ICDs could possibly deliver preventive electrical treatment after the onset of the warning T-wave alternans. Source: Canadian Heart Rhythm Society

By measuring stored data from implantable cardioverter-defibrillators (ICDs), researchers have learned that high-amplitude electrogram T-wave alternans often precedes life-threatening arrhythmias. Researchers hope ICDs might one day deliver preventive electrical treatment after the onset of the warning T-wave alternans, and just before the arrhythmias.

Previous studies have used T-wave alternans from the routine surface electrocardiogram to predict whether patients are at sufficient long-term risk of ventricular tachycardia (VT) or ventricular fibrillation (VF) to warrant implantation of an ICD.

“We use T-wave alternans in a different way,” lead author Charles Swerdlow, MD, told Cardiovascular Business News. “We ask this question: Can we predict in real-time which high-risk patients—those already implanted with a defibrillator—will have serious arrhythmias based on signals recorded from their ICDs?”

The short and preliminary answer is yes, Swerdlow said. But the researchers, from Cedars-Sinai Medical Center in Los Angeles and Medtronic in Minneapolis, had to overcome several technical hurdles before they could test their theory.

They first validated that T-wave alternans recorded from inside the heart represent the same phenomenon as those measured on the surface ECG. Then, the researchers devised a method to accurately analyze the shorter period of electrical data from the ICD that precedes arrhythmias, as compared to the easier analyzation of longer periods of available data from surface ECG, such as from a treadmill test.

Swerdlow and colleagues then demonstrated that the electrical processes of the ICD signal did not introduce artifacts that would falsely be characterized as T-wave alternans.

For this study, which appears in the May issue of Heart Rhythm Journal, the researchers sought to determine if there were any identifiable electrical patterns from the ICD data that preceded serious arrhythmias. They enrolled 10 patients after reviewing records of more than 400 patients with Medtronic ICDs. All were males, average age 68 years, with average left-ventricular ejection fraction of 0.29.

Researchers recorded pre-onset intracardiac electrograms for 10 to 15 seconds before detection of VT/VF, depending on the ICD model. For control data, six patients subsequently had a telemetry Holter recording for two minutes during baseline sinus rhythm and atrial pacing at 105 bpm.

Results indicated that 80 percent of the 101 analyzed T-wave alternans prior to VT/VF exceeded a threshold of 30 microvolts compared to only 11 percent of 207 sinus-rhythm controls and 2 percent of 345 atrial-pacing controls.

“Our data provide the first evidence that high amplitude intracardiac electrogram T-wave alternans immediately precedes spontaneous VT/VF in humans,” Swerdlow said. “The mean value of T-wave alternans prior to VT/VF exceeded that of control recordings by a factor of four to five.”

Swerdlow and colleagues should be “commended on their novel and systematic approach for measuring T-wave alternans in pre-onset electrograms from ICD storage and for an important pathophysiological observation,” Stefan H. Hohnloser, MD, from the section of clinical electrophysiology at J. W. Goethe University Hospital in Frankfurt, Germany, wrote in an accompanying editorial

There are two more stages to this study, Swerdlow said. The next step is to validate these findings in a larger cohort of patients. Researchers have been collecting data for more than a year and by the summer will be analyzing intracardiac electrograms from about 80 patients.

The second step is to implement software—which the researchers developed—in ICDs implanted in humans that will continuously measure T-wave alternans in real-time and determine whether it is actually detecting T-wave alternans and how often those T-waves predict serious arrhythmias, Swerdlow said.

“Such an approach might be able to warn patients to cease activities such as driving prior to VT/VF and to initiate adaptive pacing algorithms to prevent VT/VF,” he said.

If successful, it would be the first kind of preventive electrical treatment delivered by an ICD, Swerdlow said, adding that such a product is perhaps three to five years away.

The new ICD would require no new hardware, since it is software driven. Additionally, Swerdlow said that they developed the software so the computational techniques are efficient and do not require a lot of power.

“There will be a minimal amount of power drain,” he said. 

Medtronic provided no financial support for this study.