Asthma: exacerbation: normal oxygen saturation does not exclude respiratory failure

Clinical bottom line (level 4)

  1. Around 10% of patients with acute severe asthma have respiratory failure
  2. Patients with an oxygen saturation < 92% are more likely to have respiratory failure.
  3. Patients with an oxygen saturation > 92% are probably less likely to have respiratory failure, but this cannot be safely excluded.
Carruthers and Harrison: Thorax 1995; 50 : 186-188
Expires November 2003

The study

Setting: respiratory unit, acute hospital, UK

89 patients (aged 13 to 79, ?% male) with acute severe asthma

Independent unblinded reference standard, applied in all patients from a consecutive appropriate spectrum.
Reference standard:
  • arterial blood gas - respiratory failure diagnosed if pO2 < 8 kPa and/ or pCO2 > 6 kPa
Diagnostic test: pulse oximetry: abnormal if SaO2 < 92%

The evidence

pre-test probability of respiratory failure: 9.0%, (95% CI: 3.0% to 15%)

diagnostic test respiratory failure no respiratory failure LR+
(95% CI)		 post-test probability LR-
(95% CI)		 post-test probability
SaO2 92% or lower 5 12 4.2
(2.0 to 8.9) 		29% 0.44
(0.18 to 1.1) 		4%
total 8 81

Comments

  1. No attempt was made to measure blood gases on air or standard oxygen concentrations (79% taken on room air).
  2. Acute severe asthma defined according to British Thoracic Society guidelines (PEFR < 50%, pulse > 100, RR > 25, unable to complete full sentences in one breath)

Citation

  1. Carruthers DM, and Harrison BD: arterial blood gas analysis or oxygen saturation in the assessment of acute asthma?. Thorax 1995; 50 : 186-188
Search Terms: acute asthma in Cochrane
Contributor: Chris Ball, November 2001
Reviewer:

Clinical Question.
Patient acute asthma
Intervention or Exposure pulse oximetry
Outcome respiratory failure