Baker left Wrigley Field to have a chest X-ray performed to rule out pneumonia. Bench coach Chris Speier managed in his place for a 6-5 win over the Cubs in 11 innings. Speier will also manage for Baker on Thursday.
"I guess the good thing is it's not pneumonia, but the bad thing is he's got an irregular heartbeat," Reds head trainer Paul Lessard said. "Basically he had a heavy feeling. He wasn't breathing well. We just decided to look into it a little further. He's had a history of an irregular heartbeat for a while, so it doesn't really bother him. That's why he was kind of poo-pooing it."
Baker, 63, was seen by a member of the Cubs' medical staff -- Dr. Stephen Adams -- before departing for Northwestern Memorial Hospital. He was instructed to get some rest.
There were no outward signs that Baker has felt ill lately. He was in the clubhouse chatting with a couple of his players during the afternoon before the team took the field for batting practice.
The players were informed of Baker's condition during a team meeting.
"You know I had to worry about my daddy. I can't have daddy passing out on us, man," second baseman Brandon Phillips said. "I'm glad he's all right. He's going to be in our prayers. I'm going to pray for him when I get back to the hotel.
"Hopefully he's back soon. I miss him and the toothpicks and the wristbands. I miss it all."
Speier and other members of the club front office spoke to Baker after the game, and he reported that he was feeling fine.
"He sounded good," Speier said. "They're just doing their due diligence and running some tests. He'll be back shortly."
On Tuesday, in a 3-1 Reds win over Chicago, Baker managed the 3,000th game of his career.