Researchers at Cedars-Sinai Medical Center assisted in developing the computer programs that make sophisticated decisions in a new type of implantable cardioverter-defibrillator (ICD) that for the first time treats rhythm problems originating in the upper chambers of the heart, as well as those in the lower chambers.
With colleagues at several other cardiac centers in the United States and other countries, Cedars-Sinai physicians, directed by cardiologist and research scientist Charles D. Swerdlow, M.D., also conducted the ICD's first clinical study. This is the first study to demonstrate that an implantable device can reliably detect irregular heart rhythms for extended periods of time, up to days at a time.
To arrive at their conclusions and support their findings, Dr. Swerdlow and his colleagues devised a novel method of telemetering or "downloading" data from the implanted devices to external monitors. ICDs and other implanted electronic devices have limited data storage capacity, less than one minute per arrhythmia episode. This limitation is an ongoing source of frustration for physicians and researchers developing new devices and analyzing the software that makes them work.
To circumvent this problem, Dr. Swerdlow's team adapted external "Holter" monitors that are routinely used to track patients' heart rhythms. These special Holter monitors served as external storage units for detailed data about every heartbeat sent to them via telemetry from the implanted ICDs.
"This was the first study to use this integrated, hybrid approach to permit investigators to analyze the ability of an implanted device to detect rhythms continuously for extended periods of time, allowing us to know when arrhythmias were occurring and if the devices were missing anything," Dr. Swerdlow said.
The new ICD which has already been made available in Europe and is now awaiting final approval from this country's Food and Drug Administration uses more sophisticated detection software to accomplish several tasks that previously could not be performed.
In the past, all ICDs treated only life-threatening ventricular arrhythmias abnormal heartbeats of the lower chambers of the heart that can cause cardiac arrest. Within a few seconds of detecting a faulty heart rhythm, they administer a shock to the heart in an attempt to re-establish a normal rhythm and save the patient's life.
"Until now, defibrillators have only been required to detect abnormal rhythms for a few seconds because they were intended to treat ventricular arrhythmias that killed people rapidly. Therefore, if they detected a problem, they were obligated to treat immediately," according to Dr. Swerdlow, who has been involved in the development of the new ICD for the past five years.
Although the new device monitors ventricular signals, it also tracks the trickier rhythms of the heart's upper chambers the atria and is sufficiently sophisticated to decide whether a shock should be provided immediately, delayed or not given at all.
"It has always been more difficult to detect atrial signals than ventricular signals," said Dr. Swerdlow. "The atrial signals are of smaller amplitude or size, and they can be extremely small during atrial fibrillation (extremely rapid, irregular and disorganized rhythm of the atria). The ventricular signals are always present and they're bigger. The ICD has to keep track of the ventricular signals because while it is watching for atrial fibrillation, the device has to be prepared to respond if the patient has a ventricular rhythm disturbance at the same time."
Because atrial fibrillation often stops spontaneously after minutes or hours without causing serious symptoms or posing a threat to life the new ICD technology instantly assesses the type and severity of the abnormal rhythm. If a life-endangering situation exists, it provides the necessary shock. But because the shocks are uncomfortable for patients and can actually be dangerous in rare instances the device withholds treatment in non-life-threatening cases but continues to monitor the rhythms.
"If you have a serious arrhythmia and you're going to be dead in 20 or 30 seconds, you really can't schedule your treatment," said Dr. Swerdlow. "On the other hand, if what you have is uncomfortable palpitations and you don't want to get a shock in the middle of your board meeting or while you're giving a lecture, you can go home, lie down and activate the device."
This treatment flexibility results from the study's finding that the ICD can accurately detect arrhythmias over an extended period of time. Patients can program or manually trigger treatment at a time that is convenient for them.
"Therapy can be safely delayed without concern that a continuous arrhythmia will be misinterpreted as one that stops and starts. That allows people to go home and go to sleep and have the shock delivered in their sleep because for some patients it doesn't hurt as much when they're sleeping. Those who don't like to be awakened from sleep by a shock may self-administer treatment at a different time," Dr. Swerdlow said.
Developers of the new ICD, produced by Medtronic, a medical device manufacturer based in Minneapolis, faced a host of challenges. One was the data storage dilemma that was answered by the novel use of the Holter monitors. Furthermore, the device had to detect atrial fibrillation (AF) signals of various sizes or amplitudes, distinguishing those that spontaneously stopped and restarted from those that were continuous. For those that stopped and restarted by themselves, it should withhold treatment. For those that were continuous and needed intervention, it should deliver a shock, called cardioversion.
The device also had to discriminate between atrial fibrillation and atrial tachycardia or atrial flutter (AT), rapid, more organized heartbeats that should be treated by antitachycardia pacing a series of painless electrical impulses that slow down and regulate the heartbeat instead of cardioversion.
In addition, the implanted computer that monitors the signals and makes the decisions, as well as the treatment device that may be called upon to deliver a shock of several hundred volts (via a transformer), all has to operate from a power source consisting of two 2.8-volt batteries.
The results of the clinical trial of 80 patients showed that the ICD is sensitive and accurate, appropriately differentiating between atrial signals that truly start and stop and those that only seem to start and stop because the smallest signals occasionally escape detection. The results of the study were published in the Feb. 29 issue of "Circulation."
"Because you can't treat what you can't identify, detection is both the gun sight and the trigger lock of an ICD. It tells you what to target and it tells you when not to deliver a treatment," Dr. Swerdlow said. "We don't want people to get unnecessary treatment and on the other hand, we don't want to miss any necessary treatments because of underdetection. We don't want to miss arrhythmias that last a long period of time because episodes of atrial fibrillation that last long enough can cause blood clots to form in the atrium. If you shock a patient after too long and the blood is not adequately anticoagulated, there is a risk of stroke."
According to the journal article, during the period of Holter monitoring, the ICD was 100 percent accurate in detecting instances of atrial tachycardia/flutter and atrial fibrillation when they occurred. It was also extremely precise in avoiding inappropriate detection. During the 1,290 hours of Holter monitoring, only six inappropriate detections occurred a 99.99 percent ability to specify between true AT/AF episodes and similar rhythms.
Furthermore, of 166 episodes appropriately detected as atrial tachycardia, 45 percent were effectively treated by the device's antitachycardia pacing capability. In fact, one of the conclusions drawn from the study is that "discrimination of AT versus AF on the basis of the regularity of the atrial rhythm permits successful, painless antitachycardia pacing for some rhythms that would otherwise require painful cardioversion."
In addition to providing patients with greater treatment flexibility and control, the new technology offers a new level of safety assurance time to double-check the accuracy of detection, according to Dr. Swerdlow.
"Unlike treatment of ventricular arrhythmia, where you have to commit to treatment in 10 or 20 seconds, here the ICD can reassess the situation in five or 10 minutes. Obviously, you can collect a lot more data, cross-check it, and do better confirmations in five to 10 minutes than you can in 10 to 20 seconds. That permits what we call a higher degree of detection specificity making sure we only treat real arrhythmias, as opposed to misinterpreting electrical signals."
The new ICD is intended only for patients who have highly symptomatic atrial rhythm disturbances, with or without ventricular rhythm disturbances. In fact, according to Dr. Swerdlow, it is appropriate only for a small minority of patients diagnosed with atrial arrhythmias because many problems can be managed by medication alone.
Patients who currently have ICDs implanted to manage ventricular fibrillation will not benefit from this new device unless they have atrial arrhythmia also.
"All defibrillators have batteries that wear out, typically four or five years after implant," said Dr. Swerdlow. "Because ventricular defibrillators have been around for many years, there are thousands of patients each year whose defibrillators are being replaced. Therefore, in patients who have both atrial and ventricular arrhythmias, when a ventricular defibrillator needs to be replaced, this new ICD the first to treat both might be an option."
If the FDA approval process continues as expected, the Medtronic Jewel AF could be available in the United States in several months. Scientists at medical centers in Germany, the Netherlands, and the United States, as well as engineers at Medtronic, Inc., the manufacturer of the ICD, participated in the study, which was sponsored by the manufacturer.
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