Acute Exacerbation of Asthma
Introduction
Asthma is characterised by episodes of reversible airflow obstruction caused by bronchospasm, increased mucous production and mucosal inflammation of the small airways. The result is collapse of small airways, particularly on expiration causing increased intrathoracic pressure and eventual cardiopulmonary collapse if severe and untreated.
Precipitants of an acute exacerbation of asthma include exercise, cold air, allergens, smoking, NSAIDs or Beta-Blockers.
Most patients who are diagnosed with asthma are prescribed a ‘preventer’ inhaler as well as a ‘rescue’ inhaler. Typically, preventers contain either a Long Acting Beta Agonist, Long Acting Muscarinic Antagonist, Inhaled Corticosteroid or a combination of either. Relievers are typically Short Acting Beta Agonists or Short Acting Muscarinic Antagonists. The long-term management and prevention of acute asthma exacerbations is beyond the scope of this article.
Peak Expiratory Flow Rate
Peak Expiratory Flow Rate (PEFR) measurement is a useful tool to quantify the degree of airflow obstruction in an acute exacerbation and is a relatively quick, simple and cheap test performed at the bedside. A video describing the technique of PEFR measurement can be found below in Further Reading.(1)
PEFR is measured and then compared with either the patient’s known personal best PEFR when well or with a pre-determined value based on the patient’s age and height (see graph below). The result is a % of what PEFR they can achieve now versus what they should be capable of achieving either when well or based on a population mean value for their age, sex and height. This % value is important when classifying acute severity of disease and guiding management.
Clinical Features & Classification Severity
Typically, an acute exacerbation of asthma presents with a relatively rapid onset of shortness of breath, difficulty completing sentences, cough and wheeze. Patients may be tachypnoeic, tachycardic and hypoxic.
The severity of an exacerbation is graded as Mild, Moderate, Severe and Life Threatening. This classification is largely based on clinical, rather than biochemical or radiological parameters and is broken down below.(2),(3)
Indicators of Life Threatening Asthma on Arterial Blood Gas sampling include:
PaO2 <8.0kPa
PaCO2 normal
With tachypnoea, CO2 is being expired much more quickly and as such should be low on an ABG. A normal or raised PaCO2 indicates the patient is tiring and moving less air.
An exacerbation of asthma that requires mechanical ventilation or presents with a raised PaCO2 is defined as near fatal.
Differential Diagnosis
The differential diagnosis for acute severe asthma includes;
Acute Exacerbation of COPD
Pulmonary Embolism
Pneumothorax
Infection/Pneumonia
Clinical Investigations
Bedside Investigations
o PEFR
Apart from classifying the severity of Asthma as shown above the PEFR is also useful to monitor response to emergency treatment in the ED
o ABG
In particular looking for Hypoxia (PaO2 <10kPa), Normocapnoea/Hypercapnoea (PaCO2 >4.6kPa) and Acidosis (pH <7.35).
o ECG
Most likely finding is Sinus Tachycardia. Remember that Tachycardia is also a side effect of Salbutamol.
Laboratory
Full Blood Count, Urea & Electrolytes, CRP, D-Dimer (if PE considered and if test indicated)
Not useful for immediate management of acute severe asthma, however necessary for further investigation/ruling out other causes of dyspnoea.
Radiology
Chest X-Ray
Useful to help rule out Pneumothorax or Pneumonia as a cause of the patient’s presentation.
Up to 75% of x-rays are normal in an acute exacerbation of asthma.(4)
May see hyperinflation
Greater spacing between ribs
Horizontal appearance of ribs
>10 posterior ribs visible above diaphragm
Management & Disposition
All Patients:
Assessment, Vital Signs, PEFR (if able) & Laboratory/Radiology Investigations as indicated
Severe to Life Threatening Exacerbations should be managed in Resus
All patients with severe and life-threatening exacerbations of asthma should be admitted for further treatment or observation. Depending on response to treatment, moderate exacerbations may be suitable for discharge with appropriate follow up in the community. Each presentation to ED with Acute Asthma represents an opportunity for education on prevention and inhaler technique. Consider discussing an Asthma Action Plan(5) with the patient if being discharged home.
Mechanical Ventilation in life threatening Asthma
This is reserved for patients who do not respond to other therapy described above or who have such poor respiratory effort that they need assisted ventilation. It should only be commenced with senior support +/- ICU input.(6)
References
1. PEFR Measurement and Explanation – GeekyMedics https://www.youtube.com/watch?v=jdA8KU_D9JU
2. Scottish Intercollegiate Guidelines Network, British Thoracic Society. SIGN158 – British Guideline on the management of asthma. Revised Edition July 2019. Available at https://www.sign.ac.uk/sign-158-british-guideline-on-the-management-of-asthma
3. HSE National Asthma Programme. Management of the Acute Adult Asthma Patient. 2012. https://irishthoracicsociety.com/wp-content/uploads/2017/05/Adult_Emergency_Asthma_Guideline_Final-v_1-0-new.doc
4. Asthma Radiopedia Reference Article https://radiopaedia.org/articles/asthma-1?lang=gb
5. Asthma Action Plan – Asthma Society of Ireland https://www.asthma.ie/document-bank/asthma-action-plan-0
6. Brenner B, Corbridge T, Kazzi A. Intubation and Mechanical Ventilation of the Asthmatic Patient in Respiratory Failure. Proc Am Thorac Soc . 2009. 6;371-379
This blog was written by Dr. James Condren and was last updated in October 2020