"He is a bit better but not in a state to be released from the hospital. Doctors are keeping an eye on him," a senior officer at the Presidency correctional home (jail) told PTI.
Sixty two-year-old Karnan was admitted to state-run SSKM hospital last evening after he complained of "uneasiness" and chest pain.
It was learnt that the doctors at the hospital conducted a few medical tests on the former judge today too and the results are being awaited.
He was brought from Chennai to Kolkata on Wednesday afternoon and was taken to the Presidency correctional home.
Karnan, who has earned the dubious distinction of being the first sitting high court judge to be awarded a jail term by the apex court, retired from service on June 12 as a fugitive.