|
|
News |
|
|
|
|
|
|
T-wave alternans test may predict ICD candidates
most, least likely to need devices
|
January 8,
2007 |
|
Washington,
DC - The microvolt T-wave alternans (TWA) test
(Cambridge Heart, Bedford, MA) does more than identify primary-prevention
implantable cardioverter-defibrillator (ICD) candidates at highest risk for
sudden death (SD)—it can predict which of them won't gain much from having a
device, according to a large cohort study [1].
Abundant
evidence suggests that a "nonnegative" TWA finding points to an
elevated SD risk that may be lowered with an ICD, but the new study's authors
say it's the first to show the important flip side of the screening equation:
that ICDs probably won't prolong life for patients who are high risk by
traditional criteria but pegged as lower risk by the noninvasive test. Unlike
some prior TWA studies, the current one enrolled only patients with ischemic
heart disease.
Current
eligibility criteria for primary-prevention devices are widely seen as so
encompassing they could put an untenable burden on reimbursement resources.
"The challenge for policymakers and clinicians alike is to find effective
risk-stratification strategies that further define which patients are most and
least likely to benefit from ICD therapy," write the authors, Dr
Theodore Chow (Ohio Heart and Vascular Center, Cincinnati) and associates.
"Ideally, such a strategy would identify patients who receive little to no
benefit, thereby making the intervention more cost effective when implemented
and allowing society to lower costs without sacrificing life," they write.
"Our
findings suggest that [microvolt] TWA indeed may identify such a low-risk
subgroup, with as many as one third of patients deriving minimal benefit from
prophylactic ICD implantation."
The
group's findings appear in the January 2/9, 2007 issue of the Journal of the
American College of Cardiology.
|
|
||||||||||
|
|
|||||||||
|
|
||||||||||
The
TWA test monitors for minute beat-to-beat variations in the
electrocardiographic T-wave during exercise stress. According to convention,
"nonnegative" findings combine those that are either positive or
"indeterminate" for TWA. Both results are considered predictive of
increased risk. The test, performed using Cambridge Heart's ECG-analysis
algorithm for the risk stratification of ICD candidates, is reimbursable under
Medicare and other third-party payers but not considered essential to the
screening process.
Coauthor
Dr Paul S Chan (University of Michigan, Ann Arbor) told heartwire that
the large sample size and 51% ICD prevalence gave the analysis enough
statistical power "to study the benefit of actual ICD therapy within
subgroups of TWA findings." The prospective study, he said, began before
the release of the SCD-HeFT data that led directly to expanded,
LV-function-based primary-prevention ICD eligibility criteria.
Of
the group's 768 patients with an LVEF <35% and no history of
sustained ventricular arrhythmia, 67% tested nonnegative for TWA. As previously
reported by heartwire, a 2006 analysis found that the cohort's nonnegative
patients had more than twice the adjusted risk of both all-cause and arrhythmic
death over a mean of 18 months compared with those testing negative [2]. In
the new analysis, which follows the cohort for another nine months, ICD therapy
was significantly and independently associated with decreased mortality among
nonnegative patients but not those who tested negative for TWA.
Multivariate
analysis, HR for outcomes associated with ICD therapy compared with no ICD
therapy
|
Hazard ratios adjusted for propensity
scores, drug therapies, and a range of demographic and clinical
characteristics.
*p=0.003
†p=0.005
To download table as a slide, click on
slide logo below
To
reduce statistical confounding, risks were adjusted in part through the use of
propensity scores based on variables, mostly from ECG and electrophysiologic
tests, which were considered likely to influence decisions to treat with ICDs,
according to Chow et al.
In an
accompanying editorial [3], Drs Andrea M Russo and Francis E Marchlinski
(University of Pennsylvania Health System, Philadelphia) observe that
"inserting ICDs in all patients who currently meet guidelines based on LV
function leads to implantation of devices in many patients who will never need
them." Based on the current study, they write, the TWA test "appears
promising in predicting patients who might be most likely and least likely to
benefit from ICD therapy."
They
continue, "Although [microvolt] TWA testing has a low specificity and
predictive accuracy, there is a high negative predictive value. Patients with
reduced LV function who are being considered for prophylactic ICD implantation
but have a negative [microvolt] TWA test might avoid, or at least postpone, ICD
insertion."
Interviewed,
Chan echoed the editorial's call for more definitive data before the test could
be recommended for routine use. The findings, he said, "need to be
validated in subsequent cohort studies and probably in actual randomized
trials. I think we've reached the point where people agree that T-wave
alternans [screening] can identify patients who are high risk or low risk. The
real question now is whether we have clinical equipoise to perform a clinical
trial randomizing patients who screen negative to receive defibrillators or
not." That step will ultimately be necessary, he said, for the test
"to really gain clinical acceptance in the cardiology community."
|
