Clinical Editor's Corner: Kern

Comparing FFR Tools: New Wires and a Pressure Microcatheter

Morton Kern, MD
Clinical Editor; Chief of Medicine, 
Long Beach Veterans 
Administration Health Care System, Long Beach, California; 
Associate Chief Cardiology, Professor of Medicine, 
University of California Irvine, 
Orange, California
mortonkern2007@gmail.com

Morton Kern, MD
Clinical Editor; Chief of Medicine, 
Long Beach Veterans 
Administration Health Care System, Long Beach, California; 
Associate Chief Cardiology, Professor of Medicine, 
University of California Irvine, 
Orange, California
mortonkern2007@gmail.com

Use of fractional flow reserve (FFR) has increased dramatically over the last several years, largely due to the acceptance of an ischemia (FFR)-guided approach to coronary intervention for best outcomes based on the FAME and FAME II studies (Figure 1). These and other outcome studies demonstrate both the clinical and economic value of using a physiologic approach and treating only the ischemia-producing lesions, rather than treating all lesions using the angiographic-only approach.

Along with the growth in use of FFR, more interventional cardiology industry partners have entered the medical device market. This increasing industry activity reinforces how important FFR is, and that it is not just a footnote in the cath lab. Addressing the concerns of operators and labs about the technical limitations of FFR guidewire systems, five companies now make products for FFR, all aimed at improving the procedure. I thought it would be of value to review the FFR guidewires/catheters now available, and how they may improve workflow in the cath lab. As a disclaimer, I do consult for all our industry partners that make pressure/flow products. I have tried to present the technology in an objective fashion without commercial bias. If I have omitted some information or mistakenly attributed features to one or another product, I apologize in advance. 

A historical note 

In 1977, translesional pressure measurement was incorporated into the very first angioplasty procedure (PTCA). The 4.5 French (F) percutaneous transluminal coronary angioplasty (PTCA) balloon ‘DG’ catheter of Dr. Andreas Gruentzig, the developer of coronary angioplasty, had a pressure measuring capability through a second lumen: one lumen for inflating and deflating the balloon, and one lumen through to the tip with a pressure port just proximal to the fixed, short-tip guidewire. The use of pressure measurement was intended to overcome the poor and often uninterpretable angiographic images available at that time. The pretreatment gradient and subsequent reduction of the gradient was a marker of success and added significantly to the angiographic appreciation of a PTCA endpoint. The concept of FFR was invented in 1993, four years after Dr. Gruentiz’s untimely death, but he would have understood its value, having recognized from the beginning that translesional pressure was better than angiography alone. However, the size of the PTCA catheter and the small fluid-filled lumen produced suboptimal pressure signals. Following the initial PTCA procedures, angiography improved, guide catheters and wires were miniaturized, and guidewire-mounted pressure and flow sensor technology was introduced. FFR was found to be superior to resting gradient data for lesion assessment. Unfortunately, it has taken over two decades to appreciate the ‘visual (angiographic)-functional (FFR, ischemia) mismatch’ and measure FFR, which is now becoming an important standard practice in our cardiac cath labs.  

Limitations of FFR technology 

Despite strong outcome data, FFR is still underutilized. Areas for improvement of FFR technology fall into four categories: 1) signal stability [i.e. drift]; 2) wire handling characteristics; 3) use of multiple wires for complex or multi-vessel assessment; 4) use of IV adenosine hyperemia. To this last point, novel resting pressure indices, like iFR, or submaximal hyperemia, like contrast FFR, have been proposed to improve FFR workflow.  These alternative methods, along with a non-invasive, hyperemia-free alternative, offered by cardiac computed tomography angiography-derived FFR (FFRCT), are subjects for a future editor’s page. We will now review the specifics of FFR equipment.

The limitations noted above are addressed with three technological innovations: 1) increased signal stability with better piezo-resistive and optical sensors, couplers and signal processors; 2) improvements in wire construction with novel (e.g. nitinol, cobalt chromium, laser-cut hypotube) cores; and 3) rapid pressure sensor placement using a monorail microcatheter pressure system.   

Pressure wires/catheters

Several companies make pressure wire/catheter products. Each wire has unique handling characteristics and value-added features. These features are summarized in Figure 2. Current pressure wire sensors are either piezo-electric or optical. Pressure wires also differ from regular workhorse wires, having to incorporate the thin wires or optical fibers that transmit the pressure signals (Figure 3). 

Piezo-electrical/resistive sensors 

The piezo-electric sensor wires are made by St. Jude Medical and Philips Volcano. These two companies have the longest track record for use in the cath lab. The improved torque control of the guidewire arises from its specific core wire composition and construction. The sensor is located at the proximal end of the radiopaque flexible wire tip (about 3 cm long). Although the past performance of pressure wires has been criticized as lower than that of everyday workhorse guidewires, the latest versions appear to be highly competitive. Unique features of the St. Jude Medical pressure system include the wireless connection and thermodilution blood flow measurement for resistance calculations. For Philips Volcano, proprietary software enables the pressure system to compute the instantaneous wave-free ratio value, iFR. In addition, Philips Volcano manufactures a special Doppler-tipped combination pressure and flow wire, useful to study the microcirculation.  

How do piezo-electrical/resistive sensors work?

Piezo-electrical/resistive pressure sensors are the most commonly used and widely understood. Piezoelectric pressure sensors measure dynamic pressure. They are typically not suited for static pressure measurements. Based on piezoelectric technology, various physical quantities can be measured; the most common are pressure and acceleration. For pressure sensors, a thin membrane over a large base is used, ensuring that an applied pressure specifically loads the elements in one direction. Deformation of the crystal generates an electrical charge, which is transmitted along thin wires inside the guidewire. Piezo-electrical sensors have other special capabilities that include fast response, ruggedness, high stiffness, extended ranges, and the ability to measure quasi­static pressures. These are standard features associated with quartz pressure sensors.

Advances in transistor technology make this type of transducer a staple in the industry. The disadvantage of any electrically functioning equipment is the potential for signal interference at connector points or sensor interfaces. Current piezo-electric technologies are noted to have very low signal drift rates. Acceptable FDA signal drift is <5mmHg/10 minutes, but all FFR guidewires/catheters used in the cath lab specify drift of <7mmHg/hour.  

Optical sensor guidewires

Two companies, Opsens Medical and Boston Scientific, make pressure wires using optical fibers and sensors. The incorporated thin optical fibers permit construction around a larger, specialized core (e.g. nitinol, cobalt chromium), making wire torque closer to that of workhorse wires (Figure 4).

How do optical sensors work?

Optical sensing has improved over the years through advances both in basic components research and manufacturing, the optical fiber covering, the optical connectors and the optical sensing/sensors instrument. Optical sensors used in pressure guidewire use a diaphragm design similar to piezo-resistive sensors. The difference resides in the way of measuring the membrane deflection, which is optical rather than electrical. As blood pressure increases, the membrane deflects inward, which induces a phase delay between two light beams created within the sensor assembly. This so-called Fabry-Pérot interferometer (Figure 5a) has the effect of modulating the frequency content of the light signal, as opposed to modulating the light intensity. This is a major difference compared with earlier versions of optical pressure guidewires, which were based on intensity modulation, making them sensitive to any and all effects impacting light intensity such as fiber/wire bending, connection, light source aging.

A regular fiber is coupled to a diaphragm assembly at the end of the fiber. Light is sent to the end of a fiber via a fiber coupler and received by a photodiode/phototransistor. Optical pressure sensor technology purportedly overcomes electrical signal drift, with <1mmHg/h reported. Optical sensors are immune to electrical interference and do not conduct electricity. An optical sensor is rarely affected by temperature and moisture, factors that can produce signal drift, although less than piezo transducers. Some optical sensors have unique or proprietary construction, such as different adhesives or dual ridges of the face plate (Figure 5b), which reduces the effects of temperature-induced pressure shifts. 

An optical sensor monorail microcatheter

A unique pressure sensor system is the microcatheter RXi design by ACIST Medical. The RXi pressure catheter also uses fiber-optic technology. The major advantage of this rapid exchange, monorail design is that it permits use over any operator’s choice of favorite guidewire for lesion access and measurement of FFR. The microcatheter is slid across the target lesion, leaving the working wire in place. The catheter has a low and elliptical profile about 1.5x the diameter of a guidewire, with a .020 x .025-inch shaft (<2 French at the lesion-crossing location). The distal tip containing the optical pressure sensor measures .027 x .036 inches (<3 French) and tapers back to the shaft (Figure 6). The effect of the elliptical construction on flow resistance is intended to produce a minimal contribution to lesion severity despite the additional cross-sectional area. 

Why does signal drift occur?

All pressure sensors, no matter what design, have some degree of signal drift. Fortunately, for our current use, this drift is often small and infrequent in the lab. Why does signal drift occur? Pressure signal drift comes from changing sensor sensitivity due to temperature changes, moisture, microbubbles on the transducer, or from electrical interference caused by blood or fluid at the wire/signal couple interface. Signal drift may be related to temperature shift from room temperature to patient body temperature, variance of instrument calibration, blood and saline remnants on the connector, or microscopic air bubbles trapped inside the sensor capsule. 

Pressure signal drift may also be related to the aortic pressure measurement part of the FFR procedure. Changing the height of the aortic pressure transducer, capillary forces within the catheter, wedging of the guide catheter in the coronary ostium, a loss of pressure through the Y connector when open or through a wire introducer tool, are all possible causes of a varying pressure signal. The aortic pressure transmitted through some contrast injectors with incorporated transducers may become damped. As an aside, damping does not necessarily affect FFR, as the system consoles always calculate the mean value. Damping may have a bigger effect on iFR, because the mean of the entire waveform is not used.

Special features

Each pressure sensor system offers special features (Figure 7) and may be sold as part of a package, including intravascular imaging with intravascular ultrasound (IVUS) or optical coherence tomography (OCT), or synchronized signal overlay with the angiogram during pressure wire pullback. St. Jude Medical’s special features include wireless pressure signal connection, temperature-sensing capability to measure coronary flow reserve (CFR) and microvascular resistance, and absolute flow (Figure 8a). Philips Volcano offers a unique pressure/flow combination wire (Figure 8b) and ability to calculate the instantaneous wave-free pressure ratio (iFR). ACIST Medical provides a rapid exchange catheter for use with the best, personally selected percutaneous coronary intervention (PCI) guidewires and incorporates optical technology. Opsens wire systems have improved wire handling with nitinol construction and employ an optical sensor. Boston Scientific is the newest company to the market. Their system features an Asahi tip, laser-cut hypotube, and a free-spinning handle. Initial experience suggests highly maneuverable wire handling coupled to optical pressure sensor technology.  

The bottom line

There has never been a better time to use FFR equipment that can be tailored to the needs of your lab’s budget and workflow. For the operators and patients needing the best PCI results, this is very good news.

Reference

  1. Diletti R, Van Mieghem NM, Valgimigli M, Karanasos A, Everaert BR, Daemen J, et al. Rapid exchange ultra-thin microcatheter using fibre-optic sensing technology for measurement of intracoronary fractional flow reserve. EuroIntervention. 2015 Aug;11(4):428-32. doi: 10.4244/EIJY15M05_09.

Disclosure: Dr. Kern reports he is a consultant and speaker for St. Jude Medical and Philips Volcano, and a consultant for Boston Scientific, Opsens, ACIST Medical, and Merit Medical.

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