Without even noticing, it is something you normally do about twelve to eighteen times per minute: breathing. People at rest breathe about five to eight liters of air in and out every minute. When exercising heavily this can increase to a staggering hundred liters of air per minute, or even more! Breathing actually is a very complex process. So, what exactly happens with the air you inhale? And why do you experience shortness of breath with an illness like idiopathic lung disease (ILD)?
So let’s take a look at air, figuratively speaking. The air we breathe is made of a mixture of mostly nitrogen (about 78%), oxygen, a little bit of carbon dioxide and some other gasses. Nitrogen is an inactive gas and has almost no influence on our breathing. However, we do use the oxygen and produce carbon dioxide. The air we inhale has 21% oxygen and 0.03% carbon dioxide. In the air we exhale this composition has changed to about 16% oxygen and 5.6% carbon dioxide. This means our body uses about 5% of the oxygen from the air and produces carbon dioxide which is excreted. But how does this work?
When you inhale, the air flows from your nose or mouth through the trachea, bronchi and bronchioles until it ends up in the alveoli. The alveoli look like little grape-like structures and are made up of tiny air sacs. And there are many of these air sacs to be found in the lungs: we have hundreds of millions of them and their total surface area is about fifty to a hundred square meters. That’s as big as half a tennis court! Within the walls of the alveoli is tissue called the interstitium. Surrounding the alveoli are tiny blood vessels, called the capillaries. They form a kind of net around each air sac. The oxygen in the air you breathe passes through the alveoli walls and the interstitium into these tiny blood vessels (see Figure 2). Once in your blood, the oxygen is picked up by red blood cells and transported to all parts of our body where it is used by our cells. Upon consuming the oxygen, cells produce carbon dioxide. This waste product travels in the opposite direction from oxygen: when released into the blood stream, it is brought to the lungs where it passes from the capillary blood into the air sacs and, finally, is exhaled.
The concentration of carbon dioxide in the alveoli is an important trigger for respiration. When this concentration reaches a certain level, a signal is sent to your brain. In response, your brain sends a signal to your diaphragm and the muscles in your chest, causing you to inhale. Inhaling is an active process. This means your muscles are working actively to make your chest cavity bigger. When inhaling calmly, the diaphragm is tightened, which causes the chest cavity (and lungs) to expand. As a result, air flows into your lungs, see Figure 3.
This has to do with the air pressure which becomes lower inside your lungs upon expanding, which causes air to flow in. When breathing heavily, for example when exercising but also when breathing is difficult for you, several muscles around the ribcage work together with the diaphragm to expand your chest cavity even further.
Exhaling, in contrast, is a passive process. When you exhale calmly, the elasticity of your lungs ensures your lungs and chest cavity ‘bounce’ back and regain their original size. You can think of a rubber band that you pull: upon releasing the rubber band, the elastic fibers enable the band to take its original shape and size. This way, air is pushed out of your lungs. When you exhale heavily and forcefully, certain muscles around your ribcage help to even further reduce the size of your chest cavity.
So what goes wrong in your lungs in ILD? In ILD the interstitium of the alveoli has scarred and thickened. This makes it difficult for oxygen to pass into the capillaries. The scarring also makes the lungs lose their elasticity and become stiff and difficult to inflate when inhaling. Literally, stiff lungs hold less air than normal, healthy lungs. Because of this stiffness, they have less capacity to fill up. This low lung capacity, together with the difficulty of oxygen passing through the scarred and thickened interstitium, causes shortness of breath. To make things worse, the amount of scarring can increase over time. This means the lungs get even stiffer, which further lowers the filling capacity of your lungs and further decreases the amount of oxygen that can reach the blood. How fast this all happens varies per person.1
You can probably imagine that when your lungs are stiff and cannot reach their filling capacity, this makes you feel short of breath. And that when the oxygen has difficulty reaching your bloodstream, this makes you want to breathe even harder. This breathlessness, due to the lower levels of oxygen in your blood, is caused by an involuntary response of your brain. It reacts to the increasing carbon dioxide levels and decreasing oxygen levels. Your nerves keep on signaling your lungs to work harder, but because of ILD, your lungs are not able to comply. As a patient, you are aware of this imbalance between what is asked of your lungs, while the effort they are making is not enough. This increases your sensation of unsatisfied inspiration.2 On top of that, because it is more difficult for you to breathe, the breathing itself causes you to use up more oxygen, even at rest. All this causes your breathing to become more rapid and shallow and this gets even worse when exercising.
Unfortunately, there is no effective treatment for breathlessness in ILD. However, you might benefit from patient support groups or pulmonary rehabilitation programs. These can teach you how to improve your energy level, reduce the shortness of breath and can give you every day practical advice on how to keep your oxygen levels as high as possible. This may have a positive effect on your well-being and in the end, make you feel more in control of your condition.3