Mr Hickey was taken ill in Rio this morning as he was being served with an arrest warrant
Pat Hickey is in a stable condition and will remain in hospital for another 24 hours.
Mr Hickey was taken ill this morning as Brazilian police were serving him with an arrest warrant as part of an investigation into alleged ticket touting.
A statement just issued by the Olympic Council of Ireland (OCI) said he was admitted to Samaritano Hospital in Rio de Janeiro for "investigation of chest pain" and due to his "previous cardiac history he will remain in hospital for a further 24 hours".
Earlier it was announced that Mr Hickey would step aside temporarily as President of the OCI following his arrest this morning.
"In light of this morning’s developments and his ill health, Mr Hickey has taken the decision to step aside temporarily as President of the OCI and all other Olympic functions (IOC member in Ireland, EOC President, ANOC Vice President) until this matter is fully resolved," the previous OCI statement read.
"Mr Hickey will of course continue to cooperate and assist with all ongoing enquiries."
Earlier today, Sports Minister Shane Ross said that in light of the arrest he will be returning to Dublin as soon as possible - and consult with officials and the Attorney General as to the best course of action for the government.
In a statement, Mr Ross said: "Today, I learned from our ambassador here in Brazil that a second Irish citizen has been arrested in connection with the Brazilian authorities' investigation into alleged ticket touting. I understand that person has accepted consular assistance and our Consul General is engaging with him."
"Given the seriousness of this matter, and in the interests of taking swift and decisive action, I will be returning to Dublin as quickly as possible."
"I will immediately consult with my officials, with Minister of State Patrick O'Donovan, and with the Attorney General with a view to considering the options open to the Government and decide the best course of action," he added.
Earlier, police in Brazil held a press conference where Mr Hickey's passport, Olympic credentials and flight ticket were shown to the media.
The IOC - of which Mr Hickey is a member - also held a press conference this afternoon.
IOC presidential spokesman Mark Adams said it is important to remember no wrongdoing has been proven yet.
The OCI has denied any wrongdoing whatsoever in the ticket touting scandal.
It has seen the arrest of another Irish man, Kevin Mallon, who allegedly had hundreds of tickets earmarked for team Ireland.
Pro 10 Sports Management, the OCI's official Authorised Ticket Reseller (ATR), have said they have acted fully and properly within the reseller guidelines.
Meanwhile footage obtained by ESPN Brazil shows the moment Mr Hickey was arrested in a Rio hotel:
Bloomberg journalist Tariq Panja tweeted an image of the arrest warrant issued by police in Rio:
Arrest warrant for Patrick Hickey was sigmed by judge yesterday. Says he needs to be kept in preventative custody pic.twitter.com/PGm1vd4LsJ— tariq panja (@tariqpanja) August 17, 2016
Labour Party leader Brendan Howlin has meanwhile hit out at Sports Minister Shane Ross.
"It has become abundantly clear that Shane Ross’ only interest in this matter is in trying to keep his own hands clean. And it’s equally clear that he has little understanding of the job he was appointed to do," Mr Howlin said.
"The Olympic Council of Ireland is funded by the Sports Council. And under the Sports Council Act 1999, that body is empowered to request any information it wishes from bodies in receipt of funding, and may withhold funding if this information is not received."
"Pat Hickey is entitled to due process, but as Ireland’s reputation suffers further damage, it is about time that Shane Ross got out of the way and tasked the Sports Council with taking ownership of this issue," he added.
Additional reporting by Jack Quann, Andrea Gilligan and Elaine Power