The 23-year-old MacLean was playing in Owen Sound, Ont., on Monday night when stricken, according to the Phoenix Coyotes.
"Brett received CPR on site and was taken to a local hospital by ambulance where he was treated," Coyotes general manager Don Maloney said in a statement. "He was then transported via an air ambulance to [University] Hospital in London, Ont., where he was admitted to the cardiac ICU. Our thoughts and prayers are with Brett and his family. We request that everyone please respect their privacy at this time. We will provide an update on Brett's status when information is available."
The Port Elgin, Ont., native was drafted 32nd overall by the Coyotes in 2007 from the Oshawa Generals, where his teammates included current NHL players John Tavares, Cal Clutterbuck and Michael Del Zotto.
In his final junior season, he notched 61 goals and 58 assists in 61 games for the Generals.
MacLean rejoined the Phoenix organization last season after a brief stint in the Winnipeg system.
He has played 13 games with the Coyotes in his career and five with the Jets, scoring three goals and two assists overall. Most of his pro career has been spent in the American Hockey League.