Asthma is a Type I Hypersensitivity condition whereby allergens induce hyper-inflammatory cascades resulting in bronchospasm and mucous hypersecretion, presenting with wheeze, breathlessness, and productive cough. In more severe presentations, patients can present with reduced breathing or breath sounds (silent chest), cyanosis, reduced consciousness or collapse.

Peak expiratory flow rate (PEFR) is the volume of air forcefully expelled in one quick exhalation breath. It tells us about the adequacy of ventilation, and any airflow obstruction. We measure this by using a peak flow meter, the units of measurement is litres per min (L/min).

Classification by severity (British Thoracic Society 2019)