Is the barium enema exam of the large intestine safe for heart patients?

Compared to what? Ca: Cancer? Any procedure or test comes with risk, so if it isn't needed, then best not to do it. However, whoever ordered the Barium enema did so for a reason & the risk-vs-benefit calculation was done based on symptoms, history, & a preliminary diagnosis. All-in-all, barium is safer than colonoscopy using Mag citrate or other laxatives that cause electrolyte problems & require sedation. Enema is OK.