For patients presenting to the Emergency Department with acute dyspnea, emergency physicians will be asked to categorize the diagnosis as acute decompensated heart failure or non-cardiogenic shortness of breath a) after the initial clinical assessment, and b) after performing lung ultrasound (LUS) for LUS arm or after chest radiography (CXR) and natriuretic peptide (NT-pro BNP) results for CXR arm. All patients will undergo CXR, those enrolled in the LUS arm, after sonographic evaluation. After discharge, the cause of patient's dyspnea will be determined by independent review of the entire medical records performed by two emergency physicians. In case of disagreement, a third expert physician will review entire medical records, and adjudicate the case.

Study protocol After the initial standard work-up, which includes past medical history, history of the present illness, physical examination, ECG, and arterial blood gas analysis, the emergency physician responsible for patient care will be asked to categorize the diagnosis as ADHF or non-cardiogenic dyspnea.

Then, the patient will be assigned to one of the experimental arms. In the LUS arm, the same emergency physician will perform LUS, and express the new integrated presumptive etiology ("LUS-implemented" diagnosis). All patients will then undergo CXR.

In the CXR arm, patients will undergo CXR, and the new integrated etiology will be record after CXR and NT-proBNP results will be available.

After hospital discharge, two expert emergency physicians, blinded to LUS results, will independently review the entire medical record, and indicate the final diagnosis. In case of disagreement, a cardiologist will review the medical records, and adjudicate the case.

Statistical analysis The accuracy of each diagnostic tool will be expressed as sensitivity, specificity, predictive values and likelihood ratios obtained using 2 x 2 tables. "Positive" and "negative" results will be considered, for each test, the diagnosis of ADHF or non-cardiac dyspnea, respectively. Receiver operating characteristic (ROC) and area under curve (AUC) statistics will be also shown.

Inclusion Criteria:

• acute dyspnea as chief complaint
• presence of an emergency physician skilled in lung ultrasound at evaluation time

Exclusion Criteria:

• mechanical ventilation ongoing at enrolment time
• dyspnea clearly related to a different aetiology (e.g. trauma, anxiety, etc)