A 70-year-old patient died at a hospital in Yamanashi Prefecture in December after air was mistakenly introduced into his bloodstream during surgery, sources said Monday.

The University of Yamanashi Hospital in the town of Tamaho has confirmed that the incident took place, but denies that the air in his bloodstream killed the man.

The patient suffered a stroke and died nine days after the operation.

The sources said the man was hospitalized in late November to have surgery to remove an aneurysm near his heart, using an artificial heart-lung machine.

During the Dec. 3 operation, the clinical engineer operating the machine mistakenly mixed about 20 ml of air into the man's blood.

The man suffered a stroke shortly after the operation and was moved to the intensive care unit. His condition worsened on Dec. 10 and he died two days later.

Medical experts say there is a high risk that a person could suffer a stroke if a certain amount of air is mixed into his or her blood due to the narrowness of blood vessels in the brain.

The doctor in charge informed the man's family of the mistake. No autopsy was conducted in keeping with the family's request, and the cause of his death was identified as multiple organ failure, the sources said.

Hospital officials said they had reported the incident to the Japan Council for Quality Health Care -- a third-party entity that examines and helps improve the quality of health-care services in the country -- and do not believe the incident constituted medical error.

The officials said they cannot comment on the specifics of the case, citing the patient's privacy.