- Bruce Levine, Chicago baseball beat reporter
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Silva was examined by Colorado's team doctors before being sent to St. Joseph's Hospital in Denver, where he will spend the night for observation.
"He stepped back, we thought something was wrong," acting manager Alan Trammel said. "He has an abnormal heart rate. He's staying overnight for observation is all that I know. He won't be going back [to Chicago] with us tonight."
The team said that Silva's pulse was high and a result of a syndrome called PSVT, which causes an abnormally high heart rhythm. Paramedics corrected the pulse on the way to the hospital, where Silva was said to be back to normal. Silva will see a team cardiologist when he returns to Chicago.
"It caught a lot of us off guard," Trammell said. "We didn't know of any history of that for him."
A Cubs trainer went to the mound after Silva issued a one-out walk to Jonathan Herrera, the second batter in the Rockies' lineup. Silva remained in Sunday's game and allowed a single to Carlos Gonzalez and a ground-rule RBI double to Troy Tulowitzki.
Trammell and the trainer went back out to the mound and Silva, who was experiencing shortness of breath, was replaced by left-hander James Russell.
"Whether it is at the altitude, I don't know," Trammell said. "We saw something after Herrera's at-bat when he backed off the mound. We thought something was wrong."
Silva allowed two runs in one-third of an inning, his shortest start of the season. The Cubs went on to lose 8-7.
Silva has hit the wall in his last four starts, pitching a combined 7 2/3 innings while allowing 20 hits and 14 earned runs over those games.
Information from The Associated Press contributed to this report.