DENVER (AP) — Right-hander Carlos Silva left the Chicago Cubs' game against the Colorado Rockies on Sunday in the first inning with an abnormal heart rate.

The Cubs said he will remain in a Denver hospital overnight for observation.

"It caught a lot of us off guard," acting manager Alan Trammell said. "We didn't know of any history of that for him."

A Cubs trainer went to the mound after Silva walked the second batter he faced, Jonathan Herrera. Silva stayed in and allowed a single to Carlos Gonzalez and a ground-rule RBI double to Troy Tulowitzki.

Trammell and the trainer went back to the mound, and Silva 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 recorded one out in his shortest outing of the season, yielding two runs and two hits.