"The cause has been diagnosed as acute exacerbation of chronic obstructive pulmonary disease (COPD) with bilateral pneumonitis and anaemia," it said.
Bhattacharjee has been suffering from progressive lung problem COPD for years. He also has diabetes and multiple cardiac ailments.
"He is being treated with necessary intravenous antibiotics, oxygen and non-invasive ventilation (bipap) and other medications. In the last 12 hours, his blood pressure -- which was low initially (80/60 mmHg) -- has risen to an acceptable range."
"Blood gas analysis report on Saturday morning shows decreased carbon-dioxide level, his vital parameters are stable now," the bulletin said. The doctors are investigating the cause of anaemia.
"He is kept under close monitoring in ICU under a seven-member expert medical team, which includes critical care specialists, general physicians, cardiologists and gastroenterologist," the hospital said.
State Governor Jagdeep Dhankar and Chief Minister Mamata Banerjee visited the 75-year-old Bhattacharjee at the hospital on Friday night.
Bhattacharjee, who was the Bengal Chief Minister during 2000-2011, undertook an all-out industrialisation drive for reducing unemployment in the state but the measures proved counter-productive.
As his government tried to acquire large tracts of land for setting up industries, the farmers revolted, and the Trinamool Congress fanned the discontent to gain popularity and finally came to power in 2011 ending 34 years of Left Front rule.
Bhattacharjee was last seen in public on February 3, when he went to the Brigade Parade ground to attend a rally called by the Left Front.