A day after Pope Benedict XVI announced his resignation, the Vatican has acknowledged that the pontiff has had a pacemaker for years.
But Vatican spokesman Federico Lombardi reiterated that he was not stepping down because of any specific illness.
His last public appearance will be his final mass in Saint Peter's Square on 27 February, Fr Lombardi said.
The pontiff would have no role in the running of the church after his resignation, he added.
The unexpected development - the first papal resignation in nearly 600 years - surprised governments, Vatican-watchers and even the Pope's closest aides.
Cardinal Joseph Ratzinger became Pope Benedict XVI in 2005 after John Paul II's death.
The BBC's David Willey in Rome says that in theory there has never been anything stopping Pope Benedict or any of his predecessors from stepping aside.
Under the Catholic Church's governing code, Canon Law, the only conditions for the validity of such a resignation are that it be made freely and be properly published.
But resignation is extremely rare: the last pontiff to step aside was Pope Gregory XII, who resigned in 1415 amid a schism within the Church.
According to a report in Italy's Il Sole 24 newspaper, the Pope had surgery to replace a pacemaker just under three months ago.
At a news conference at the Vatican, Father Lombardi confirmed that the batteries in the pacemaker, which had been fitted several years ago, had been replaced in the routine operation.
"That hasn't affected his decision [to resign] in any way and simply he felt that his strength was diminishing with the advancement of age," Father Lombardi said.
Earlier the pontiff's brother, Georg Ratzinger, said the Pope had been advised by his doctor not to take any more transatlantic trips and had been considering stepping down for months.
"When he got to the second half of his 80s, he felt that his age was showing and that he was gradually losing the abilities he may have had and that it takes to fulfil this office properly," he told the BBC from his home in Regensburg, Germany.
He said the resignation therefore was part of a "natural process".
The Vatican now says it expects a new pontiff to be elected before Easter.
Father Lombardi said the Pope would not intervene in the election of a successor, who will be chosen by members of a 117-strong conclave held in the Sistine Chapel at the Vatican.
"`He will not interfere in any way,'' AP news agency quotes the Vatican spokesman as saying.
Analysts say Europeans are still among the favourites, including the current Archbishop of Milan, Angelo Scola, and Christoph Schoenborn, a former Austrian student of the current Pope.
But strong candidates could emerge from Africa and Latin America, which both have very large Catholic populations. Among the names being mentioned are Ghana's Cardinal Peter Turkson and Cardinal Francis Arinze of Nigeria.
Father Lombardi said the Pope would continue with his diary as usual until the day he officially retires on 28 February.
He is due to officiate at an Ash Wednesday service at the Vatican.
"The last general audience [on 27 February] will be held in the square since a lot of people will come," AFP news agency quotes Father Lombardi as saying.
After that the Vatican has said he will retire to the papal residence at Castel Gandolfo before moving into a renovated monastery used by cloistered nuns for "a period of prayer and reflection".
"He'll stay in Rome and will certainly have some duties and of course will continue to educate himself intellectually and theologically," Georg Ratzinger told the BBC.
"Where he's needed he will make himself available, but he will not want to want to intervene in the affairs of his successor," he said.
At 78, Cardinal Joseph Ratzinger was one of the oldest popes in history at his election.
He took the helm as one of the fiercest storms the Catholic Church has faced in decades - the scandal of child sex abuse by priests - was breaking.
The pontiff said in his Monday's statement: "After having repeatedly examined my conscience before God, I have come to the certainty that my strengths, due to an advanced age, are no longer suited to an adequate exercise of the Petrine ministry."
A theological conservative before and during his time as pontiff, he has taken traditional positions on homosexuality and women priests, while urging abstinence and continuing opposition to the use of contraceptives.
His attempts at inter-faith relations were mixed, with Muslims, Jews and Protestants all taking offence at various times, despite his efforts to reach out and make visits to key holy sites, including those in Jerusalem.