You attend a 30-year-old male who has been involved in a knife fight. He has a single stab wound to the left anterior chest at the 4th intercostal space, parasternal. He is conscious, tachycardic (140 bpm), hypotensive (75/50 mmHg), and has distended neck veins despite hypovolaemia. Heart sounds are muffled. What is the most likely diagnosis and what is the immediate concern?