Arterial blood gas analysis or oxygen saturation in the assessment of acute asthma?

A study was undertaken to determine if arterial blood gas estimation is always necessary in the assessment of patients presenting to hospital with acute severe asthma, or whether oxygen saturation as measured by pulse oximetry is a reliable screening test for predicting those in respiratory failure.

A prospective study was conducted in a specialist respiratory medical unit. Arterial blood gas tensions and pulse oximetry were measured in 89 consecutive patients admitted with acute severe asthma. Respiratory failure was defined as PaO2 < 8.0 kPa or PaCO2 > 6 kPa.

When oxygen saturation was 92% or higher (72 patients) respiratory failure was found in three (4.2%) cases. In the 82 patients with a saturation of 90% or higher six patients (7.3%) had respiratory failure.

In the initial assessment of acute severe asthma an oxygen saturation of > 92% suggests that respiratory failure is unlikely and therefore arterial blood gas measurement is unnecessary. This study is only relevant to the assessment of asthmatic patients at presentation. Other parameters of severity must be continually assessed in all asthmatic patients admitted to hospital irrespective of the initial SaO2, and blood gases measured when clinically indicated.