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JACC: CARDIOVASCULAR INTERVENTIONS
VOL. 3, NO. 8, 2010
© 2010 BY THE AMERICAN COLLEGE OF CARDIOLOGY FOUNDATION
ISSN 1936-8798/$36.00
PUBLISHED BY ELSEVIER INC.
DOI: 10.1016/j.jcin.2010.07.004
EDITORIAL COMMENT
If You Want to Stent . . .
Do Intravascular Ultrasound!*
John McB. Hodgson, MD
Wilkes-Barre, Pennsylvania
Most invasive cardiologists are familiar with the common
catheterization lab “joke” that if you want to justify placing
a stent, use intravascular ultrasound (IVUS) to assess the
lesion and that if you prefer medical therapy, use fractional
flow reserve (FFR) to assess the lesion. Nam et al. (1) has
nicely provided a study that documents the validity of this
statement. In their retrospective study involving the use of
either FFR or IVUS for intermediate lesion assessment, the
rate of stenting was 3 times higher in the IVUS assessed
See page 812
group. This result is expected based on our understanding of
fluid dynamics and coronary lesions. I will address that
further. The most important finding of this study, however,
is that once again, physiologically (FFR)-guided decisions
regarding percutaneous coronary intervention (PCI) are safe
and yield excellent patient outcomes. After all, ensuring
excellent patient outcome with appropriate resource utilization (i.e., cost-effectiveness) is our prime responsibility.
where ⌬P is the pressure drop across a stenosis, As is the
minimal cross sectional area inside the stenosis, l is length,
and V is blood flow velocity through the tube.
Thus, longer lesions, tighter lesions, and conditions of
high flow will lead to greater pressure (energy) loss across a
stenosis. Fractional flow reserve accurately determines this
pressure loss and is well validated as a method able to
predict myocardial ischemia. On the other hand, IVUS
measures only 1 component of the Bernoulli relationship:
As.. Therefore, it should be no surprise that IVUS is less
well suited to assessing the physiologic significance of a
given stenosis. Further, IVUS cannot account for another
important physical factor: the blood flow requirements of
the subtended myocardial perfusion bed. Fractional flow
reserve is not only lesion-specific, but also accounts for the
variable myocardial blood flow requirements and resulting
impact of an upstream stenosis on this specific myocardial
bed. For example, a 70% stenosis in a vessel subtending a
small diagonal or a previously infracted mid-anterior descending territory will have less physiologic impact than an
identical lesion in a mid-anterior descending subtending a
healthy territory. Because the latter myocardium requires
higher flow (and ⌬P is exponentially related to flow), the
FFR will be lower, even though the lesion is identical. This
disparity has been described in 3 studies (3–5) detailing the
poor relationship between FFR and IVUS minimal lesion
area even in the best-case scenario of single-vessel, single-
A Little Physics (Sorry)
The nature of flow through tubes of various sizes has been
well understood for hundreds of years. In the 18th century,
a Dutch-born mathematician and physicist Daniel Bernoulli
(1700 to 1782) (Fig. 1) discovered the principle that bears
his name while conducting experiments concerning the
conservation of energy. In 1738, he published his observations in the book Hydrodynamic (2) (Fig. 2). The principle
he described relevant to stenotic fluid-carrying tubes is
summarized (and very much simplified) in the following
equation:
⌬P ⬇ 1/AS ⫻ l ⫻ V2,
*Editorials published in JACC: Cardiovascular Interventions reflect the views of the
authors and do not necessarily represent the views of JACC: Cardiovascular Interventions or the American College of Cardiology.
From the Department of Cardiology, Geisinger Health System, Wilkes-Barre,
Pennsylvania. Dr. Hodgson has received speaker fees (⬎$10,000) from Volcano and
educational grants (⬎$10,000) from Volcano and RADI (St. Jude) Medical.
Figure 1. Sketch of Daniel Bernoulli
Daniel Bernoulli was born February 8, 1700, Groningen, the Netherlands,
and died March 17, 1782, Basel, Switzerland.
Hodgson
Editorial Comment
JACC: CARDIOVASCULAR INTERVENTIONS, VOL. 3, NO. 8, 2010
AUGUST 2010:818 –20
819
is associated with excellent clinical outcome (7). In the future,
application of IVUS-based lesion compositional analysis may
provide additional differentiating information.
Clinical Application
The clinical uses of both IVUS and FFR have been steadily
growing, with acceleration in recent years due to multiple
trial publications showing utility for lesion evaluation, PCI
deferral, and PCI guidance. The study by Nam et al. (1)
further assists us in understanding how to incorporate these
tools into every day practice. Based on existing data, I
believe rational integration would include:
• Routine use of FFR for intermediate lesions (40% to 70%
visual stenosis) to determine if PCI is needed (i.e., use
physiology tools to determine physiology). There should
be no concern for adverse long-term patient outcome for
those in whom PCI is deferred.
• Use of IVUS for lesion definition and PCI guidance in
patients for whom the need for PCI is clear (i.e., use
anatomic tools to guide anatomic processes).
• In cases where FFR documents the need for PCI, IVUS
can be used for PCI guidance.
Figure 2. Faceplate From Hydrodynamic
Faceplate from Daniel Bernoulli’s book Hydrodynamic.
lesion stenosis (r2 between 0.4 and 0.6). Although it is
technically possible to measure all of the necessary physical
properties of a stenosis with IVUS and calculate the pressure
loss and FFR, this is not clinically practical (6). Thus, it has
been clear for nearly 300 years that the best way to
understand the physical limitation imposed by a stenosis is
to measure the pressure loss across it during flow, that is,
FFR.
Unfortunately, many of us have forgotten even the most
basic of physics: the area of a circle (␲r2). Understanding the
relationship between vessel diameter and its cross-sectional
area is critical to proper interpretation of IVUS in the
catheterization lab. The area of a healthy 2.5-mm vessel is
4.9 mm2. Thus, the finding of a lesion with cross-sectional
area of 4 mm2 in this vessel should not prompt concern (or
PCI). This lesion is only a 28% area stenosis! It is well
accepted that “significant” coronary lesions must be ⬎50%
diameter stenosis (equivalent to approximately 75% area stenosed). Similarly, a 3.0-mm vessel has an area of 7.1 mm2, so
a lesion with minimal area of 4.0 mm2 yields only a 44% area
stenosis. It should be obvious that a single IVUS minimal
lesion area cannot be applied to all size vessels to determine
significance. Alternatively, it is true that deferring PCI in a
lesion with an IVUS-defined minimal area greater than 4 mm2
A recent review by Magni et al. (8) highlights the complementary nature of IVUS and FFR. The era of “competition”
between physiology and anatomy must end. At Geisinger, we
have incorporated both FFR and IVUS into our ProvenCare
process for ensuring optimal PCI. In this process, where the
cost is fixed and the procedure comes with a “warranty,” we
require that every aspect of the procedure is evidence-based and
uniformly applied (9).
Reprint requests and correspondence: Dr. John McB. Hodgson,
Department of Cardiology, Geisinger Health System, 1000 East
Mountain Boulevard 36-10, Wilkes-Barre, Pennsylvania 18711. Email: [email protected].
REFERENCES
1. Nam C-W, Yoon H-J, Cho Y-K, et al. Outcomes of percutaneous
coronary intervention in intermediate coronary artery disease: fractional
flow reserve–guided versus intravascular ultrasound–guided. J Am Coll
Cardiol Intv 2010;3:812–7.
2. Hydrodynamic. Encyclopedia Britannica. 2010. Encyclopedia Britannica Online. Available at: http://www.britannica.com/EBchecked/topic/
658890/Hydrodynamica. Accessed: July 1, 2010.
3. Briguori C, Anzuini A, Airoldi F, et al. Intravascular ultrasound criteria
for the assessment of the functional significance of intermediate coronary
stenosis and comparison with fractional flow reserve. Am J Cardiol
2001;87:136 – 41.
4. Takagi A, Tsurumi Y, Ishii Y, et al. Clinical potential of intravascular
ultrasound for physiological assessment of coronary stenosis. Relationship between quantitative ultrasound tomography and pressure-derived
fractional flow reserve. Circulation 1999;100:250 –5.
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Hodgson
Editorial Comment
5. Costa MA, Sabate M, Staico R, et al. Anatomical and physiologic
assessments in patients with small coronary artery disease: final results of
the physiologic and anatomical evaluation prior to and after stent
implantation in small coronary vessels (PHANTOM) trial. Am Heart J
2007;153:296.e1–7.
6. Takayama T, Hodgson JM. Prediction of the physiologic severity of
coronary lesions using 3-D IVUS: validation by direct coronary pressure
measurements. Catheter Cardiovasc Interv 2001;53:48 –55.
7. Abizaid A, Mintz G, Mehran R, et al. Long-term follow-up after
percutaneous transluminal coronary angioplasty was not performed
based on intravascular ultrasound findings: importance of lumen dimensions. Circulation 1999;100;256 – 61.
JACC: CARDIOVASCULAR INTERVENTIONS, VOL. 3, NO. 8, 2010
AUGUST 2010:818 –20
8. Magni V, Chieffo A, Colombo A. Evaluation of intermediate coronary
stenosis with intravascular ultrasound and fractional flow reserve: its use
and abuse. Catheter Cardiovasc Interv 2009;73:441– 8.
9. Casale A, Paulus R, Selna M, et al. “ProvenCare SM”: a provider-driven
pay-for-performance program for acute episodic cardiac surgical care.
Ann Surg 2007;246:613–23.
Key Words: fractional flow reserve 䡲 intravascular ultrasound 䡲 percutaneous coronary intervention.