Diagnostic accuracy of capnovolumetry for the identification of airway obstruction - results of a diagnostic study in ambulatory care

Abstract

Background: One of the known weaknesses of spirometry is its dependence on patients' cooperation, which can only partially be alleviated by educational efforts. Therefore, procedures less dependent on cooperation might be of value in clinical practice. We investigated the diagnostic accuracy of ultrasound-based capnovolumetry for the identification of airway obstruction.

Methods: Consecutive patients from a pulmonary outpatient clinic were included in the diagnostic study. As reference standard, the presence of airway obstruction was evaluated via spirometry and bodyplethysmography. Capnovolumetry was performed as index test with an ultrasound spirometer providing a surrogate measure of exhaled carbon dioxide. Receiver operating characteristic (ROC) analysis was performed using the ratio of slopes of expiratory phases 3 and 2 (s3/s2) ≥ 0.10 as primary capnovolumetric parameter for the recognition of airway obstruction. Logistic regression was performed as secondary analysis to identify further useful capnovolumetric parameters. The diagnostic potential of capnovolumetry to identify more severe degrees of airway obstruction was evaluated additionally.

Results: Of 1400 patients recruited, 1287 patients were included into the analysis. Airway obstruction was present in 29% of patients. The area under the ROC-curve (AUC) of s3/s2 was 0.678 (95% CI 0.645, 0.710); sensitivity of s3/s2 ≥ 0.10 was 47.7 (95% CI 42.7, 52.8)%, specificity 79.0 (95% CI 76.3, 81.6)%. When combining this parameter with three other parameters derived from regression analysis (ratio area/volume phase 3, slope phase 3, volume phase 2), an AUC of 0.772 (95% CI 0.743, 0.801) was obtained. For severe airway obstruction (FEV₁ ≤ 50% predicted) sensitivity of s3/s2 ≥ 0.10 was 75.9 (95% CI 67.1, 83.0)%, specificity 75.8 (95% CI 73.3, 78.1)%; for very severe airway obstruction (FEV₁ ≤ 30% predicted) sensitivity was 86.7 (95% CI 70.3, 94.7)%, specificity 72.8 (95% CI 70.3, 75.2)%. Sensitivities increased and specificities decreased considerably when the combined capnovolumetric score was used as index test.

Conclusions: Capnovolumetry by way of an ultrasound spirometer had a statistically significant albeit moderate potential for the recognition of airway obstruction in a heterogeneous population of patients typically found in clinical practice. Diagnostic accuracy of the capnovolumetric device increased with the severity of airway obstruction.

Trial registration: The study is registered under DRKS00013935 at German Clinical Trials Register (DRKS).

Keywords: Airway obstruction; Area under the curve (AUC); Asthma; COPD; Capnovolumetry; Diagnostic study; ROC analysis; Sensitivity; Specificity.

Fig. 1

Tracing of a volume-based capnogram during a single expiration. The capnogram is divided into four phases: phase 1, the CO₂-devoid volume of the dead space; phase 2, transition between airway and alveolar gas; phase 3, alveolar part; phase 4, final emptying of the lung (in tidal breathing normally absent or inconspicuous). (A) Parameters of capnographic measurement: s2 and s3 represent the slopes (concentration vs. volume) of the expiratory CO₂-curve in phase 2 and phase 3, respectively. The volume expired during phase 2 or phase 3 is termed ‘volume phase 2’ or ‘volume phase 3’. In order to compute the parameters ‘area phase 2’ and ‘area phase 3’ a horizontal line at the end-tidal CO₂ concentration is drawn. The area above the CO₂ concentration curve bounded by the horizontal line in phase 2 and phase 3, respectively, represent the parameters ‘area phase 2’ and ‘area phase 3’. These areas are complements of the areas under the curve. The angle alpha (α) is formed by the slopes of phases 2 and 3

Fig. 2

Flow-chart of the selection process leading to the final population of 1287 patients. A total of consecutive 1400 patients underwent capnovolumetry. Patients who turned out to have had bronchial provocation challenges or bronchodilator testing prior to capnography due to organizational reasons were excluded from analysis (n = 45). Moreover, patients who did not undergo bodyplethysmographic and spirometric measurements (n = 61) were excluded. Five patients were excluded due to low quality of their bodyplethysmographic data, and two patients based on invalid capnovolumetric measurements

Fig. 3

ROC-curves for the recognition of airway obstruction. The AUC for the ratio of slopes s3 and s2 (s3/s2) was 0.678 (95% CI 0.645, 0.710). The AUC for the combined capnovolumetric score derived from the area-to-volume ratio of phase 3, the logarithm of the slopes of phase 3, the volume of phase 2, and the logarithm of the ratio of slopes of phases 3 and 2 was 0.772 (95% CI 0.743, 0.801)