Study objective: To determine whether the plateau phase of the expiratory capnogram (dCO₂/dt) can detect bronchospasm in adult asthma patients in the emergency department and to assess the correlation between dCO₂/dt and the peak expiratory flow rate (PEFR) in spontaneously breathing patients with asthma and in normal, healthy volunteers. Methods: We carried out a prospective, blinded study in a university hospital ED. Twenty adults (12 women) with acute asthma and 28 normal adult volunteers (15 women) breathed through the sampling probe of an end-tidal CO₂ monitor, and the expired CO₂ waveform was recorded. The dCO₂/dt of the plateau (alveolar) phase for five consecutive regular expirations was measured and a mean value calculated for each patient. The best of three PEFRs was determined. The PEFR and dCO₂/dt were also recorded after treatment of the asthmatic patients with inhaled β-agonists. Results: The mean±SD PEFR of the asthmatic subjects was 274±96 L/minute (57% of the predicted value), whereas that of the normal volunteers was 527±96 L/minute (103% of the predicted value) (P<.001). The mean dCO₂/dt of the asthmatic subjects (.26±.06) was significantly steeper than that of the normal volunteers (.13±.06) (P<.001). The dCO₂/dt was correlated with PEFR (r=.84, P<.001). In 18 asthmatic subjects the pretreatment and posttreatment percent predicted PEFRs were 58%±17% and 74%±17%, respectively (P<.001), whereas the dCO₂/dt values were .27±.05 and .19±.07, respectively (P<.005). Conclusion: The dCO₂/dt is an effort-independent, rapid, noninvasive measure that indicates significant bronchospasm in ED adult patients with asthma. The dCO₂/dt value is correlated with PEFR, an effort-dependent measure of airway obstruction. The change in dCO₂/dt with inhaled β-agonists may be useful in monitoring the therapy of acute asthma.

[Yaron M, Padyk P, Hutsinpiller M, Cairns CB: Utility of the expiratory capnogram in the assessment of bronchospasm. Ann Emerg Med October 1996;28:403-407.