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.[59]
Lack of response indicates either a severe exacerbation or an alternative diagnosis.
Result
variable clinical improvement depending on severity
peak expiratory flow or FEV₁
Test
Reproducible spirometry and peak expiratory flow (PEF) are difficult to assess reliably in young children, even when well, and therefore rarely used. However, this may be the only test available in primary care.
Studies have shown that during acute exacerbations reproducible measurements are also difficult in older children.[63]
Should be measured with the patient seated and the best value of three attempts taken.
Reduction in PEF, expressed as percentage of predicted value or personal best, reflects severity of the exacerbation. According to the Global Initiative for Asthma (GINA) guidelines, for children aged 6-11 years, a PEF >50% best or predicted corresponds to a mild or moderate exacerbation, and a PEF ≤50% best or predicted corresponds to a severe or life-threatening exacerbation.[7]
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 blood gases (ABGs) are reserved for life-threatening exacerbations to assess partial pressure of carbon dioxide (PaCO₂) retention/respiratory acidosis, which is a sign of impending respiratory failure.[7]
Free-flowing venous blood gases, taken at the time of intravenous cannulation, provide an approximation of arterial PaCO₂.[61]
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.
Result
low PaCO₂ (<35 mmHg); rise with impending exhaustion
chest x-ray
Test
Not indicated routinely in acute exacerbations of asthma.
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.
Result
normal or variably abnormal
fractional exhaled nitric oxide (FeNO)
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