Asthma: acute exacerbation: FEV 1 did not predict abnormal blood gases accurately.

Clinical bottom line (level 4)

  1. Two thirds of patients with acute asthma exacerbations were likely to have an abnormal pCO 2 . Hypercapnia was far less common, and was very unlikely if FEV 1 >30% predicted.
  2. Patients with FEV 1 <20% predicted were more likely to have an abnormal pCO 2 . Higher FEV 1 values could not safely exclude abnormal pCO 2 levels.
  3. Blood gas levels of many patients improved following treatment. However, improvement in FEV 1 did not predict improvement in blood gas levels.
Nowak et al: Journal of the American Medical Association 1983; 249 (15): 2043-2046
Expires November 2002

The study

Setting: university hospital, USA

86 patients (aged mean 25 years, 63% female) acute exacerbation of asthma (PEFR 30% to 40% predicted)

Excluded if
  • <16 or >40 years old
  • other cardiac or other lung disease


    • All patients had terbutaline 0.25 mg subcutaneously and iv aminophylline 5-6 mg/kg loading dose, followed by 0.9 mg/kg/hour.
    Independent unblinded reference standard, applied in all patients from a non-consecutive appropriate spectrum.
    Reference standard:
    • arterial blood gas:
      • pCO 2 :positive if more than 33 mmHg (4.4 kPa)
      • change in pO 2 following therapy
      • change in pCO 2 following therapy
    Diagnostic test:
    • FEV 1 -best of three
    • change in FEV 1 following therapy

    The evidence

    pre-test probability of normal or elevated pCO 2: 64%, (95% CI: 54% to 74%)
    pre-test probability of hypercapnia: 14%, (95% CI: 7.4% to 21%)
    pre-test probability of improvement in pO2 following therapy: 77%, (95% CI: 67% to 86%)
    pre-test probability of improvement in pCO2 following therapy: 67%, (95% CI: 57% to 77%)

    diagnostic test normal or elevated pCO 2 low pCO 2 LR
    (95% CI)    		 post-test probability
    predicted FEV1 <20% 36 6 3.4
    (1.6 to 7.2)     		86%
    predicted FVE1 20-30% 12 7 0.97
    (0.42 to 2.2)     		63%
    predicted FEV1 >30% 14 22 0.36
    (0.21 to 0.61)     		39%
    total 62 35


    diagnostic test normal or elevated pCO 2 low pCO2 LR
    (95% CI)    		 post-test probability
    predicted FEV1 <20% 12 30 2.4
    (1.7 to 3.4)     		29%
    predicted FVE1 20-30% 2 17 0.70
    (0.18 to 2.7)     		11%
    predicted FEV1 >30% 0 36 0.0
    (0.0 to 0.44)     		0%
    total 14 83


    diagnostic test rise in pO 2 no change or fall LR+
    (95% CI)    		 post-test probability LR-
    (95% CI)    		 post-test probability
    rise in FEV1 52 15 1.1
    (0.89 to 1.3)     		78% 0.46
    (0.084 to 2.6)     		60%
    total 55 17


    diagnostic test rise in PCO2 no change or fall LR+
    (95% CI)    		 post-test probability LR-
    (95% CI)    		 post-test probability
    rise in FEV1 48 23 1.0
    (0.89 to 1.2)     		68% 0.74
    (0.13 to 4.1)     		60%
    total 51 25

    • No patient with a FEV1 >30%, or PEFR <200 l/min was hypercapnic.

    Comments

    1. Patients with asthma exacerbations are usually tachypnoeic, and consequently would be expected to have a low pCO2. A pCO2 within normal limits indicates some degree of respiratory failure.
    2. The study suggests that arterial blood gases need to be taken in the majority of patients with asthma exacerbations, and should be repeated to monitor improvement.
    3. No correlation between subsequent recovery and initial arterial blood gas measurement was found. However, follow-up was short (48 hours) and no adjustment for confounding factors were made, so this result is uncertain.

    Citation

    1. Nowak RM, Tomlanovich MC, Sarker DD, et al: Arterial blood gases and pulmonary function testing in acute bronchial asthma: predicting patient outcomes. Journal of the American Medical Association 1983; 249 (15): 2043-2046
    Search Terms: reference from asthma chapter: Lee, Hsu, Stasior; Quick Consult Manual to Evidence-Based Medicine: publ Lippincott-Raven, 1997
    Contributor: Chris Ball and Clare Wotton, June 2000
    Reviewer: Mitsuhiro Kamei

    Clinical Question.
    Patient asthma
    Intervention or Exposure FEV1
    Outcome diagnosis of abnormal pCO2