Silva was pulled Sunday after facing four batters in Colorado, spent the night at a Denver hospital and flew back to Chicago on Monday. He was being examined by cardiologist Mark Upton.
Silva was placed on the disabled list Monday to give him time to get treatment. But during his throwing session Wednesday, he showed he hasn't lost his stuff.
"He throws a pretty good sinker," said Carlos Zambrano, who played catch with Silva under the watchful eye of athletic trainer Mark O'Neal. "I thought I had a good sinker, but I think he's close or similar to my sinker."