Author + information
- John McB. Hodgson, MD⁎ ()
- ↵⁎Reprint requests and correspondence:
Dr. John McB. Hodgson, Department of Cardiology, Geisinger Health System, 1000 East Mountain Boulevard 36-10, Wilkes-Barre, Pennsylvania 18711
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 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.
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: where ΔP 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 ΔP 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-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 (πr2). 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 is associated with excellent clinical outcome (7). In the future, application of IVUS-based lesion compositional analysis may provide additional differentiating information.
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.
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).
Dr. Hodgson has received speaker fees (>$10,000) from Volcano and educational grants (>$10,000) from Volcano and RADI (St. Jude) Medical.
↵⁎ 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.
- American College of Cardiology Foundation
- Nam C.-W.,
- Yoon H.-J.,
- Cho Y.-K.,
- et al.
- ↵(2010) Hydrodynamic: Encyclopedia Britannica. Encyclopedia Britannica Online. http://www.britannica.com/EBchecked/topic/658890/Hydrodynamica, Accessed: July 1, 2010.
- Takagi A.,
- Tsurumi Y.,
- Ishii Y.,
- et al.
- Costa M.A.,
- Sabate M.,
- Staico R.,
- et al.
- Abizaid A.,
- Mintz G.,
- Mehran R.,
- et al.