Bronchoconstriction

Bronchoconstriction is the constriction of the airways in the lungs due to the tightening of surrounding smooth muscle, with consequent coughing, wheezing, and shortness of breath. Bronchoconstriction can also be due to an accumulation of thick mucus.

Physiology
Bronchoconstriction is due to an activation of the parasympathetic nervous system. Postganglionic parasympathetic fibers will release acetylcholine next to the Reissessen muscle, a smooth muscle layer surrounding the bronchi. These smooth muscle cells have M3 type muscarinic receptors on their membrane. The activation of these receptors by ACh will activate an intracellular Gq protein, that in turn will activate the PLC pathway, that will end in an increase of intracellular calcium concentrations and therefore contraction of the smooth muscle cell. The muscle contraction will cause the diameter of the bronchus to decrease, therefore increasing its resistance.

Causes
The condition has a number of causes, the most common being emphysema as well as asthma. Exercise and allergies can bring on the symptoms in an otherwise non-symptomic individual.

Emphysema
With emphysema the shortness of breath due to effective bronchoconstriction from excessive very thick mucous blockage (it is so thick that great difficulty is encountered in expelling it resulting in near exhaustion at times) can bring on panic attacks unless the individual expects this and has effectively learned pursed lip breathing to more quickly transfer oxygen to the blood via the damaged alveoli resulting from the disease. The most common cause of emphysema is smoking and smoking cessation is mandatory if this incurable disease is to be treated. Prevention of bronchoconstriction by this pathway is vital for emphysema sufferers and there are several anticholinergic medications that can greatly improve the quality of life for these individuals. In combination with mucous thinning agents such as Guaifenesin significant improvement in breathing can be accomplished.

Exercise-induced bronchoconstriction
More generally termed exercise-induced asthma, the preferred and more accurate term exercise-induced bronchoconstriction better reflects underlying pathophysiology. It is also preferred due to the former term giving the false impression that asthma is caused by exercise.

In a patient with EIB, exercise initially follows the normal patterns of bronchodilation. However, by three minutes, the constriction sets in, which peaks at around 10-15 minutes, and usually resolves itself by an hour. During an episode of this type of bronchorestriction, the levels of inflammatory mediators, particularly leukotrienes, histamine, and interleukin, increase. TH2-type lymphocytes are activated, with an increase in T cells expressing CD25 (IL-2R), and B cells expressing CD 23, causing increased production of IgE. After exercise, the conditions will fade within one to three minutes. In most sufferers of EIB, this is followed by a refractory period, of generally less than four hours, during which if exercise is repeated, the bronchorestriction is less emphasised. This is probably caused by the release of prostaglandins.

The underlying cause of this type of bronchoconstriction appear to be the large volume of cool, dry air inhaled during strenuous exercise. The condition appears to improve when the air inhaled is more fully humidified and closer to body temperature.

This specific condition, in the general population, can vary between 7 and 20 percent. This increases to around 80 percent in those with symptomatic asthma. In many cases, however, the constriction, even during or after strenuous exercise, is not clinically significant except in cases of severe to moderate emphysema.

Allergen-induced bronchoconstriction
While a different cause, this has very similar symptoms, namely the immunological reaction involving release of inflammatory mediators.

Inhalation of allergens in sensitized subjects develops into bronchorestriction within 10 minutes, reaches a maximum within 30 minutes, and usually resolves itself within one to three hours. In some subjects, the constriction does not return to normal, and recurs after three to four hours, which may last up to a day or more. The first is named the early asthmatic response, and the latter the late asthmatic response.

Bronchioconstriction can occur as a result of anaphylaxis, even when the allergen is not inhaled.