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Measurement of expired carbon dioxide, oxygen and volume in conjunction with pretest probability estimation as a method to diagnose and exclude pulmonary venous thromboembolism
Author(s) -
Kline Jeffrey A.,
Hogg Melanie
Publication year - 2006
Publication title -
clinical physiology and functional imaging
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.608
H-Index - 67
eISSN - 1475-097X
pISSN - 1475-0961
DOI - 10.1111/j.1475-097x.2006.00672.x
Subject(s) - medicine , expiration , receiver operating characteristic , confidence interval , tidal volume , likelihood ratios in diagnostic testing , exhalation , pulmonary embolism , emergency department , nuclear medicine , expired air , pre and post test probability , surgery , anesthesia , respiratory system , psychiatry
Summary Background: The classical alveolar pCO 2 –pO 2 relationship predicts that pulmonary embolism (PE) causes a low ratio of pCO 2 /pO 2 at end expiration. Our purpose was to define a simple protocol to collect expired pCO 2 /pO 2 to diagnose PE. Emergency department patients with suspected PE were enrolled. Clinical pretest probabilities for PE were estimated prior to diagnostic testing using the Canadian score and clinicians’ unstructured estimate. Patients provided three 30‐s periods of tidal breathing, separated by three deep exhalations. Expired pCO 2 , pO 2 and breath volume were measured. All patients underwent standardized objective testing for PE including 90‐day follow‐up. Diagnosis (PE+) required anticoagulation for image‐proven PE within 90 days. Results: Of 200 patients enrolled, 178 were included in final analysis (24 PE+). The mean coefficient of variability for the deep‐exhaled and end‐tidal pCO 2 /pO 2 ratios were 6·8 ± 6·7 and 7·5 ± 6·8%, respectively. Mean pCO 2 /pO 2 ratios were stable throughout the collection periods in both methods. When compared with the deep‐exhaled ratio, the end‐tidal mean ratio demonstrated slightly better diagnostic utility by the area under the receiver operating characteristic curve. The end‐tidal ratios were divided into four interval likelihood ratios and coupled with pretest probability from the two methods to generate three sets of posttest probabilities. Receiver operating characteristic analysis demonstrated good overall diagnostic performance (areas under the curves >0·88) for both posttest probability sets. Conclusion: This preliminary work demonstrates that the end‐tidal pCO 2 /pO 2 averaged from 30 s of breathing can produce clinically relevant likelihood ratios for the diagnosis and exclusion of PE.