Keeping the Beat

Imaging-guided technique for complex arrhythmias could save lives

» Web extraScars tell stories of battles fought and won. For many people, they are a source of pride, but when it comes to the heart, these scars are often a harbinger of troubles to come.
Read about how scar tissue is formed after a heart attack.

The leading cause of hospitalization in this country continues to be heart disease and stroke, and every seven minutes someone dies from one of them, according to the Heart and Stroke Foundation of Canada. One cause of cardiac arrest is triggered when the electrical circuitry of the heart malfunctions. This sparks an abnormal heart rate called an arrhythmia, which may make a heart to beat too slowly, too quickly or irregularly. Arrhythmias can cause blood flow in the brain and body to decrease, resulting in heart palpitations, chest discomfort or pain, dizziness, fainting or even sudden death. Some arrhythmias have no symptoms or warning signs and may not be serious, while others may be life-threatening.

A team of Sunnybrook Research Institute (SRI) cardiovascular scientists is investigating therapies and techniques that could save more people from dying of arrhythmic events. Two of them—physicist Dr. Graham Wright and cardiologist Dr. Eugene Crystal—are senior co-authors of research papers that aim to enhance certain areas of heart disease management and treatment; Wright is the technical lead and Crystal the clinical lead.

One of those papers, which was published earlier this year, revealed the findings of a study of 43 Sunnybrook patients who had received an implantable cardioverter defibrillator (ICD) after having a heart attack. A small device that’s placed in the chest, an ICD uses electrical pulses or shocks to help control arrhythmias, especially those that can cause sudden cardiac arrest. The patients, who were followed-up for as long as 46 months, with a median follow-up of 30 months, were seen every three months or sooner in cases where the ICD delivered shocks.

“Many patients with chronic damage to the heart, which usually develops after a heart attack, are at risk of sudden cardiac death when the heart stops and death occurs in minutes,” says Wright, who is the director of the Schulich Heart Research Program at SRI and a professor at the University of Toronto. “We’re trying to come up with more specific criteria to ensure that we’re putting an ICD in the patients who would most benefit from one.”

An ICD is an expensive device that costs tens of thousands of dollars; data from a few years ago in Ontario cite that 1,500 ICDs were being implanted every year, costing the health care system roughly $50 million annually. The current criterion for putting an ICD in patients is that the heart pumps about one-half or less of the normal amount of the blood to the body each cardiac cycle. That criterion isn’t foolproof, however, and cardiologists err on the side of caution by putting ICDs into some patients who may not actually need them, while missing some patients who might benefit.

“The problem is that we have no good tools to estimate who will die suddenly,” says Crystal, who is a specialist in electrophysiology, that branch of cardiology that focuses on the heart’s electrical system and diagnoses and treats arrhythmias. “Right now, nine out of 10 people who get an ICD may not need it for many years. Our hope is that in the long run, with larger studies that build on our research, we can tell which patients will benefit the most from an ICD.”

One cause of arrhythmia is the scarring of heart tissue from a prior heart attack. In the study, scar tissue was detected in all 43 patients. The research paper that Wright and Crystal co-authored examined the potential utility of cardiac magnetic resonance imaging (MRI) to identify patients with such scarring. “There is something in the scar that helps us understand who is more prone to having further arrhythmia events,” says Crystal.

Although there have been previous studies in this area of cardiovascular research, Wright and Crystal have developed more precise ways of measuring scar tissue. “We focused on developing a quantitative measure that more accurately describes how much scar and healthy tissue there is,” says Wright. “This measure appears more specific in predicting the arrhythmia events.”

There is a broader multicentre study being led out of Calgary that is testing the more conventional MRI approach and in which SRI will be participating. A substudy is also looking at the potential utility of the more advanced methods. “We’d like to be able to identify better who is going to have an arrhythmia event and who isn’t,” says Wright. “By developing more precise ways to measure the scar tissue, we hope to be able to save more lives.”

Another study on which Crystal and Wright collaborated aimed to take the above research one step further: to see if it was possible to prevent arrhythmia events from happening in the first place, so that an ICD wouldn’t need to be implanted. That meant trying to determine what was causing the irregular heartbeat events. There are myriad causes, including blocked arteries, high blood pressure, diabetes, and an overactive or underactive thyroid gland. This preclinical study focused on identifying the scarring of heart tissue from a prior heart attack.

Based on a 2012 survey led by Dr. Eugene Crystal of 19 EP centres:

  • EP centres in university-affiliated teaching hospitals: 17
  • Full-time EP specialists: 67
  • Part-time EP specialists: 21
  • People served by a Canadian EP specialist: 420,000
  • People served by a U.S. EP specialist: 127,500
  • Ablations performed annually: 8,041
  • Average ablations a Canadian EP specialist performs each year: 104
  • Ablations an EP specialist must perform each year as per the Canadian Cardiovascular Society to maintain their competency: 50

It had already been established that MRI is a powerful tool to detect scar tissue in the heart, but the existing technology isn’t without limits. “One of the challenges of using MRI is that the heart is a moving target because it’s beating, so it’s hard to get a clear high-resolution image,” says Crystal. A procedure called cardiac ablation can correct heart rhythm problems; it uses long flexible catheters or tubes inserted through a vein in the groin and threaded to the heart to correct the structural problems causing the arrhythmia.

Cardiac ablation works by scarring or destroying the tissue in the heart that’s triggering the abnormal rhythm. In some cases, it prevents faulty electrical signals from travelling through the heart, stopping the arrhythmia. Once the abnormal heart tissue that’s causing the arrhythmia is identified, the cardiologist will aim the catheter tip at the area of abnormal heart tissue. Energy will travel through the catheter tip to create a scar or destroy the tissue triggering the arrhythmia. In other cases, ablation blocks the electrical signals travelling through the heart to stop the abnormal rhythm and allow signals to travel over a normal pathway instead.

Although cardiac ablation can be successful the first time, some people need repeat procedures. “The problem is that it’s hard to identify those regions of scar where you might want to ablate,” says Wright. “Ideally, with the appropriate ablations, you get rid of the potential for further arrhythmia events. We want to put in an electrical signal that will burn or damage the suspect tissue, but we’re not always sure where that tissue is.”

Typically, ablation to prevent sudden cardiac death is only done in patients who have ICDs, and it often doesn’t work because the techniques aren’t accurate or specific enough. Crystal and Wright have developed a technique using MRI that seems to be able to identify where the damage is and the size of the scar; if so, then it could transform how such procedures are done. “We think that our measurement sees the ablated region based on temperature-sensitive changes in the state of iron in the blood and heart muscle,” says Wright. “It’s very cool physics.”

The preclinical study was first performed on non-beating hearts, then on beating hearts. To take it to a human-trial phase, the scientists need to be able to get to the point where they’re comfortable scanning patients with ICDs in MRI machines, which they hope will happen in the next couple of years. The cardiovascular research group has submitted one-half dozen patents for approval, including one for the new MRI technology. “SRI is a world leader in imaging-guided cardiovascular therapies,” says Crystal. “We have multidisciplinary collaborations with imaging specialists and experts in various areas of cardiovascular research.”

As for the potential impact of being able to use MRI in this way, the first group to benefit would be patients with ICDs who are experiencing regular arrhythmia events and shocks, which degrade their quality of life—they’re always anxious that a shock may be coming, because although the shocks are brief, they can be painful. “The goal would be to get rid of the patients’ stress and reduce the incidence of heart failure,” says Crystal.

Another ambitious but not unrealistic longer-term objective is not to have to put an ICD in a patient at all, because ablating the scar tissue would prevent further arrhythmia events from occurring. “If the procedure worked and you only had to do it once, then it would be a one-time cost to the health care system that would improve patient outcomes,” says Wright. “The MRI technique we’ve developed holds a lot of promise for more specific treatment.”

This research is funded by the Canada Foundation for Innovation, Canadian Institutes of Health Research, GE Healthcare, Mitacs, and the Ontario Ministry of Research and Innovation.

Dr. Eugene Crystal