Human error, and not a medical device, was to blame for the deaths of two patients at the Hospital of St. Raphael, the U.S. Food and Drug Administration said Tuesday in a preliminary decision.

The FDA said that the device — a meter that regulates oxygen flow — was erroneously connected to a nitrous oxide outlet instead of an oxygen outlet.

The mistake resulted in the deaths of two women, who inhaled fatal doses of the anesthetic instead of oxygen during heart procedures.

The FDA also found that dim lights in a procedure lab and a missing safety pin on the oxygen flow meter were factors in the deaths.

“We have no evidence the device itself failed,” said FDA spokeswoman Sharon Snider. The FDA believes human error also led to the safety pin being broken off the device.

The FDA, which regulates medical devices, is continuing its investigation. A final ruling from the FDA is expected later this year. The agency did not issue a written report Tuesday.

Doris Herdman, 72, of Southington, and Joan Cannon, 68, of Wallingford, died in January when they were accidentally given the nitrous oxide during a heart procedure.

–Associated Press