Identifying Airway Disease
Capnogram shape in obstructive lung disease.
Anesth Analg. 2005 Mar;100(3):884-8
Krauss B, Deykin A, Lam A, Ryoo JJ, Hampton DR, Schmitt PW, Falk JL.
Division of Emergency Medicine, Children's Hospital-Boston, MA
262 people at a pulmonary laboratory received capnograms and pulmonary function tests(forced expiratory volume in 1 second (FEV1)). People with obstructive disease(OD) had capnograms that “were significantly different” from normal and Reactive Airway(RD) patients.
The authors write: “These differences were progressive, increasing with disease severity… Differences between OD capnograms and normal and RD capnograms, correlating to changes in FEV(1), were sufficiently large enough to suggest that the capnogram could be used to discriminate between OD and normal."
Patients were categorized as either Severe OD, moderate OD, RD or normal based on their FEV1 and FEV1/FVC scores.
• FEV1: Forced Expiratory Volume in 1 Second - This is the amount of air that you can forcibly blow out in one second, measured in liters. Along with FVC it is considered one of the primary indicators of lung function.
• FEV1 / FVC - This is the ratio of FEV 1 to FVC. In healthy adults this should be approximately 75 - 80%.
Here is an excellent description of what happens in the plateau phase of expiration:
“The plateau phase of the capnogram reflects the passage of air from progressive emptying of the aveoli. Normally alveoli are equally ventilated and all have similar CO2 concentrations, so the capnogram has a nearly constant value throughout this phase of exhalation. In OD, some terminal bronchi are narrowed, resulting in local hypoventilation of alveoli and an associated increase in their CO2 concentration. Alveoli attached to unobstructed terminal bronchi are relatively hyperventilated and have diminished C02 concentrations. During exhalation, unobstructed alveoli empty ahead of obstructed ones, leading to a progressive increase in CO2 concentration during exhalation. This uneven emptying of alveoli gives the alveolar plateau a characteristic sloped appearance in OD.
“In RD, the airflow in the earliest portion of exhalation approximates that of subjects with normal lung function. Thus, the initial expiratory phase, in the first 250ms, is unchanged. Subsequent flow rates are progressively reduced because of because of diminished lung volume, resulting in diminished FEV1 in RD. However because the terminal bronchi are unobstructed and homogeneous, the alveoli empty evenly, leading to a low alveolar slope and angle. Taken together, these effects result in capnograms that are similar to those obtained from normal subjects, despite severe decreases in FEV1 in the presence of RD.”
Bottom Line: A simple study to show that simply by looking at the wave form, it was possible to detect who had obstructive airway disease and who did not. This helps confirm the utility in using capnography in addition to clinical judgement in determining if obstructive airway disease is present.
Utility of the expiratory capnogram in the assessment of bronchospasm
Ann Emerg Med. 1996 Oct;28(4):403-7. Links
.Yaron M, Padyk P, Hutsinpiller M, Cairns CB.
Division of Emergency Medicine, University of Colorado Health Sciences Center, Denver, USA.
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 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 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 beta-agonists may be useful in monitoring the therapy of acute asthma.