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 (FEV 1 ≤ 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 (FEV 1 ≤ 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).

Capnovolumetry in combination with clinical history for the diagnosis of asthma and COPD

“…Regarding the limitations of the study, it has to be mentioned that the present study was a secondary analysis based on previous results 1 . It included information not previously used and followed a different methodological path focusing on decision algorithms.…”

“…The analysis used data from a previous study performed in a private clinical practice in Augsburg, Germany, in which capnovolumetry was performed as index test, while the presence or absence of airway obstruction was evaluated via spirometry and bodyplethysmography as a reference standard 1 . The physician-based diagnoses relied on a comprehensive assessment of the patients' files and lung function data.…”

“…For the explanation of parameters, see ref. 1 . Log 10 (s3/s2) is the logarithm of the ratio s3/s2, log 10 (s3) the logarithm of slope of phase 3.…”

“…Therefore, it is of special interest in conditions where spirometry is unreliable due to insufficient cooperation by the patients 5 or lack of experience of the personnel in guiding the maneuvers or even concerns regarding the accuracy of spirometers 6 . Previous studies have shown a moderate diagnostic accuracy of this method regarding airway obstruction 1 and an acceptability of spirograms for clinical use in only about 60% of patients in a primary care setting 6 . However, the establishment of a clinical diagnosis also includes clinical history, signs, and symptoms, which can be covered by a set of standard questions.…”

“…Capnovolumetry has been proposed as a method to obtain information on the functional state of patients with obstructive airway diseases [1][2][3][4] . Capnovolumetric measurements are not time consuming and relatively easy to perform since the patient only needs to perform quiet tidal breathing over about 10 breathing cycles.…”