmospheric pressure (0). Since the alveolar pressure is greater than the atmospheric pressure, exhalation will result.

How do we change the pleural pressure? By changing the size of the chest cavity. This is accomplished by contracting or relaxing the surrounding musculature - the diaphragm, and to a lesser extent, the intercostal muscles. (Other muscles can also come into play when a particularly forceful inhalation or exhalation is required.) For instance, when the diaphragm contracts, the chest cavity space is increased, and thus the pleural pressure decreases.

Normally, exhalation is a passive process (as mentioned above). In other words, we put effort into inhaling, but once we stop contracting the 'inhalation' muscles (mainly the diaphragm), the chest cavity (and thus the lung) recoils spontaneously without our intervention. This is due to the lung's elastic properties - like a stretched a rubber band (in inspiration), the chest cavity and lungs spontaneously contract (in expiration) once the force that kept them expanded is removed.

In emphysema, however, there is damage to the elastic walls of the distal respiratory tree. As a result, elastic recoil is diminished. To exhale now requires us to actively increase the pleural pressure. This does increase the alveolar pressure as before, but also tends to narrow the intrathoracic airways. The two cancel each other out to some degree, and so expiration is only partially effective. More air than usual is trapped within the lung. It can't be expelled, because increasing the pleural pressure past this point is counterbalanced by narrower airways.

This is why their chest diameter increases - there's more air in their lungs. (Note that this extra air doesn't help them: any air that is not fully exhaled can't collect oxygen, and is of no use.)

Lastly, emphysema usually coexists to some degree with chronic bronchitis. In this condition, the airways are inflamed, swollen and loaded with mucus. All of this increases the difficulty that the patient has exhaling, resulting in further 'air trapping' as above.