Utility of the expiratory capnogram in the assessment of bronchospasm

Study objective: To determine whether the plateau phase of the expiratory capnogram (dco(2)/dt) can detect bronchospasm in adult asthma patients in the emergency department and to assess the correlation between dco(2)/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 dco(2)/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 dco(2)/dt were also recorded after treatment of the asthmatic patients with inhaled beta-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 dco(2)/dt of the asthmatic subjects (.26 +/- .06) was significantly steeper than that of the normal volunteers (.13 +/- .06) (P<.001). The dco(2)/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 dco(2)/dt values were .27 +/- .05 and .19 +/- .07, respectively (P<.005). Conclusion: The dco(2)/dt is an effort-independent, rapid, noninvasive measure that indicates significant bronchospasm in ED adult patients with asthma. The dco(2)/dt value is correlated with PEFR, an effort-dependent measure of airway obstruction. The change in dco(2)/dt with inhaled beta-agonists may be useful in monitoring the therapy of acute asthma.