Definition

  • variable airflow obstruction and airway hyper responsiveness
    • variable means that the airflow obstruction is temporary and reversible
    • inflammation causes hyper responsiveness and this further cause more inflammation leading into a viscous cycle.

Cause

  • inflammation of the airways due to hypersensitivity reaction which further obstruct the airways during expiration.

Epidemiology

  • 4th May = World Asthma Day
  • 300 million people (2004)
  • the prevalence of asthma is increasing throughout the world, and the rate appears to increase as communities adopt western lifestyles and become urbanized

General Risk Factors

  • parents with asthma
  • respiratory infection during childhood
  • low birth weight (due to smaller airway)
  • obesity (fat under diaphragm which limits diaphragm contraction)

Explaination for exercise (Exercise Induce Asthma)

  1. Exercise induce asthma; the cause is unclear
    1. Exercise increases oxygen demand. Usually a person will breathe through their mouth at a higher rate (normally, nasal breathing will humidify and heat the air as alveoli needs to be protected from cold, dry air). Mouth breathing will then lead to lost of heat and water from the tracheobranchial tree. Rapid breathing during exercise also reduces the humidity of the airways. Cold, dry air has high tonicity. This will cause fluid movements from the surrounding capillary (which is hypotonic) to diffuse into the airways. This causes the airway to narrow.

· Under normal condition, parasympathetic (vagal) innervations will cause bronchoconstriction. On the other hand, sympathetic (B2 adrenergic receptor) increase bronchodilation. During exercise, the increase of air-flow is required thus inhibiting the bronchoconstriction and promotes bronchodilation. This is why asthmatic patient do not get breathlessness during exercise but complains of shortness of breath hours later (due to the gradual build up of fluid)

Explanation for cold air

Besides increasing tonicity, cold air also causes bronchospasm.

GERD

The explanation is unclear. Unable to link between GERD and asthma. Some possible theories;

  1. asthma triggers GERD
    1. bronchodilators can relax the LES (lower esophageal sphincter)
    2. small amount of the medication is swallowed when patient inhale the bronchodilators.
  2. GERD triggers asthma. There are two possible mechanism
    1. Acid reflux will cause injury to the lining of the throat, airways and lungs, making inhalation difficult and often causing a persistent cough. It may also cause the formation of a fistula.
    2. Acid reflux into the esophagus will stimulate a nerve reflex somehow which cause the the airway to constrict in order to prevent the acid from entering. This leads to shortness of breath.

Occupational asthma

  • A lung condition caused by inhaling workplace fumes, gases or dust
  • Can develop if you never had asthma before or had childhood asthma that later cleared. It can also worsen any pre-existing asthma

Cause

    1. Direct irritation of the allergens which induces pre-existing asthma.
    2. Allergic sensitization which arise from the same concept of pathophysiology of asthma.

Risk factors