Besides maintaining optimal electrical stimulation to the heart, these CRT-D (cardiac resynchronization therapy with defibrillation) devices are giving cardiologists a new view of subtle changes in a key measurement of heart health -- heart rate variability. Patients with little variability -- whose hearts are unable to appropriately react to external stimuli by regulating their beating action -- are known to be at higher risk of suffering a heart attack. The new insight from CRT-D devices is possible because they record detailed data on heart function 24 hours a day.
These new findings are important for two reasons, the researchers said. First, the collected information appears to more accurately identify high risk patients who would benefit from early and aggressive therapy. Secondly, the devices provide cardiologists with objective information about the health status of their patients, information that can be frustratingly difficult to obtain in a typical clinical setting in this medically diverse group of patients, the researchers said.
The results of the study were presented by Duke cardiologist Roosevelt Gilliam, M.D., March 7, 2005, at the annual scientific session of the American College of Cardiology in Orlando. "When you talk to heart failure patients, many times their perceptions of how they feel do not match with their actual clinical status, which can make it difficult for cardiologists to get a true idea of how the disease is progressing," said Gilliam, chief of electrophsysiology at Duke. "This study shows that changes in heart rate variability just might be better in picking out those people at highest risk."
In their analysis of 1,411 heart failure patients who received the CRT-D device, the researchers found a strong correlation between the heart rate variability changes within two weeks of implantation and mortality one year later.
"This approach may play a significant role in targeting a subset of heart failure patients for whom we need to be more aggressive if we don't see early improvement in heart rate variability," Gilliam said. "We would expect that as their heart function improves after implantation, their heart rate variability would improve as well."
Heart failure is a condition marked by the inability of the heart muscles to pump enough oxygen and nutrients in the blood to the body's tissues. Also known as congestive heart failure, its many causes include infections of the heart, coronary artery disease, high blood pressure, previous heart attacks and valve problems. An estimated 4.7 million Americans suffer from the condition, with 400,000 new cases reported each year. Roughly one-half of patients die within five years of diagnosis.
Although there is no cure, drugs can improve the strength of the heartbeat (digoxin), relax blood vessels (ACE inhibitors)or remove the excess buildup of fluid in the lungs (diuretics). Implantable devices are the latest options in preventing arrhythmias that can lead to sudden death, with CRT-Ds being the latest technology.
CRT-Ds perform two main functions. First, the devices electrically stimulate both sides of the heart in coordinated fashion, which optimizes the contractability of already weakened heart muscle. Second, the defibrillator "shocks" the heart back into normal rhythm whenever the heart beats irregularly.
In addition to maintaining the heart, the devices also collect heartbeat-by-heartbeat data that can be downloaded from the device and entered into a computer. One use of the data in this trial was to create a "footprint," or visual representation, of a patient's heart rate variability over time. Patients with a footprint of 30 percent or less in heart rate variability were almost two-and-half times more likely to die after one year.
"A person with normal heart rate variable would have a very wide footprint, while those with sick hearts would have a narrow footprint that represents the reduced variance in heart rate response," Gilliam explained. "These footprints provide us with an unbiased - distinct from the patient's perception - look at how the patient is truly doing. It is an objective finding that you can look at and easily understand."
Gilliam pointed out that the cardiologists are "in uncharted waters" when it comes to the new devices, their role in the treatment of heart failure, and how to make best use of the new data which they can provide. Additional trials will be needed to tease out the relationships between different patient characteristics, the use of the new technology, and patient outcomes, Gilliam said.
The CRT-D devices were developed by Guidant Corp., Indianapolis, which funds the Heart Failure-Heart Rate Variability Registry, an ongoing national study. Gilliam has no financial ties to Guidant.