The operation is underway now. A press conference - later canceled - was scheduled to take place Wednesday at 15:30.
Sīle told LSM that the president is undergoing "a heart operation, which is a serious operation", stressing that it's not a simple procedure.
An announcement by the President's Chancery reads that "The team of cardiologists, heart surgeons and all the involved specialists has decided that it's very important now to eradicate the source of the infection, and a heart operation is the way of doing that."
Vējonis was taken to Gailezers Hospital on Monday and transferred to Pauls Stradins Clinical University January 19.
The presidential powers are temporarily transferred from Wednesday to parliamentary speaker Inara Murniece.
Latvian Television News Service tweeted later in the day that the operation has been concluded, citing Lāsma Sīle as the source. Further comments on the president's health will be given tomorrow as doctors want to wait at least 24 hours after the operation.
"The operation on President Vējonis has concluded; doctors have done what they planned, and the source of the infection has been removed."
Valsts prezidenta Vējoņa operācija noslēgusies; ārsti plānoto ir izdarījuši un infekcijas avots ir likvidēts.— LTV Ziņu dienests (@ltvzinas) January 20, 2016
In a later tweet, LTV's reporters said they had information that one of Vejonis' heart valves had been replaced with an artificial valve.
Following the operation, the President was in a "stable post-operative condition" a later statement from the hospital said.
Meanwhile offiicals and members of the public took to social media to wish him well.
Prime Minister Laimdota Straujuma said she would be meeting stand-in President Murniece to continue business as usual.
LTV reported Thursday morning that the president had awoken and talked with the doctors, according to Atis Veinbergs, the main doctor at Stradins Hospital.
Veinbergs didn't specify for how long the Latvian president will have to undergo further treatment. Even though the infection was eliminated, it had spread throughout the president's organism.
The president is now in intensive care. He will be needing rehabilitation afterwards.
Vējonis himself said that he's feeling okay and appropriately for a post-surgery period, according to Veinbergs.