A spokesman from the Texas hospital where Harry Whittington, 78, was recovering said some of the birdshot from the accident had lodged near his heart.
Mr Whittington was moved into intensive care and ordered to stay in the Corpus Christi hospital for at least another seven days to monitor his condition.
Doctors left the birdshot in place and said Mr Whittington’s life was not in immediate danger.
Mr Cheney has not spoken publicly about the incident.
The news gave a serious edge to the incident that the White House had begun making light of on Tuesday after tense exchanges with the press the day before over failure to disclose the shooting for almost a full day.
Dr David Blanchard, the hospital's emergency room chief, said at a news conference doctors noted an irregular heartbeat during a morning examination of Mr Whittington and it was determined he was having a minor heart attack.
He was moved into the hospital's intensive care unit and underwent cardiac catheterization for his quivering heart.
"We picked up an irregular heartbeat. At no time did he have any chest pain, the classic signs of a heart attack or anything like that," Dr Blanchard said.
Mr Whittington, a Republican stalwart in Texas, was peppered by an estimated 200 pellets on Saturday when Mr Cheney whirled to shoot a covey of quail on a southern Texas ranch but hit Mr Whittington instead.
Mr Cheney spoke to him by telephone from Washington on Tuesday after watching a televised hospital news conference on Mr Whittington's health setback.
"The vice president wished Mr Whittington well and asked if there was anything he needed," Mr Cheney's office said in a statement.
Doctors and officials had told the press that Mr Whittington suffered only minor wounds, but Dr Blanchard said on Tuesday, "We knew he had some birdshot very near to the heart from the get-go."
At least one of those pellets had gotten close enough to the heart to cause "irritability," which led to Tuesday's heart attack, the doctor said.
President George W. Bush's chief spokesman, Scott McClellan, served notice that there would be no official, formal White House account of how the incident occurred.
The story developed into a controversy because news of the shooting only came out on Sunday, nearly 24 hours after it occurred, when ranch owner Katharine Armstrong, also a Republican stalwart and lobbyist, called the Corpus Christi Caller-Times to report it.
Neither the White House nor Cheney's staff disclosed the accident, saying the primary concern had been making sure Mr Whittington received proper care.
In an off-camera exchange with reporters Mr McClellan made light of he shooting, saying a college football team due to meet Mr Bush would be wearing bright orange school colors "not because they're concerned that the vice president may be there."
"Although that's why I'm wearing it," joked the spokesman, poking at a tie with a bright orange pattern.
The sheriff's department in Kenedy County, where the Armstrong ranch is located, said on Monday it had found no misconduct in the incident.