Background: The classical alveolar pCO2-pO2 relationship predicts that pulmonary embolism (PE) causes a low ratio of pCO2/pO2 at end expiration. Our purpose was to define a simple protocol to collect expired pCO2/pO2 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 pCO2, pO2 and breath volume were measured. All patients underwent standardized objective testing for PE including 90-day follow-up. Diagnosis (PE+) required anticoagulation for imageproven 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 pCO2/pO2 ratios were 6·8 ± 6·7 and 7·5 ± 6·8%, respectively. Mean pCO2/pO2 ratios were stable throughout the collection periods in both methods. When compared with the deepexhaled 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 pCO2/pO2 averaged from 30 s of breathing can produce clinically relevant likelihood ratios for the diagnosis and exclusion of PE.
- Receiver operating characteristic
ASJC Scopus subject areas
- Physiology (medical)