COLORADO SPRINGS, Colo. -- Former Apollo astronaut James Irwin, who suffered a cardiac arrest while jogging Friday, regained consciousness and spoke with his family, but was still disoriented, doctors said.

Irwin, 56, who became the eighth man to walk on the moon during the 1971 mission of Apollo 15, was upgraded from critical but stable condition to serious but stable condition on Sunday.

'He is definitely awake. He has been speaking with his family, and he's breathing on his own,' Penrose Hospital nursing supervisor Debby McElvaney said. 'It will take several days to see a marked improvement, however.'

Irwin was removed Sunday from a ventilator used to assist his breathing, but remained attached to a cardiac monitor, said nursing supervisor Betty Hagge.

Hagge described Irwin as 'disoriented,' and said that was a slight improvement from Saturday, when he opened his eyes, but did not move them.

Irwin, who suffered heart attacks in 1973 and 1977 and underwent bypass surgery between the two attacks, collapsed while jogging Friday on a street corner near his Colorado Springs home. A fire department rescue team used a defibrillator to shock his heart back into action.

Doctors first thought Irwin had suffered another heart attack but changed their diagnosis to cardiac arrhythmia, an irregularity in the rhythm of the heart's beating that stops the flow of blood from the heart to the body.

Arrhythmia does not damage the heart, said Hagge.

'It was not a heart attack,' said Hagge. 'He had cardiac arrhythmia, and there was no further damage to the heart.'

His friend and co-author, Monte Unger, said Irwin believed jogging would strengthen his heart. Irwin's personal secretary, Cathy Duncan, said he tried to run 6 miles a day.

Irwin, a native of Pittsburgh, resigned from NASA in 1972. He has spent the last decade writing books and speaking about his fundamentalist Christian faith and had been planning another trip to Turkey's Mount Ararat in August to resume his search for Noah's Ark.

Irwin's only spaceflight came aboard Apollo 15 in 1971, when he drove about the moon's surface in the first lunar roving vehicle.

After the mission, NASA reprimanded Irwin and his crewmates for taking souvenir stamped envelopes to the moon and offering them for sale.