There are many causes of shortness of breath but only a few reasons. The causes have to do with the diseases or conditions associated with it. The reasons for shortness of breath relate to the mechanisms that produce it.
What is dyspnea
Dyspnea for the most part is a synonym for shortness of breath or a sensation of breathlessness. But it also applies to other descriptions. They include an unpleasant urge to breathe or the sensation of an inability to take a deep breath. It is not clear whether the varied descriptions are truly different or if they are all just one sensation expressed differently.
The main factors involved are the following:
- Increased work of breathing
- Hypoxia – low oxygen in tissues
- Hypercapnia – high carbon dioxide in tissues
- Reduced oxygen carrying capacity
- Low pH
When disease is the cause it might be due to factors within or outside of the lungs. Factors within the lungs that increase the work of breathing do so by blocking air flow through airways or by reducing the lungs’ compliance. Lung compliance is the ability of the lungs to expand or stretch during inward breathing. It is a type of elasticity. Factors outside of the lungs that increase the work of breathing do so by causing a decrease in the lungs’ compliance.
The mechanism is not well-understood. But according to past research the sensation of dyspnea caused by increased work of breathing involves the transmission of signals to sensory sites in the brain. The signals originate in mechanical receptors. Those receptors are within the lungs and the external structures of breathing. The external structures are breathing muscles and tendons of the chest wall as well as the diaphragm. The signals travel through afferent nerves from their sites of origin to areas of the brain where the experience of dyspnea occurs.
Chemoreceptors are involved in producing dyspnea in response to low amounts of oxygen (O2) and high levels of carbon dioxide (CO2) in the body. It appears that they do it in two ways. One way is by stimulating the breathing center in the brain in response to low O2 or high CO2. This in turn leads to an increase in the work of breathing. The other way is via an unknown mechanism, possibly involving signaling to the cerebral cortex and other advanced regions of the brain involved in the perception of low O2 and high CO2.
Low O2 that causes dyspnea can result from reduced ventilation, oxygenation, or O2 pressure of inspired air. The most common cause of the latter is high altitudes well above sea level. High CO2 is the result of reduced ventilation.
Low pH or an increased acidity of blood also correlates with dyspnea. This is so whether the disorder is respiratory or metabolic acidosis. Diabetic ketoacidosis is but one example of the latter. The reasons low pH seems to cause shortness of breath are possibly the same or similar to those for low O2 and high CO2.
The amount of O2 in the body can be low even when heart and lung function are normal if the oxygen carrying capacity of blood is low. The most common reason for the latter is anemia, regardless of the cause. It is often-overlooked as one of the reasons for shortness of breath.