Food and Drug Administration doctors determined last December that a particular kind of pacemaker wiring, implanted in more than 18,000 heart patients, might be fatally defective. They rated the risk of wire failure at nine on a scale of zero (no risk) to 12.

Yet in January the FDA approved a mildly worded letter that the wires' manufacturer, Medtronic Inc., sent to doctors across the nation. The letter merely told doctors they should monitor pacemaker patients implanted with No. 4002 wires twice a month for three years and once a month thereafter -- a routine monitoring schedule.

An FDA doctor speaking for the agency concedes that the manufacturer's letter did not go far enough. Ironically, he said one reason for approving the letter was the FDA medical panel's concern over the danger; they didn't want to delay alerting doctors by haggling over wording.

In defense of FDA approval of the weak letter, the FDA doctor said it probably raised warning flags among heart specialists who received it. A letter with more explicit language, he said, might have been considered insulting by experts.

When the Associated Press broke the story early this year, Medtronic, of Minneapolis, was quoted as saying the wiring problem was not life-threatening.

But internal FDA documents obtained by our associate Stewart Harris give a different picture. In an analysis of the problem wires, eight FDA doctors signed a worksheet that said a break in the wires' insulation "may result in failure to sense and/or pace," and added ominously: "Failure to sense and/or pace could result in cardiac arrhythmias or . . . death."

Medtronic's letter to physicians aknowledged that "complication rates continue to be higher than that of other Medtronic leads." But it said the likelihood of total failure in 4002 wires "is low."

According to the internal FDA document, failure of the insulation could render the wires useless, meaning that patients who are totally dependent on their pacemakers to stimulate their hearts probably would die. Luckily, most patients are not totally dependent on pacemaker impulses.

The FDA analysis said that of 18,618 patients implanted with 4002 wires at the time, 13,900 were assumed to be at risk. The FDA doctors concluded that the "product deficiency presents moderate to high risk of adverse health consequences," and gave it a nine rating on the 12-point risk scale.

A Medtronic spokeswoman said the risk to patients is "minimal." Cracking in the 4002 wires' insulation will not result in total failure, but merely cause the pacemaker to skip a beat now and then, she said.

"They are trying to minimize a very serious situation," FDA and industry critic Charles Stein, a pacemaker patient, told us. "Medtronic and the FDA are living the big lie."

Stein, who runs the Pacemaker Recall Databank in Orlando, Fla., and publishes a pacemaker newsletter, wants warnings delivered to pacemaker patients rather than their doctors. And he questions why the FDA relies on manufacturers to issue warnings about their products.