A 22-year old male was brought to the shock trauma bay after sustaining a gunshot to the chest. His blood pressure on arrival was 78/36 with a pulse of 78 and two large-bore IVs were inserted to begin fluid resuscitation. He is unresponsive with decreased breath sounds and is immediately intubated. He is not a heavy man, but his heart sounds are muffled and you note distention of his jugular veins. Ultrasound does not reveal any fluid collections in the abdomen, but does reveal fluid in the pericardium. After a liter of fluid is given, his pressure is now 50 systolic by palpation and pulse is 52. What is your diagnosis?