Manager Dave Roberts said after the Dodgers' 8-5 victory that Jansen was at the team hotel earlier in the day and didn't feel right. Jansen called the trainer and was taken to the hospital.
Jansen has dealt with an irregular heartbeat in the past. Roberts said the right-hander had his medication with him on the trip. He will visit his cardiologist in Los Angeles.
"We're holding out hope it's not too serious," Roberts said. "In talking to the trainer right now, his mood is fine."
Roberts didn't know whether the team would make a roster move in Jansen's absence. Los Angeles plays three more games at Coors Field before returning home.
The Dodgers used Scott Alexander to close the ninth Thursday. Jansen is tied for the National League lead with 32 saves this season.
"Anytime you're talking about the heart and irregular heartbeat, you've got to be very careful with it," Roberts said. "You're talking about altitude [in Denver]. That's why we want to be proactive and get him back [to Los Angeles.]"
Jansen spent a month on the disabled list during the 2011 season because of blood-thinning medication he was taking to treat an irregular heartbeat. In 2012, he was out again after suffering an irregular heartbeat during an Aug. 27 appearance in Colorado. He underwent a procedure on his heart on Oct. 23, 2012, to return it to normal rhythm.
"Once he sees the doctors, the cardiologists, we'll know more about his timetable then," Roberts said.