Maisy was lucky enough to be owned by a pair of physicians who recognized her abnormally rapid breathing at an early age. Radiographs (x-rays) made the diagnosis, as you can see below:
The top film is a lateral view (from the side) and shows not only the air-filled loops of bowel focally surrounding the heart, but also that there is no division between the diaphragm and heart.
The bottom film is a ventral dorsal view (from front to back) confirms the above findings, showing that the bowel is definitely within the pericardial sac (as opposed to being beside it, which might have the same appearance on a single film)
As Maisy got older, she would tire with any exertion, and so her owners decided to do surgery. Maisy did very well. All of the abdominal organs could be reduced back into the abdomen without a problem -- there were not significant adhesions -- and the edges of the hernial ring were trimmed, after which the hernia could be closed completely.
Maisy recovered without complication and never looked back.
Four months later, she remains a normal, happy, healthy girl...running, playing, and loving life.
For more information on peritoneal pericardial hernias, check here.