Investigations
1st investigations to order
arterial blood gas (in hospital)
Test
If the patient has any features indicating a life-threatening asthma exacerbation (including SpO2 <92% on pulse oximetry), measure arterial blood gas. No other investigations are immediately required prior to starting management.[13]
Also perform an arterial blood gas if:[13]
PEF <50% at 15-20 minutes of treatment
There are signs of severe asthma or PEF <50% at 60 minutes of treatment
Repeat arterial blood gas within 60 minutes of starting treatment, if:[13]
initial partial pressure of oxygen (PaO2) <8 kPa (60 mmHg) unless subsequent SpO2 >92%, or
partial pressure of carbon dioxide (PaCO2) normal or raised, or
the patient deteriorates.
Result
the following on arterial blood gas indicate more severe asthma:[13]
‘normal’ or raised PaCO2 (>4.6 kPa [35 mmHg])
severe hypoxia (PaO2 <8 kPa [60 mmHg])
low pH (or high H+)
peak flow (in the community and in hospital)
Test
Measure PEF (before initiating treatment, if possible to do this without inappropriately delaying management) to help assess severity and direct decisions about management.[1][13]
Consider PEF in the context of other markers of severity. PEF alone does not determine the severity of an exacerbation. See Diagnosis recommendations.
Practical tip
Bear in mind that PEF depends on effort from the patient and may therefore vary in patients depending on the effort involved.
Measure PEF 15 to 30 minutes after starting treatment and continue to monitor PEF according to the patient’s response to treatment.[13] Repeat PEF measurements after bronchodilator therapy and at least four times throughout the patient’s hospital stay, if admitted.[13] Different peak-flow meters give different readings, so use the same (or a similar type of) meter where possible.
FEV1 is a valid measure of airway calibre, but PEF is considered more clinically useful and is more readily available in acute care.[13]
How to use a peak flow meter to obtain a peak expiratory flow measurement.
Result
life-threatening asthma: PEF <33% of best or predicted
acute severe asthma: PEF 33%-50% of best or predicted
moderate asthma: PEF >50%-75% of best or predicted
pulse oximetry (in the community and in hospital)
Test
Assess and record oxygen saturation on air using pulse oximetry.[13]
If the patient has any features indicating a life-threatening asthma exacerbation, measure arterial blood gas. No other investigations are immediately required prior to starting management.[13]
Pulse oximetry should be closely monitored during acute management.[1] Use pulse oximetry to determine the adequacy of oxygen therapy and the need for arterial blood gas measurement.[13]
Patients with SpO2 <92% (irrespective of whether the patient is on air or oxygen) or other features of life-threatening asthma require arterial blood gas measurement.[13]
Treat aggressively any patients with saturations <90%.[1]
Bear in mind that pulse oximetry will not detect hypercapnia (however, SpO2 <92% is associated with a risk of hypercapnia).[13] Also note that pulse oximetry may overestimate oxygen saturation in patients with dark skin colour.[1]
Do not delay supplemental oxygen when indicated if pulse oximetry is not readily available but commence monitoring as soon as available.[13]
Result
life-threatening asthma: SpO2 <92%
acute severe asthma: SpO2 ≥92%
moderate asthma: SpO2 ≥92%
chest x-ray (in hospital)
Test
Do not routinely request a chest x-ray as an investigation but consider if a complicating or alternative pulmonary pathology is suspected.[1][13]
Request a chest x-ray if there is:[13]
Suspected pneumomediastinum or pneumothorax
Suspected pneumonia/consolidation
Life-threatening asthma
Failure to respond to treatment satisfactorily
Requirement for ventilation.
In the emergency department, order a chest x-ray if there are signs of acute severe asthma or PEF <50% after 60 minutes.[13]
Result
often normal, even in a life-threatening exacerbation
Investigations to consider
full blood count (in hospital)
Test
Indicated in acute exacerbations when complicating factors (e.g., pneumonia) are suspected from history and examination. Assess for evidence of infection and/or eosinophilia.
Result
eosinophilia (0.3 × 109/L or greater)
raised WBC count could indicate evidence of infection
urea and electrolytes (in hospital)
Test
Assess for electrolyte abnormalities which may need correction.
Result
electrolyte abnormalities
in particular, look for deranged potassium, which often falls with beta-2-agonist use
C-reactive protein (in hospital)
Test
Assess for evidence of infection.
Result
elevated CRP suggests presence of infection
theophylline levels (in hospital)
Test
Check in a patient already taking oral theophylline.
Result
elevated if theophylline toxicity
ECG (in hospital)
Test
Check to rule out arrhythmia and as baseline in case aminophylline is needed.
Result
features of arrhythmia; baseline values
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