Thursday, March 23, 2006

Microvolt T-wave Alternans Approved - Now what?

On 21 March 2006, the microvolt T-wave alternans as a reimbursible test to screen patients with reduced heart muscle function that might be at high risk for ventricular arrhythmias that can lead to sudden cardiac death.

So what is "microvolt t-wave alternans testing (MTWA)?" Well, the test is performed by placing special high-resolution electrodes designed to reduce electrical interference on a patient's chest prior to a period of controlled exercise. These electrodes detect TINY beat-to-beat changes in a portion of the EKG (the "T-wave") that represents a "resetting" current after the heart is stimulated. The changes are in the order of one millionth of a volt ("microvolt" range). A special mathmatical method of detecting these changes called "spectral analysis" is used to measure the timing change and amplitude change of the electrical signals. A positive test was present if the MTWA onset heart rate was <=110, negative if the maximum MTWA onset heart rate was >=105 and all others considered indeterminate. Indeterminate tests occurred because of noise (slightly less than 10% of the time), skipped heart beats (ectopy), inability to achieve a heart rate of 105, or inability to sustain t-wave alternans during testing.

One of the better recent prospective studies to evaluate T-wave alternans was recently published by Bloomfield et al. in the Journal of the American College of Cardiology (Vol 47, No 2, 456-63): "Microwave T-wave Alternans and the Risk of Death or Sustained Ventricular Arrhythmias in Patients with Left Ventricular Dysfunction." This is a follow-up investigation that expands the applicability of the test published in Circulation in 2004 which looked at just ischemic cardiomyopathy patients (weak heart muscles from coronary disease). In Bloomfield et al.'s recent study, 549 patients were studied, only 1/2 of which had ischemic coronary heart disease. In this study, either positive or indeterminate tests were lumped together and compared to a negative test. Bottom line: A negative test means there is a 2.5% chance of death or arrhythmia in 2 years, vs a 12.3% (positive test) to 17.5% (indeterminate test) risk of death or cardiac arrhythmia.

What was interesting was the numbers.... 549 patients (average ejection fraction 25%), 189 had normal (negative) tests (with 4 events), 360 patients had abnormal MTWA tests (with 47 events). "Abnormal" MTWA tests included either positive (162 patients) or indeterminant (198 patients) tests.

So will ICD implant rates in patient's with low EF be decreased if this test is widely deployed? I don't think so. Given the LARGER total number of patients with positive or indeterminate tests, relative to the number of patients with negative tests in this study. If this test is widely implemented in stress labs, while the patients with negative tests might be spared an ICD, the increased screening using this test might actually INCREASE the number of implanted devices, since far more patients will have positive or indeterminate tests AND CMS seems to be willing to pay for the testing.....

I guess the next question is will smaller cardiology offices be willing to shell out the $30,000 to purchase this machine and the $75 for electrode patches per test? You have to do a lot of T-wave alternans testing to see a return on that investment if the CMS reimbursement rate is low....


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