Boston Scientific has updated its instructions for doctors using catheter technology to treat atrial fibrillation with extreme cooling, following reports of four fatalities and other serious complications, the FDA publicized Wednesday.

The company, which has approximately 9,400 employees in Minnesota, highlighted the risks of developing an atrioesophageal fistula, a rare and often fatal complication resulting from injury to the esophagus during cryoablation procedures using POLARx and POLARx FIT catheters, which are thin, tubular devices physicians can advance to the heart through blood vessels.

During a cryoablation procedure, a balloon expands inside the pulmonary vein, using extreme cold to freeze tissue that transmits the signals that cause the heart to quiver in atrial fibrillation.

Seven reports of injuries and four fatalities are linked to the catheters. The FDA has classified the instruction change as a Class I recall, the most serious type, reserved for situations when there's a reasonable chance that using a device without following updated instructions would lead to serious health problems or death.

The recall does not require doctors to stop using the catheters — existing inventory can be used with the new instructions, Boston Scientific said.

Patients who have already had a procedure using one of the affected catheters don't need to take any action.

The Heart Rhythm Society said in a safety alert that the POLARx system, commercially introduced in 2020, has been used in about 69,000 cases worldwide, citing the company's data. The society urged its members using the system to read and follow the company's instructions.

"At present, there is no indication that their system is not safe and effective, if used in accordance with the instructions for use," the society's alert to doctors said.

Cryoablation and radiofrequency ablation, another AFib procedure cauterizing tissue around the pulmonary vein point by point, present a risk that doctors can inadvertently damage other organs like the esophagus, doctors say.

Pulsed field ablation—which uses a high voltage impulse to create pores in cells in the area surrounding the pulmonary vein—presents a smaller chance that the procedure affects adjacent organs, they say. Boston Scientific and other medical technology are racing to commercialize this new technology.