 In patients presenting with acute dyspnea in a pre-hospital setting, the early and correct diagnosis may present a significant clinical challenge. Physical examination, chest radiography, electrocardiography, and standard biological tests often fail to accurately differentiate heart failure HF from pulmonary causes of dyspnea. Timely differentiation of HF from other causes of dyspnea may permit the early institution of appropriate medical therapy. Brain natriuretic peptide BNP and amino-terminal pro-brain natriuretic peptide NT-proBNP have been proposed as early markers of HF and demonstrated to be useful for diagnosing and excluding HF in the emergency department. A combination of BNP or NT-proBNP testing and standard clinical assessment has been suggested to be superior to either tool used in isolation. Some previous studies have also suggested that quantitative capnometry QC may be useful in differentiating between cardiac and obstructive causes of respiratory distress. Therefore, the investigators hypothesized that a new combination of NT-proBNP testing, standard clinical assessment, and partial pressure of end-tidal CO2 PetCO2 would optimize evaluation and differentiation of acute dyspnea in a pre-hospital setting. 