Holmgren said he did not know if something happened in the game to cause the elevated rate.
"It wouldn't go down, even after rest," said Holmgren, referring to Hartnell's condition on Friday night. He said it took an "abnormal" amount of time for the rate to settle.
According to righthealth.com, the heart may start beating faster for several reasons, including exertion, dehydration, stress, anger, blood-sugar fluctuation, excessive caffeine consumption, or as a reaction to certain medications.
Holmgren and coach Peter Laviolette both used the word scary to describe what happened Friday.
Hartnell underwent some preliminary testing on Saturday, Holmgren said.
The Flyers were "mildly concerned because he's a young guy," Holmgren said, adding that Hartnell had some "twitching in his arms" Friday and that trainer Jim McCrossin examined him and discovered the high heart rate.
"We'll continue to monitor it and run some tests on him and get to the bottom of it," the general manager said.
Before Holmgren addressed reporters, Hartnell downplayed the reason he had missed Sunday's practice and never revealed the heart problem.
"Just a maintenance day, just a little sore," he said. "Soreness everywhere. First couple weeks of the season, everyone is having a tweaked muscle here and there, and it's better to be safe than sorry."
Hartnell had 24 goals and 25 assists while playing last season on the Flyers' best line, along with Danny Briere and Ville Leino.
Vets released. The Flyers released veteran forwards Michael Nylander and Adam Mair from tryout contracts on Sunday.
Counting players placed on waivers and two injured prospects, the Flyers have 29 players left in camp. They figure to open the season with 22.
Because of a groin injury, Nylander, who had been Jaromir Jagr's linemate with the Rangers and Washington, never played in any games in camp. Mair played one game.
Nylander, who will turn 39 next Monday, is now healthy, but the Flyers want to take a longer look at young prospects like Sean Couturier, Brayden Schenn, Matt Read, Zac Rinaldo, and Tom Sestito.
Mair, 32, played in just one exhibition.
"At the end of the day, we're at [the maximum] 50 contracts," Holmgren said, "and I don't see how we can move a guy out right now and make room for either one of those guys. We're quite happy with the way some of these other young kids are playing."
Holmgren said he was exploring the option of having Nylander "play at a lower level."
Breakaways. Defenseman Chris Pronger, rehabbing from back surgery, skated with the team longer on Sunday than at any point in camp. . . . Matt Walker and Erik Gustafsson, candidates to make the roster on defense, are expected to play Monday against the visiting Rangers. . . . The NHL will hold a news conference at Citizens Bank Park Monday at 1:30 p.m. to talk about the Jan. 2 Winter Classic between the Flyers and Rangers.
Contact staff writer Sam Carchidi at email@example.com or @BroadStBull on Twitter.