Investigations
1st investigations to order
response to treatment with a short-acting beta-2 agonist (SABA)
Test
Reversible bronchoconstriction is a hallmark feature of asthma, and the initial response to bronchodilator therapy can be a useful guide to the accuracy of asthma as the diagnosis when objective confirmation cannot be undertaken.[60]
Lack of response indicates either a severe exacerbation or an alternative diagnosis.
Result
variable clinical improvement depending on severity
peak expiratory flow (PEF)
Test
Recommended in older children and adolescents (e.g., aged 6-11 years), provided it does not unduly delay treatment.
Can be difficult to assess during exacerbations, even in older children, and may stay normal in many attacks.[62] If attempted, measurement should be performed with the patient seated and the best value of three attempts taken.
The degree of reduction in PEF, expressed as a percentage of the predicted value, reflects exacerbation severity. For children aged 6-11 years, a PEF >50% best or predicted indicates a mild or moderate exacerbation and a PEF ≤50% best or predicted indicates a severe or life-threatening exacerbation.[7]
Rarely used during the initial assessment or monitoring of acute asthma exacerbations in children (e.g., aged 5 years and younger).
Result
decreased
Spirometry (FEV₁)
Test
Only measure lung function in the acute setting if it will not unduly delay treatment.[7]
When used for children aged 6-11 years, an FEV₁ >50% best or predicted indicates a mild or moderate exacerbation and a FEV₁ ≤50% best or predicted indicates a severe or life-threatening exacerbation.
Can also be useful in identifying differentials when the diagnosis is not clear and may be used in older children with mild or moderate exacerbations to confirm the diagnosis before treatment.
Note that spirometry is rarely used in the acute setting, even for older children.
See Asthma in children.
Result
decreased
oxygen saturation
Test
Should be performed immediately.[7]
Pulse oximetry is a way of screening oxygenation non-invasively.
Significant hypoxaemia with an SpO₂ <90% is infrequent during asthma exacerbations and, if present, represents severe airflow limitation. Pulse oximetry may overestimate oxygen saturation in people with dark skin colour.[7]
Result
oxygen saturation values measured by pulse oximetry vary with degree of exacerbation: mild/moderate: 90% to 95% (on air); severe/life-threatening: <90% (on air)
Investigations to consider
arterial or venous blood gases
Test
Arterial, venous, or capillary blood gases are reserved for life-threatening exacerbations to assess the partial pressure of carbon dioxide (PaCO₂) and identify retention/respiratory acidosis, which is a sign of impending respiratory failure.[7]
Consider measuring ABGs if the peak expiratory flow or FEV₁ is <50% predicted, there is no response to treatment, or the patient is deteriorating.[7]
Free-flowing venous blood gases, taken at the time of intravenous cannulation, provide an approximation of arterial PaCO₂.[63]
PaCO₂ levels drop due to increased minute ventilation related to pronounced tachypnoea. PaCO₂ rises again as exhaustion sets in, into the normal range (35-45 mmHg); this is a sign of impending respiratory failure.
Not indicated routinely in acute exacerbations of asthma.
Result
low PaCO₂ (<35 mmHg); rise with impending exhaustion
chest x-ray
Test
May be used to exclude other diagnoses in patients presenting with their first episode of asthma, particularly if clinical features are atypical, and in patients with severe exacerbations who have had an atypical response to initial treatment.
Also used to assess focal signs on examination suggestive of pneumonia or pneumothorax.[7]
Findings include hyper-inflation, pneumothorax, atelectasis, pneumonia, or lobar collapse with consolidation.
Not indicated routinely in acute exacerbations of asthma.
Result
normal or variably abnormal
Emerging tests
fractional exhaled nitric oxide (FeNO)
Test
Where the equipment is available, the UK National Institute for Health and Care Excellence (NICE) recommends FeNO as part of the initial treatment and assessment of acutely unwell children, but this recommendation is not consistent across current international guidelines.[64]
Not indicated routinely in acute exacerbations of asthma. Not indicated routinely for assessing exacerbation severity or guiding acute asthma treatment.[7][56]
One randomised controlled trial concluded that using FeNO to guide asthma treatment did not reduce exacerbations among children.[65]
Result
May be elevated
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