The xray shows gas outlining the heart, with the pericardium being "lifted" off the heart. This is a "pneumopericardium", shown particularly well on this coronal reformatted CT:
Pneumopericardium is a surgical emergency as the development of cardiac tamponade by raised pericardial pressures must be avoided as a matter of urgency. Following the CT, he was taken to theatre and an on-table endoscopy confirmed a fistula track between the gastric pouch and pericardium, with food debris seen within the pericardial sac. He underwent a total gastrectomy and formation of an oesophagojejunostomy, and windowing of the pericardium.
Identification of pneumopericardium should prompt emergent referral to the surgeons and a CT of the chest and abdomen with oral contrast should be undertaken. Oesophago- and gastro-pericardial fistulae are recognised complications of oesophagogastric surgery and peptic ulceration.
Note: these images and a more full description with references are also found in a publication I have co-authored with surgical colleagues in BMJ Case Reports, I have permission to share these images.