Asthma: acute exacerbation: quiet chest, too dyspnoeic to talk or cyanosed increased the risk of hypercapnia.

Clinical bottom line (level 4)

  1. Around a quarter of patients with acute asthma had hypercapnia.
  2. Patients who had a quiet chest, were too dyspnoeic to talk, cyanosed or required supplemental oxygen, were more likely to be hypercapnic.
  3. Patients who were not using their accessory muscles of respiration, on beta-agonists, or who had visited the emergency department within the last two weeks were less likely to have hypercapnia.
Mountain and Sahn: American Review of Respiratory Diseases 1988; 138: 535-539
Expires November 2002

The study

Setting: university hospital, USA

170 patients (aged mean 36 years, 72% female) admitted 229 times with acute asthma

Excluded if
  • aged <18 years
  • features of COPD (heavy smoking history, cough and sputum production, chronic dyspnoea on exertion)
  • pneumonia, heart disease, insulin-dependent diabetes mellitus, renal failure
  • no arterial blood gas measurement


    • All patients had beta-agonists (inhaled or subcutaneously), iv aminophylline and most had iv steroids.
    Independent unblinded reference standard, applied in some patients from a consecutive ?appropriate spectrum.
    Reference standard:
    • arterial blood gas within one hour of presentation
    Diagnostic test: clinical signs and symptoms

    The evidence

    pre-test probability of hypercapnia: 27%, (95% CI: 21% to 32%)

    diagnostic test hypercapnia no hypercapnia LR+
    (95% CI)    		 post-test probability LR-
    (95% CI)    		 post-test probability
    quiet chest 19 4 13
    (4.6 to 37)     		83% 0.71
    (0.59 to 0.84)     		20%
    too dyspnoeic to talk 24 11 6.0
    (3.1 to 12)     		69% 0.65
    (0.53 to 0.80)     		19%
    cyanosis 13 7 5.1
    (2.1 to 12)     		65% 0.82
    (0.72 to 0.94)     		23%
    requiring supplemental oxygen 28 15 5.1
    (3.0 to 9.0)     		65% 0.59
    (0.47 to 0.75)     		18%
    RV strain on ECG 16 9 4.9
    (2.3 to 10)     		64% 0.78
    (0.67 to 0.91)     		22%
    hyperinflation on chest x-ray 39 50 2.2
    (1.6 to 2.9)     		44% 0.51
    (0.36 to 0.73)     		16%
    on oral theophylline 55 148 1.0
    (0.93 to 1.1)     		27% 0.83
    (0.35 to 2.0)     		23%
    on corticosteroids 29 54 1.5
    (1.1 to 2.1)     		35% 0.77
    (0.60 to 1.0)     		22%
    on beta-agonists 56 125 1.2
    (1.0 to 1.3)     		31% 0.39
    (0.16 to 0.96)     		13%
    use of accessory muscles 60 142 1.2
    (1.1 to 1.3)     		30% 0.11
    (0.015 to 0.76)     		4%
    previous visit to emergency department within two weeks 5 38 0.36
    (0.15 to 0.88)     		12% 1.2
    (1.1 to 1.3)     		30%
    total 61 168

    • There was no significant difference in the length of stay for patients with hypercapnia or without (~4 days).
    • 5 subjects with hypercapnia required intubation and ventilation, no subjects with normocapnia did. This equates to an 8% chance of intubation in subjects with acute asthma and hypercapnia (95% confidence intervals, 1-15%, p<0.001)

    Comments

    1. These findings suggest that hypercapnia is associated with more severe clinical asthma.

    Citation

    1. Mountain RD, and Sahn SA: Clinical features and outcome in patients with acute asthma presenting with hypercapnia. American Review of Respiratory Diseases 1988; 138: 535-539
    Search Terms: reference from review articles
    Contributor: Chris Ball and Clare Wotton, June 2000
    Reviewer: Peter Wark

    Clinical Question.
    Patient acute asthma
    Intervention or Exposure clinical signs and symptoms
    Comparison arterial blood gases
    Outcome diagnosis of hypercapnia