A 43 year-old man presents to the emergency department complaining of weakness, feeling bad, and difficulty breathing for the last 4 days. His temperature is normal, heart rate is 160 beats per minute, blood pressure is 60/40 mm Hg, and oxygen saturation is 91% on 80% FiO2 by face mask.On physical examination, his trachea is not deviated. His lung sounds are normal. But his heart sounds are muffled, distant and faint. You noted swollen veins in the neck with pressure not changing with breathing. There is an abnormal drop in systolic blood pressure when he inhales and the disappearance of radial pulse on inspiration. ECG revealed a QRS complex height that varies from one heart beat to the next. On chest x-ray, the cardiac silhouette is rounded in its lower portion and tapers at the base of the heart resembling a plastic bag filled with water sitting on a table. Jugular venous pressure waveform is remarkable for prominent x descent and absent y descent. Laboratory tests revealed normal troponin levels. Echocardiogram revealed a small right ventricle and confirmed the suspected diagnosis. Which of the following is most likely to be diagnosed in this patient?
