| In several situations, the increased
information provided by Intravascular Ultrasound literally can
change the
picture of the disease, and
affect treatment decisions. For example, in a normal artery the
intimal layer is thin -- when measured, there is little difference
between the diameter of the lumen (open channel) and the diameter
of the media (the arterial wall). In a "blocked" or diseased
artery, the intima is thickened by plaques or other tissue growth,
and the lumen diameter is reduced.
But often the plaque or tissue growth is
not evenly distributed, resulting in an eccentric shaped lumen.
This eccentric shape is
clearly shown by intravascular ultrasound. But the X-ray angiogram
only shows a "side-view" and the eccentric shape is not
seen. Depending on the angle of view, this may make the artery
look more blocked than it really is -- or conversely, may give
a false impression that the artery is only slightly blocked and
does not need to be treated. With IVUS, just a few clicks on the
console measures the area of the blockage, the size of the artery
and yields an accurate percentage of narrowing.
Another example is that sometimes the plaque pushes deeper into
the vessel wall, giving the appearance that the lumen is not significantly
blocked. Yet a significant amount of diseased plaque may exist
within the arterial wall, ready to rupture and cause a cascade
of events, resulting in a heart attack. This is called vulnerable
plaque, and cannot be visualized using standard angiography.
When is IVUS Done?
Intravascular ultrasound is done in the catheterization laboratory
in conjunction with angiography. Some cardiologists use it occasionally,
in difficult cases, or to assist in the selection and sizing
of stents and balloons. Others use it routinely, to confirm accurate
stent placement and optimal stent deployment.
How Can IVUS Make Stenting More Accurate?
One of the causes of stent thrombosis or restenosis is poor "stent
apposition" -- the stent has not been expanded to the full
width of the artery, and this under-expansion creates a "pocket" which
can collect platelets and other debris, causing a reblockage. Research
conducted using IVUS has confirmed that one of the causes of restenosis
is inadequate dilatation; that is, physicians, concerned with injuring
or dissecting the artery with a balloon inflation, have tended
to "play it safe" and end up under-sizing or under-inflating
the balloon and stent.
With the accurate measurements of both
the true diameter of the artery and the diameter of the open
lumen channel provided by IVUS,
the guesswork is taken out of choosing the correct size balloon
and stent. Using only angiography, a cardiologist may underestimate
the size
of a diseased artery.
IVUS can also measure the length of the diseased area, so the
precise length of the stent needed can be determined ahead of time,
reducing the need for overlapping stents which are known to increase
the risk of thrombosis.
Once the stent has been implanted, IVUS
can clearly show the stent struts in relation to the arterial
wall and plaque. If the stent has been undersized or if there
is any area that needs "touching up", a larger balloon
can be directed to it and expanded to fit the stent optimally.
Although IVUS was first used over 20 years
ago, the current concerns over stent thrombosis and patient outcomes
have spurred a new interest.
The recent S.T.L.L.R.
study, sponsored by Johnson & Johnson,
showed that current DES deployment techniques led to some form
of geographic
miss in 66.5% of patients. That means two-thirds of stents are
not optimally placed, which translates into negatively impacted
patient outcomes, with significantly higher restenosis, thrombosis
and
myocardial infarction rates in patients where the stent was not
placed properly. The study concluded that "a re-examination
of stent placement technique including the use of IVUS is certainly
warranted."
Modern IVUS systems are completely integrated
into the catheterization lab and with proper training, the cardiologist
can add this new
imaging technology to a standard diagnostic angiogram with a minimum
of impact on the patient.
|
