Small faults in small parts cause big concerns
Last week, the Food and Drug Administration and heart device maker Medtronic told doctors to stop using a particular component -- the wire lead -- of Medtronic’s latest generation of implanted heart defibrillators. Cardiologist Dr. William Maisel, a consultant to the FDA, explains what these devices do and what went wrong.
What are these implantable heart devices? Implantable heart devices come in two types. The more common device, a pacemaker, is designed for hearts that beat too slowly. It senses when a heart rate drops below about 60 beats per minute then administers a small jolt to the heart muscle to speed it up. The jolt is usually about 3 to 5 volts. Pacemakers are not affected by the announcement.
The other kind of device, an implantable cardioverter defibrillator (or ICD), senses when a heart is beating out of control and erratically at greater than 200 beats per minute. Defibrillators also send a shock to the heart muscle to correct the accelerated beating, but this shock is usually 750 volts -- more than six times the voltage of a wall outlet.
Pacemakers can adjust only slow heart beats, but defibrillators can detect and fix both slow and fast heart rhythms. Because they have a more complicated job, defibrillators are more complex devices.
Medtronic defibrillators, which account for about 50% of all defibrillators implanted in the U.S., use a wire that is the subject of the company’s voluntary “recall.” (Although it’s referred to as a recall, authorities aren’t suggesting removing the wires because that too poses risks.)
What is a “lead”? A defibrillator contains two parts: a computerized device that monitors heart rate and determines when to send jolts of electricity, and the lead, which is a wire covered in insulation that doctors thread through the veins. The lead stretches from the computerized device (which is usually implanted near the collarbone) to the heart, where it is secured into the heart muscle with screws.
The lead is a two-way conduit. Electrical signals from the heart travel through the lead to the device, and the device sends its shocks, when needed, back through the same lead.
In the past, leads have had a diameter of about 4 millimeters, but they are getting smaller. “The smaller the lead, the easier it is to implant and push through the blood vessels,” Maisel says. Medtronic’s is about 2.2 millimeters. Its size might be one reason the Medtronic lead is more likely to break than older leads.
Batteries power the devices but run down after five to seven years, at which point the device is replaced surgically. During replacement, the wire lead remains and is used for the next device because the lead is difficult to remove. After years sitting in a vein, blood vessel tissue grows around the lead.
If the first wire is bad, another wire can be threaded through. Although the old wire can be removed if there’s not enough room for both, 1% of the people undergoing such surgery die in the process, Maisel says.
What goes wrong with a heart device? Various things, Maisel says. In a 2006 study, he and colleagues found that of more than 2.6 million pacemakers and defibrillators implanted between 1990 and 2002, only 0.65% malfunctioned. About a quarter of the problems were due to battery issues, and a little more than a quarter were due to electrical problems.
The leads can go bad because the wire itself breaks (which is what happened in the Medtronic cases) or the insulation cracks. “What we wonder is how come they don’t break more often,” Maisel says. Not only do the wires have to handle 750 volts, but each time the heart beats, the lead moves. “That’s 500 million movements over the life of the device,” Maisel adds. “In a perfect device, that’s 500 million consecutive signals without error. It’s an amazing thing that we’re asking these wires to do.”
When a lead breaks or cracks, the device starts to receive electrical noise and thinks the heart is not beating right. A pacemaker might send a tiny shock to the heart, but with defibrillators, patients can really feel the power. “Some people describe it as being struck by lightning or kicked in the chest by a horse,” Maisel says.
Routine checkups can sometimes give clues that a device is malfunctioning. Doctors measure the size of the electrical signal coming from the heart, how much energy it takes to pace the heart and the impedance of the wire (how easily current flows through it). If measurements are off, doctors might take an X-ray to look for a wire crack. But more often than not, lead fractures are difficult to predict. “The lead breaks suddenly,” Maisel says.
What can be done? These devices are subject to a lot of bench testing, Maisel says, so it raises the question of how to improve that testing. In addition, the issue suggests better monitoring of patients is needed: “Why did we not find out about this until more than 200,000 devices had been implanted?”
Heart patients who have Medtronic defibrillators need be concerned only if their lead is a Sprint Fidelis lead numbered 6930, 6931, 6948 or 6949.
A small number of other defibrillators also use the Sprint Fidelis leads. If curious, a patient can examine the identifying information that came with the defibrillator. People who find they have a potentially problematic lead should ask their doctor how often they should come in for a checkup.
Medtronic has recommendations for physicians on how to program the device to detect potential lead failures and protect a patient from surprise shocks. They can, for example, make the device less sensitive so that if it does go bad it will be less likely to deliver a jolt.
Maisel says that in the grand scheme of things, the benefits of these devices outweigh the problems. Many people have defibrillators implanted and never have an erratic heartbeat, he says. Of 1,000 patients with Medtronic defibrillators, 100 will have their lives saved by it and 30 might have some lead problem.
“People are still better off with the device than without it,” he says.
