Introduction: Cardiac tamponade is an emergency syndrome that requires fast diagnosis and treatment; otherwise patient follows obstructive shock and cardiac arrest. Case report: A 70-year-old female was brought to the emergency department with hypoxemia. She had a history of progressive dyspnea over the past three weeks. Past medical history includes smoking. On physical examination: tachypnea, hypoxemia (SaO2 89%), jugular venous distention, arterial pressure 220/100 mmHg, heart rate rhythmic of 82 bpm. On pulmonary auscultation: diffuse and bilateral crackles. Lung ultrasound showed a bilateral B line and the echocardiogram demonstrated a pericardial effusion with signs of tamponade. A pericardiocentesis evacuated 620 ml of hemorrhagic fluid and the patient was transferred to the intensive care unit, hemodynamically stable, with SaO2 95%. At the ICU the echocardiogram, showed resolution of the cardiac tamponade and a tumor adhered to the lateral wall of the left ventricle. Chest CT demonstrated: a left lung tumor, infiltrating the pericardial sac. A pericardium biopsy demonstrated undifferentiated carcinoma. Discussion: Cardiac tamponade diagnosis requires a high level of suspicion. Respiratory failure, chest pain, and shock, observed in cardiac tamponade, are also present in different diseases. The most common finding of cardiac tamponade is dyspnea (78% of cases). Our patient had dyspnea due to pulmonary edema, secondary to left ventricle diastolic dysfunction caused by the tamponade. A bedside echocardiogram made the diagnosis of cardiac tamponade and guided the effective pericardiocentesis. Conclusion: Cardiac tamponade must be suspected in all cases of acute dyspnea. Echocardiogram is the method of choice for the diagnosis and for guiding the pericardiocentesis.