St. Jude Medical Inc has sent a letter to doctors advising them that its Riata leads that connect implanted defibrillators to the heart have a higher rate of insulation failures than previously reported.

St. Jude said in the letter that the Riata and Riata ST leads showed an insulation abrasion incident rate of 0.63 percent over nine years, higher than the 0.47 percent rate it reported a year ago. The estimated failure rate is based on an analysis of product returns and complaints for more than 227,000 Riata leads sold worldwide.

The company stopped selling the Riata leads last December.

St. Jude recommended that doctors continue to monitor devices and leads in their patients but said removing or replacing the lead was not recommended.

The letter, dated November 28, was posted on St. Jude's website.

The company said in the letter that analyses of returned products could underestimate failures. It also said a single site in Belfast, Ireland, had indicated that 15 percent of Riata leads in 165 patients experienced problems.

St. Jude said it had reports that two patients had died and one had suffered a serious injury during procedures to remove the leads. It also said it had reports of one additional death and an additional serious injury but that it had determined they were not linked to insulation issues.

Jefferies analyst Raj Denhoy said investors are concerned the problems with the Riata leads may cause physicians to reduce their use of other St Jude leads currently on the market. While the company reported a small increase in the number of incidents involving Riata leads, the rate of problems occurring is probably higher, Denhoy said.

"My sense is that if you talk to clinicians, most would tell you the number is actually higher than that," Denhoy said.