The Rockies said Nelson's heart was beating normally again after doctors administered a series of shocks during a procedure earlier Tuesday. However, any physical exertion by Nelson will be limited for at least several days. The team placed him on the 15-day disabled list.
Infielder DJ LeMahieu was recalled from the team's Triple-A affiliate in Colorado Springs to fill the roster spot of Nelson, who was batting .256 with five home runs and 23 RBI in 58 games this season.
In other moves, the Rockies placed infielder Jonathan Herrera on the disabled list with a left wrist infection, apparently caused by his wrist-watch, and recalled outfielder Andrew Brown from their Triple-A club. Right-handed pitcher Juan Nicasio also was moved from the 15-day disabled list to the 60-day disabled list, the day after undergoing surgery on his left knee.
LeMahieu and Brown were both in uniform and available for the Pirates game.
Nelson, who spent 15 days on the disabled list with a wrist inflammation earlier in the season, was in the weight room Monday afternoon going through his pregame routine when he "felt funny" and walked into the nearby training room to get checked out, Tracy said.
"They immediately detected the fact that his heart was out of rhythm and took him to the hospital," Tracy said.
Nelson has no known previous history of heart issues, Tracy added.
Herrera was taken to the hospital early Tuesday after team officials became concerned by signs of infection in his wrist. Doctors believe the source of the infection was Herrera's wrist-watch, Tracy said.
Herrera, who spent part of May and June on the disabled list because of a strained right hamstring, was released from the hospital after treatment but needs additional time to fully recover.
LeMahieu is back for a second stint this season with the Rockies. During a monthlong stay earlier, he hit .216 with a double and two RBI in 18 games.
Brown, one of the Rockies' final spring training cuts, has been having a standout season with Colorado Springs. He's batting .317 with 20 home runs and 84 RBI. Brown made his major league debut with St. Louis last season, batting .182 in 11 games. He was acquired off waivers by the Rockies last October and outrighted to the minors before rejoining the club in spring training as a non-roster invitee.