Peter Banko, the hospital administrator at a hospital in Corpus Christi, Texas, said Harry Whittington, 78, was moved back to the intensive care unit for further treatment on Tuesday morning.
Banko said there was an irregularity in the heartbeat caused by a pellet, and doctors performed a cardiac catheterisation.
Whittington expressed a desire to leave the hospital, but Banko said he would probably stay for another week to make sure more shot does not move to other organs or move into other parts of his body.
"Some of the bird shot appears to have moved and lodged into part of his heart in what we would say is a minor heart attack," Banko said.
The doctors said Whittington did not experience symptoms of a heart attack or any other problems. They left the birdshot in place and said he could live a healthy life with it there.
David Blanchard, chief of emergency care, said: "He has not had a heart attack in the traditional sense."
White House physicians who attended to Whittington at the scene after Cheney, the US vice-president, accidentally shot him were involved in the treatment, the officials said.
Whittington had initially been placed in intensive care after the accident on Saturday evening. He had been moved to a "step-down unit" on Monday after doctors decided to leave several birdshot pellets lodged in his skin rather than try to remove them.
A Texas Parks and Wildlife Department report said Whittington was retrieving a downed bird and stepped out of the hunting line he was sharing with Cheney.
"Another covey was flushed and Cheney swung on a bird and fired, striking Whittington in the face, neck and chest at approximately 30 yards," the report said.
The wildlife department issued a report on Monday that found the main factor contributing to the accident was a "hunter's judgment". No other secondary factors were found to have played a role.
Newly imposed licence
The department gave Cheney and Whittington warning citations for breaking Texas hunting law by failing to buy a $7 stamp allowing them to shoot upland game birds.
A department spokesman said warnings are being issued in most cases because the stamp requirement went into effect only five months ago and many hunters were not aware of it.
Cheney's office said on Monday night in a statement that Cheney had a $125 non-resident hunting licence and has sent a $7 cheque to cover the cost of the stamp.
"The staff asked for all permits needed, but was not informed of the $7 upland game bird stamp requirement," the statement said.