Background: The Emergency Severity Index (ESI) may not recognize high-risk patients with heart failure (HF) efficiently.Aim: This study aimed to compare the diagnostic validity and mistriage rates of the ESI plus the Capnometer (Capno) and ESI alone among dyspneic patients with HF.Method: This quasi-experimental group (random assignment) study was conducted within April 2019-February 2020. Patients were randomly assigned to the ESI+Capno and ESI groups. Triage levels, resources used, disposition and door to an electrocardiogram, and physician visit were compared among patients admitted to the Cardiac Care Unit (CCU), the Cardiac Unit (CU), or discharged from the ED. Interobserver agreement (Kappa) was used to assess the reliability of the ESI.Results: In this study, 65 HF patients were assigned to the ESI+Capno (n=36) and ESI (n=29) groups. The undertriage rates were 0% and 10% and the overtriage rates were 10% and 31% in the ESI+Capno and ESI groups, respectively. Sensitivity, specificity, and accuracy to recognize high-risk HF patients were 100%, 60%, and 90% for the ESI+Capno group and 62.5%, 42.86%, and 48.36% for the ESI group.Implications for Practice: The addition of Capno to the ESI increased the validity of triage decisions to recognize high-risk HF patients, compared to the ESI alone. It is recommended that decisions regarding triage HF patients be made after that an End-tidal Co2 is considered into the decision-making process.