This week Medtronic will begin shipping to hospitals in the United States the first pacemaker approved by the FDA as safe for most MRI scans. For consumers, it is a significant step in what is expected to be a wave of new MRI-compatible implanted cardiac devices.

But this is an example of one technology chasing another and the one being chased, the MRI scanner, is changing and is a step ahead of the new line of pacemakers. The pacemaker approved for U.S. distribution is Medtronic's first-generation pacemaker with certain limitations, while its second-generation MRI-compatible pacemaker is already in use in Europe where approval for medical devices is not as demanding as it is in the U.S. So let’s check out what this is all about -- what it means now for current and future heart patients and where it may be headed.

We are all born with a natural pacemaker that directs our heart to beat 60 to 100 times a minute at rest. The pacemaker is a little mass of muscle fibers the size and shape of an almond known medically as the sinoatrial node located in the right atrium, one of four chambers of the heart. The natural pacemaker can last a lifetime. Or it can become defective. And even if it keeps working normally, some point may not function well along the electrical pathway from the pacemaker to the heart’s ventricles which contract to force blood out to the body.

Millions of people in the world whose hearts beat too fast, too slow, or out of sync because their own pacemaker is not able to do the job right, follow their doctors’ recommendation to get an artificial pacemaker connected to their heart to direct its beating. The battery-run pacemaker in a titanium or titanium alloy case the size of a small cell phone, (why can’t it be the size of an almond?) is implanted in the upper left chest, just under the skin, with one or two insulated wire leads connecting to the heart. It can be programmed to run 24/7 or to only operate when the heart reaches a certain state of irregular beating.

With some rare exceptions, there are few drawbacks to having a pacemaker. But you need to avoid powerful magnets. No problem being around your microwave or computer and when you talk on a cell phone, just hold it on the opposite side of your body from the pacemaker. But it’s best not to walk through airport security scanners, and instead submit to a body search and hand-held scanner. The biggest "no" that accompanied your pacemaker is to avoid any magnetic resonance imaging (MRI) exam. That may not have seemed like a problem at the time you agreed to get a pacemaker implanted. But we never know what turn our health may take. And there are many conditions for which doctors seek to learn more through an MRI scan that uses a very strong magnet, radio frequencies and a computer to produce images of soft tissue, bone, and blood vessels. Because it produces such detailed images of soft tissue, it is frequently used for organs of the body such as liver, bowel, pancreas, kidneys, brain and spinal cord. The MRI, which does not require ionizing radiation, is also used in diagnosing and staging cancers and pinpointing heart problems.

But, only a few medical centers in the United States and maybe 10 worldwide have the ability to perform MRIs on patients with the pacemakers now in use. Most people wearing pacemakers who need an MRI have been unable to get one. Pacemaker companies advise against it and the American Heart Association issued a statement in 2007 that discouraged doctors from performing MRIs on pacemaker patients unless the need for the MRI was compelling enough to warrant the risk of the exam. Medicare will not pay for an MRI done on a patient who has a pacemaker. There have been a few reported deaths of pacemaker patients who got an MRI and the risks include movement of the pacemaker and re-setting of the beating pattern. The wire leads are insulated except for the exposed metal tips that screw into the heart and under the tremendous vibrating of an MRI scanner that goes on for 30 minutes to an hour and a half during the course of an exam, the tips can heat up to the point that they can burn the flesh.

So getting a pacemaker that is safe to use during an MRI is a big deal for heart patients. "This is an important step in the right direction," says Dr. Bruce Wilkoff, president-elect of the Heart Rhythm Society and Director of Cardiac Pacing and Tachyarrythmia Devices at the Cleveland Clinic. Wilkoff was the primary investigator for the EnRhythm MRI clinical trial that was the basis for the FDA’s approval of the first U.S. MRI-compatible pacemaker and he presented the study’s findings to the FDA. The study involved 464 patients in an international multicenter trial. Changes were made to the pacemaker, its leads, and the programming system, Wilkoff said, to address potential for interfering with the device’s pacing, overstimulating the patient, and overheating the tip of the leads.

Revo’s Limitations

However, this Medtronic first-generation Revo MRI SureScan Pacing System has some significant limitations.

1. The MRI-pacemaker is for new heart patients. Patients who already have a pacemaker can not get this new model unless they undergo the risky procedure of having their old pacemaker completely removed. Usually, when it comes time to replace the battery in a pacemaker (about 5-7 years), the metal case containing battery and circuitry is detached from the leads, and a new model device is hooked up to the leads. But doctors generally consider it too risky to remove the old leads from the heart for fear of tearing the heart or the veins through which the leads are inserted into the heart. Part of the design of the Revo pacemaker is its new leads and so they must be the leads that connect the pacemaker to the patient’s heart.

2. Patients must have the Revo pacemaker implanted for 6 weeks before receiving an MRI.

3. The Revo pacemaker requires a certain position of the patient inside the MRI tube so as to avoid most chest scans. This is to prevent overheating the metal tips of the leads that are attached to the heart. So heart scans are forbidden with this first generation model.

4. And Owen Faris, senior scientific reviewer for the FDA, explains that the new pacemaker won’t work for all types of MRI scans and won’t work in all MRI scanners. In his words:

In addition to the chest scan exclusion, there is a restriction on how much radio-frequency energy can be deposited into the body by the scanner. MRI scanners have two operating modes for most clinical applications. ‘Normal operating mode’ is how the scanner is normally programmed and that mode restricts the scanner to lower-energy scans (less than 2 Watts per kilogram). This is sufficient energy for most clinical MRI scans. However, for some patients and for certain scans, more power is needed. In those cases, the MRI scanner is placed in ‘First level control’ mode, which allows for greater energy deposition (up to 4 Watts per kilogram). For patients implanted with the REVO MRI pacemaker, those patients are not allowed to have these higher energy scans.

MRIs for these patients are also restricted to only allow use of 1.5 tesla MRI systems. "Tesla" is a measure of the strength of the magnetic field.

5.  Medicare does not now pay for MRI scans on a patient who has a pacemaker. Medtronic spokesperson Wendy Dougherty says that the company will not speculate on whether Medicare will cover MRIs done on patients wearing the new Revo pacemaker. The federal agency is considering a petition from a physician to cover MRIs done during an investigational study to determine the risk of MRIs involving pacemakers already in use. During the comment period for this request, Medtronic asked Medicare to restrict MRI payment to patients wearing pacemakers approved by the FDA for use with MRIs. Medicare's decision is due by March 1. Patients on Medicare would be wise to check on whether their plan will pay for an MRI before getting the test, which costs between $1,600 and $3,500 at different medical centers and offices.*

What to expect in the future?

Approximately 1.5 million Americans have cardiac pacemakers. Worldwide, 5 million people are implanted with a pacemaker or implantable cardioverter defibrillator (ICD) that shocks a chaotic shaking heart into normal working order.

Europe, where approval process of medical devices is less strict than in the U.S., began using Advisa, the Medtronic second generation MRI-compatible pacemaker, early last year. The Advisa has no restrictions on chest scans and Medtronic says this model is in clinical trials in the U.S.**

Cost of a pacemaker is $5,000 to $10,000 (just for the device, not counting the much larger charge by hospital and physicians for implanting it) and Medtronic says the Revo pacemaker will be in that range. Tao Levy, Collins Stewart market analyst, says that the Revo will cost about $1,000 more than a standard pacemaker and will boost Medtronic’s revenue in the U.S. by $50 to $60 million. Boston Scientific and St. Jude Medical are developing MRI-compatible pacemakers and other implanted devices and as other companies bring these products to market, the use is expected to grow.

But "not over the near-term," says Levy. "In a recent survey of electrophysiologists (who put pacemakers into people), the expectation is that Medtronic’s MRI-conditional pacemaker would comprise around 20% of the pacemaker implants. So the vast majority will still be regular pacemakers. Over time this might change."

"Expense and the need for proprietary leads and the coordination of personnel may prohibit widespread adoption," says electrophysiologist Dr. Westby Fisher, clinical associate professor of medicine at University of Chicago’s Pritzker School of Medicine. He is a member of the Speaker’s Bureau for both Medtronics and Boston Scientific. "Monitoring in the MRI by EKG (an electrocardiogram of the heart) is required and not all MRI machines are equipped for this feature." Using a scanner that does not have EKG capability will call for scheduling a technician or a company representative to be present when the lengthy MRI test is performed, he says, which will mean "scheduling challenges due to logistics of having the right people in the right place at the right time."

The technology of pacemakers is chasing the technology of MRI scanners. "Right now the issue is that Medtronic’s device is designed only for 1.5T powered MRI systems and there is a move to higher powered MRI machines (3T) for which the Revo is not indicated," Levy says. The 3T MRI scanners produce clearer images in less time.

Nevertheless, cardiologists and electrophysiologists and market analysts think that MRI-safety will spread beyond standard pacemakers. "I think eventually all implanted cardiac devices will be MRI compatible," says Dr. Henry Halperin, a Johns Hopkins professor of medicine who is Director of the Cardiology Bioengineering Laboratory. Not that Hopkins is in any hurry. It is one of the very few centers in the United States and in the world to run MRIs on patients with standard pacemakers and has done over 800 with no complications, Halperin says. To accomplish this, he says they re-program the patient’s pacemaker into a "safe mode" so that the force of the MRI can’t make the pacemaker send out electrical impulses that are too fast or too slow, and they reduce the MRI scanner’s power by about half. But he acknowledges that "there are still risks" and each decision to perform an MRI on a patient with a pacemaker comes down to this point: "The risk of doing the scan vs the risk of not doing the scan." That’s why Halperin says "All pacemakers should be MRI-compatible and I think they will be."

"I expect MRI compatibility will become more commonplace in implantable cardiac devices and may become the standard," agrees Dr. Thomas Callahan, an electrophysiologist at the Cleveland Clinic. "A lot of people are hoping an MRI-compatible ICD will come along."

"That is the plan," Levy says, both for ICDs and for a special kind of pacemaker used for cardiac resynchronization therapy (CRT) which resynchronizes a left ventricle that is not beating normally. "But in order to be successful, they will need to avoid any disadvantages (like a stiffer lead) and not have restrictions that might be confusing (having an MRI-safe pacemaker, but not realizing that you are getting scanned in a more powerful machine than the pacemaker was originally tested in, designed for.")

So, it seems, a good start, but there is lots of technical development to go. Will MRI-safe pacemakers eventually replace all other pacemakers?

"Most likely," says Wilkoff. "But the transition will take up to 10 years or so."

*Clarification (2/23/11): This paragraph has been edited since posting to present additional information about the prospects of Medicare coverage of MRI scans for patients with pacemakers.

**Editor's Note (2/18/11): After this blog was posted, the Medtronic spokesperson who initially said clinical trials for the Advisa pacemaker are underway in the U.S. now says, "Medtronic does not have any FDA-approved clinical trials in the U.S. for Advisa.


About the Author: Mary Knudson is a health journalist and co-author of Living Well with Heart Failure, the Misnamed, Misunderstood Condition. She writes the blog Heart Sense and teaches Writing Health Stories for the Public at the Johns Hopkins School of Medicine. You can follow her on Twitter as @maryknudson.

The views expressed are those of the author and are not necessarily those of Scientific American.