Two women bled to death when their pacemaker cables snapped, puncturing their hearts. Now doctors are struggling with what, if anything, to do for 42,000 others around the world who carry identical wires inside them.

The overwhelming concern is that more people will suddenly die. But experts are uncertain how best to prevent this, since replacing wires may be riskier than leaving them in.

``The problem is that we don't know in whom this will happen, we don't know when, and we don't even know the best way to take them out,'' said Dr. Bernard Goldman of Sunnybrook Health Science Center in Toronto.

At issue is the Accufix atrial ``J'' pacemaker lead, models 701 and 801, made by Telectronics Pacing Systems Inc. of Englewood, Colo. The lead is the electrical cable that delivers impulses to the heart to keep it beating regularly.

Telectronics' variety has been among the most popular since its introduction in 1987.

The J-shaped end attaches to the heart like a fish hook. Inside its polyurethane insulation is a coiled electrical wire, plus a 3 1/2-inch flat wire that helps the lead keep its shape.

When this piece of flat wire is bent back and forth, as may happen when people move about, it can break. Then the jagged end may work its way through the plastic. At the worst, it may then poke through the heart and jab the aorta, the main blood vessel, causing catastrophic bleeding.

So far, Telectronics knows of four instances in which people's aortas were punctured this way. Two died, while the other two survived after surgery. All four were women.

Telectronics recalled any unused wires last fall. In a letter in November to 44,500 doctors, Telectronics noted there is about a 3 percent risk of death or life-threatening complications when taking out a pacemaker lead. The manufacturer has not tried to contact patients directly.

``While the risk of extraction is small, it is clearly greater than the risk of potential failure of this lead, based on current information,'' the letter said.

Physicians hope they will get more guidance from a ``Dear Doctor'' letter that Telectronics plans to mail within a week or two after review by the Food and Drug Administration. Until then, most specialists seem to be telling their patients to wait until they know more.

Patients have generally taken the news calmly, they say. Few, though, are as well informed as Dr. Joseph Alpert of Newark, N.J., who got one of the suspect wires with his pacemaker four years ago.

``Right now, there is no indication I have a problem, so I'm sitting tight,'' said Alpert, a vascular surgeon. ``I know this can be monitored. I know it won't happen overnight. Do I like it? No, not particularly, but I don't think about it.''

Alpert's doctor is also his partner, Victor Parsonnet of Newark's Beth Israel Medical Center. Like many other surgeons and cardiologists around the world, Parsonnet began summoning his patients after Telectronics recalled the unused wires.

All pacemaker recipients receive a card with product information. From that, a doctor can tell whether a patient has one of the suspect leads.

``People have to understand that not every wire that's in place is at risk of breaking,'' Parsonnet said.

Like many other doctors, he has X-rayed all his patients with this lead. Of several hundred he has seen, five are broken. He has removed three of the broken leads so far.

However, doctors are uncertain whether the wire should be removed by opening up the chest or by threading instruments through an artery.

Recalls of faulty medical devices are routine, but devices that fail are hardly ever so dramatically life-threatening.

``What makes this different is the potential of perforation of the heart or aorta or both with hemorrhage,'' said Dr. Douglas Zipes of Indiana University. ``Patients are at risk of death in a very precipitous fashion.''

Telectronics' upcoming letter will be based on the conclusions of an expert panel that met a week ago. The manufacturer has refused to say what the letter will recommend, but experts doubt it will be wholesale removal of the leads.

Not everyone agrees that broken wires should be taken out. Looking back at old X-rays, Goldman found one patient has had a broken lead for two years with no ill effects.

Some speculate that broken wires are dangerous only at the moment they pop through the plastic insulation. If they don't hit anything at that moment, they may be unlikely to cause harm later.

``It's a very serious problem,'' said Dr. Andrew Epstein of the University of Alabama. ``Lives have been lost from it, and others will be lost from it. But whatever we do to patients carries a risk, including doing nothing.''

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Telectronics has set up a toll-free number for information: 800-349-9446.