Asthma: acute exacerbation: PEFR could not predict acidosis, hypoxia or hypercapnia.

Clinical bottom line (level 4)

  1. In patients with acute asthma exacerbation, peak expiratory flow rates could not safely exclude acidosis, hypoxia or hypercapnia.
Martin et al: Annals of Emergency Medicine 1982; 11 (2): 70-73
Expires November 2002

The study

Setting: emergency department, university hospital, USA

51 patients (aged mean 22 years, 61% female) with 89 episodes of acute asthma exacerbation (mean PEFR 26% +/- 15% predicted)

Excluded if
  • <15 or >39 years



    • Independent unblinded reference standard, applied in all patients from a consecutive appropriate spectrum.
    Reference standard:
    • arterial blood gases
    Diagnostic test: peak expiratory flow rate (PEFR)

    The evidence

    pre-test probability of acidosis: 18%, (95% CI: 10% to 26%)
    pre-test probability of hypoxia: 18%, (95% CI: 10% to 26%)
    pre-test probability of hypercapnia: 4.5%, (95% CI: 0.2% to 8.8%)

    diagnostic test pH =7.35 pH>7.35 LR+
    (95% CI)    		 post-test probability LR-
    (95% CI)    		 post-test probability
    PEFR <25% predicted 15 40 1.7
    (1.3 to 2.2)     		27% 0.14
    (0.02 to 0.95)     		3%
    total 16 72


    diagnostic test pO2 =8 kPa pO2 >8 kPa LR+
    (95% CI)    		 post-test probability LR-
    (95% CI)    		 post-test probability
    PEFR <25% predicted 12 42 1.3
    (0.92 to 1.8)     		22% 0.59
    (0.24 to 1.4)     		11%
    total 16 73


    diagnostic test pCO2 =6 kPa pCO2 <6 kPa LR+
    (95% CI)    		 post-test probability LR-
    (95% CI)    		 post-test probability
    PEFR <25% predicted 3 55 1.2
    (0.64 to 2.1)     		5% 0.71
    (0.13 to 4.0)     		3%
    total 4 85

    Comments

    1. This result is similar to a number of other studies, all of which suggest all patients with acute severe asthma attacks should have a blood gas sample taken.
    2. This was a young group of adult patients with asthma- results may not apply to the older patients with a degree of reversible airways disease.
    3. Multiple clinical features may be more helpful, especially combined with non-invasive testing (such as pulse oximetry) but this has not been studied.

    Citation

    1. Martin TG, Elenbaas RM, Pingleton SH, et al: Use of peak expiratory flow rates to eliminate unnecessary arterial blood gases in acute asthma. Annals of Emergency Medicine 1982; 11 (2): 70-73
    Search Terms: reference from British Thoracic Society Guidelines
    Contributor: Chris Ball and Clare Wotton, June 2000
    Reviewer: Mitsuhiro Kamei

    Clinical Question.
    Patient acute asthma
    Intervention or Exposure PEFR
    Comparison arterial blood gases
    Outcome diagnosis of acidosis, hypoxia and hypercapnia