Koch's cardiologist wanted to keep a closer eye on the 88-year-old, who was also showing an iron deficiency, spokesman George Arzt said.
Koch was readmitted to NewYork-Presbyterian/Columbia hospital on Monday with shortness of breath. He had been released two days earlier after being treated for water in his lungs and legs. He had initially been admitted on Jan. 19.
His hospitalization follows two other ones in recent months.
In December, he was admitted with a respiratory infection. The former mayor was also hospitalized in September after feeling weak while staying with friends in North Carolina. He was treated for anemia, was released after three days and said he planned to head straight to his office at a law firm.
In 2009, Koch spent five weeks at the hospital, undergoing a quadruple bypass in June and gallbladder surgery in July.
Koch was mayor for three terms, from 1978 through 1989, and is credited with saving the city from near-financial ruin. His trademark question to residents was, "How'm I doing?"
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