MRI-guided catheter ablation

December 10, 2000

Biomedical engineers at the Johns Hopkins University have extended magnetic resonance imaging (MRI) into cardiac surgery for the first time with a new procedure to help prevent rapid and irregular heart rates.

The novel technique uses MRI instead of X-rays to guide radio frequency cardiac ablation, a minimally invasive surgical procedure that blocks abnormal electrical pathways in heart muscle. The MRI technique makes catheter-based intervention more accurate and reliable, which should reduce overall costs, and eliminates the current hazard of radiation exposure.

MRI has been used primarily for diagnosis throughout the human body, and is increasingly being used to guide therapy. Until now, however, MRI-guided therapies have not targeted the heart. The new approach, says lead investigator Albert Lardo, Ph.D., represents "a significant shift away from X-ray-guided techniques and establishes the feasibility of performing cardiovascular intervention under MRI guidance." Lardo presented some of his findings at a scientific conference of the American Heart Association in New Orleans last month. The new technique was also described in a recent issue of the AHA journal Circulation.

In recent years, radio frequency (RF) cardiac ablation has become a standard therapy to treat basic types of arrhythmia, or abnormal heart rate. The procedure benefits about one out of every 13 arrhythmia patients each year in the United States. During RF ablation, doctors make a small incision in the arm or leg and insert a special electrode catheter, essentially a long, flexible wire, through an artery to the heart and close to the trouble spot. As radio frequency energy passes through the catheter, the tip of the wire heats up and burns the heart tissue, creating a small, elongated lesion. The lesions and subsequent scar tissue, which do not conduct electrical impulses, essentially form roadblocks for the abnormal impulses responsible for the arrhythmia.

One limitation of X-ray-guided RF ablation, however, is that it is difficult to confirm the optimal location to place the lesions. These optimal conditions are essential to the surgery's success; if the lesions have gaps, are too thin or are misaligned, the problematic electrical impulses could eventually reroute themselves, and the arrhythmia could return.

"X-ray is inherently a two-dimensional projection imaging technique," says Lardo, "which means that there is a lot of anatomic ambiguity. It's very difficult to know in 3-D where you are." Also, as the lesions are made, X-rays are deficient at distinguishing between the lesions and the heart tissue.

"Although we have bioelectric feedback, we're just kind of shooting in the dark," he says.

Not surprisingly, arrhythmia reoccurrence rates are high, says Lardo. Physicians may try to tilt the odds in their favor, sometimes creating as many as 50 to 60 lesions in only a 2- to 3-millimeter space, "but the goal is to make as few lesions as possible and still achieve conduction block," he says.

"With MRI, you can overcome all those limitations," says Lardo. "We know precisely where the catheter is so we gain a lot of confidence in being able to put the lesion in the right spot. Not only that, but we have a way to monitor the formation of the lesion s it happens, and watch it grow. If the image tells us that the lesion isn't big enough, we can go back to the same spot and deliver another burn."

Fewer and more precise and effective lesions should reduce the overall costs of the surgery, says Lardo. The time to perform the procedure should be reduced, and the need for a second procedure nearly eliminated.

MRI is generally considered a safe technique because no X-rays are used, thus eliminating any radiation exposure to the patient and physician. Also, the electromagnetic fields do not, by themselves, cause tissue damage. But despite MRI's advantages, Lardo had to overcome several hurdles to develop the new technique.

For one, the catheter could be made from only nonmagnetic materials, such as gold, platinum or copper. Nonmagnetic materials not only help reduce image distortion, but because conventional MRI uses a monstrous electromagnet, any magnetic components "would be pulled out of your hand," says Lardo.

Another problem was signal interference. When creating lesions, the ablation catheter emits radio frequency energy. At the same time, the MRI system is acquiring an image of the heart. "MRI is based on radio frequency energy," says Lardo, "so there is indeed a problem with interference."

One solution was to perform the ablation and imaging in a staged fashion, one after the other. But to take full advantage of the technology, Lardo and his team developed imaging filters that will allow both steps to be performed simultaneously.

The technique also relies on a catheter antennae. By placing the antennae next to the ablation coil in the catheter, it improves the resolution and helps eliminate the noise.

The next step is human clinical trials, which await approval from the Food and Drug Administration.
-end-
The research was supported in part by grants from The Whitaker Foundation, the American Heart Association, the National Institutes of Health, and Surgi-Vision, Inc.

Whitaker Foundation

Related Cardiac Surgery Articles from Brightsurf:

New surgical tools with smart sensors can advance cardiac surgery and therapy
Researchers developed a new class of medical instruments equipped with an advanced soft electronics system that could dramatically improve the diagnoses and treatments of a number of cardiac diseases and conditions.

How commonly do patients develop persistent opioid use after cardiac surgery?
A large, national database was used to determine how common it was for patients who hadn't used opioids before undergoing a coronary artery bypass grafting or heart valve procedure to subsequently develop persistent opioid use after surgery.

Preoperative levels of heart proteins may help predict death or cardiac complications in patients undergoing noncardiac surgery
Assessing preoperative levels of N-terminal pro-B-type natriuretic peptide (NT-proBNP), a biomarker of heart stress and structural changes, may improve risk prediction beyond the clinical risk score in patients having noncardiac surgery.

Study finds connection between cardiac blood test before surgery and adverse outcomes
The VISION study looked at whether levels of a cardiac blood test, NT-proBNP, measured before surgery can predict cardiac and vascular complications.

Mayo Clinic study: 20% of patients are prescribed opioids after cardiac device implantation surgery
One in five patients is prescribed opioids after having a pacemaker or similar device implanted, according to a large US study conducted at Mayo Clinic published in HeartRhythm.

Careful monitoring of children following cardiac surgery may improve long-term outcomes
In a medical records study covering thousands of children, a US-Canadian team led by researchers at Johns Hopkins Medicine concludes that while surgery to correct congenital heart disease (CHD) within 10 years after birth may restore young hearts to healthy function.

Chest cavity fire during emergency cardiac surgery
At this year's Euroanaesthesia Congress (the annual meeting of the European Society of Anaesthesiology) in Vienna, Austria (1-3 June), doctors present the unique case of a man who suffered a flash fire in his chest cavity during emergency heart surgery caused by supplemental oxygen leaking from a ruptured lung.

Steroid use during cardiac bypass surgery did not reduce risk of severe kidney injury
Using steroids during cardiopulmonary bypass surgery did not reduce the risk of acute kidney injury in people at increased risk of death, according to a study conducted in 18 countries published in CMAJ (Canadian Medical Association Journal).

Cardiac surgery for patients with persistent opioid use associated with higher rate of complications, increased costs
Persistent use of opioids by patients is a public health concern in the United States but not much is known about the effect of that use on patients undergoing cardiac surgery.

Blood thinner significantly reduces the risk of death after non-cardiac surgery
The study enrolled 1,754 patients in 19 countries, 51 percent of whom were male, with an average age of 70 years.

Read More: Cardiac Surgery News and Cardiac Surgery Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.