Martinez came out of Thursday's win over Tampa Bay in the seventh inning with a racing heartbeat, dizziness and cold sweats. He was originally treated in the clubhouse for dehydration but was taken to a hospital when symptoms continued.
The 38-year-old Martinez was still in the hospital on Friday afternoon as doctors tried to find the cause.
"It's a little scary," Tigers manager Brad Ausmus said. "It's not something that, at his age, you generally consider an issue or worry about."
"I think there's a little bit of an unknown factor, which is causing more anxiety than anything else," he said
The Tigers called up catcher/first baseman John Hicks from Triple-A Toledo. Ausmus plans to use the DH slot to give some rest to players dealing with minor injuries.
Miguel Cabrera, who hit a game-winning homer in the ninth inning on Thursday, was the designated hitter Friday against the Rays. Cabrera is dealing with back and groin problems and hadn't homered since May 20.