Pulse oximetry and PEFR aside, no other investigations are mandatory for patients with acute asthma exacerbations. However, adjuncts to clinical assessment become progressively more important as the severity of the episode increase.

Arterial blood gas

The British Thoracic Society guidelines recommend that ABGs should only be performed for patients with oxygen saturations below 92% and those with other life threatening features of asthma. Oxygen saturations below 92% are associated with an increased risk of hypercapnia.

Blood tests

Haematology and biochemical results will not be available in the first hour of an exacerbation and cannot, therefore, guide initial therapy. The white blood cell count may be elevated if the patient is on steroids or has intercurrent infection and hypokalaemia may be caused by β2-agonist treatment (beta-agonists decrease potassium by an average of 0.4 mmol/l). Although hypokalaemia rarely produces any clinically significant effects it is best corrected if marked or associated with ECG changes. For patients treated with oral theophyllines who are admitted with severe asthma, a serum theophylline level taken in the ED may identify toxicity or reveal non-compliance.

Chest radiograph

Chest radiographs should not be routinely requested for patients with acute asthma exacerbations but are recommended in the following settings:

  • Suspected pneumothorax, pneumomediastinum, pneumonia
  • Life-threatening features
  • Failure to respond to standard treatment
  • Patients requiring ventilation

An ECG should be performed if patients have severe symptoms, an irregular pulse or persistent tachycardia despite treatment. Sinus tachycardias and evidence of right heart strain are common. Rarely, the ECG may show changes suggestive of hypokalaemia (flat T waves, ST depression, long QT, U wave) which if confirmed should be corrected.

Point of Care Ultrasound

Point of care ultrasound undertaken by adequately trained staff may facilitate the diagnosis of pneumothorax.

  • The presence of lung sliding in both blunt trauma and critically ill ICU patients can rule out the presence of a pneumothorax
  • The absence of lung sliding alone is not specific enough to rule in a pneumothorax in critically ill ICU patients
  • Presence of a lung point is 100% specific and 100% predictive of PTX when seen, but may not be seen in large pneumothoraces