Asthma Pathophysiology


What is Asthma

Asthma is a chronic condition caused by an inflammatory response to an irritatnt, resulting in bronchospasm.

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Inflammatory Response

Antigen exposure triggers activation of antigen presenting cells.

Macrophages engulf and eliminate antigens.

Th1 cells promote inflammation contributing to bronchoconstriction and activate lympocytes.

Th2 cells are not usually found in the lungs and cause inflammatory response.  They activate IgE and eosinophils.  They are also needed to proliferation and differentiation of B cells. 

Eosinophils release neuropeptides which cause tissue injury and hyperresponsiveness.

IgE cells bind to an antigen and then bind to mast cells causing the mast cells to release histamine, which causes inflammation, bronchoconstiction and airway oedema.

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Smooth muscles of the bronchiolles swell, causing lumen to narrow, reducing airflow to the lungs.

This increases air resistance and decreases flow rate, trapping air and resulting in decreased diffusion of the alveoli.

This causes a VQ mismatch, resulting in low saturations as oxygen exchange is impaired.

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Bronchiolles produce excess mucous in attempt to trap the antigen.

This causes increased secretions and can result in mucous plugs.

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Respiratory Acidosis

VQ mismatch and low saturations result in hypoexemia, causing hyperventilation.

This causes pH and O2 to decrease and CO2 to increase, leading to respiratory acidosis

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Lumen narrowing and mucous plugs causes airway obstruction.

This causes the characteristic 'wheeze' sound as the air is inspired and expired.

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Heart Rate

Heart rate increases to compensate for lack of oxygen in the blood.

Increasing heart rate aids in oxygenation and waste removal.

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Hypoxia and AVPU

Hypoxia caused by lack of oxygen in the blood causes cyanosis which can present as blue tinged lips.

This can alter neurological status, resulting in a score of V on AVPU.

Low oxygen in the blood can also cause a low blood glucose level.  This can further alter AVPU as glucose is not being transported to the brain.

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Blood Shunting

The body begins to compensate due to hypoxia as there is desaturated haemoglobin in the blood.  This causes cyanosis.

Blood beings shunting centrally which causes increased CRT and cold peripheries.

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Respiratory Rate and Work of Breath

Lungs are unable to hold as much air due to inflammation and mucous build up.   Work of breathing increases.

Receptors in the hypothalamus pick up on low oxygen and increased carbon dioxide levels and increase respiratory rate to clear the carbon dioxide.

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Airway Remodelling

Epithelial tissue peels away due to injury, resulting in mucous not being moved from the airway due to lack of cilliated cells.

Smooth muscle in the bronchiolles becomes thickened, causing the lumen to be narrower in its relaxed state.

This makes future attacks more severe.

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