As an adjunct to PCI, the physiological significance of an intermediate coronary lesion is assessed using a pressure wire to generate an FFR (fractional flow reserve) or iFR (instant wave-free ratio). This video will teach you how to identify the key components of a pressure wire system and the meaning of the numbers that are generated by it.
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[00:00:00] In order to assess physiological importance, we often use a pressure wire in the cath lab. When we're in the cath lab, the patient is lying flat on the table during the procedure. We see a stenosis in a coronary artery and angiographically, it looks moderate, let's say 60%. We know that patients with coronary artery stenosis get symptoms of angina and they exert themselves as the myocardial blood and oxygen
[00:00:30] demand goes up, and the diseased vessel is unable to match this demand with an adequate supply. Functional or stress test dobutamine stress echo, exercise tolerance or treadmill tests, or myocardial perfusion scans, all addresses by delivering some form of either physical or chemical stress to the patient and assessing the response. How do we then assess a stenosis for physiological importance or
[00:01:00] severity in the cath lab with a patient recumbent in a resting state? We can't take the patient off the table to perform a stress test in the middle of the procedure. Here's where pressure wire assessments come in. A pressure wire, very simply, is a pressure transducer on a wire that enables it to be delivered down a coronary artery and across a stenosis. It is composed of a floppy tip and a stiffer shaft with the transducer situated
[00:01:30] at the joining of these two as shown by the arrow. We can deliver the pressure wire via a guiding catheter down the artery and position it distally to the stenosis as we would any other angioplasty guidewire. Because of the nature of these wires having a pressure transducer, they are often not as deliverable as conventional angioplasty wires. User potential issues with deliverability, the operator needs to take great care
[00:02:00] and remember that this is still a diagnostic procedure. The patient does not want to end up with a dissected coronary artery for a stenosis that turns out not to be physiologically significant. Pressure wires are now available on a number of manufacturers and are improving all the time in terms of how deliverable and steerable they are. Put very simply, if we know the pressure in the aorta from the guiding catheter known as Pa,
[00:02:30] and we know the pressure in the artery after the stenosis from the pressure wire transducer known as Pd, then we can determine the difference between these two values and therefore, the pressure gradient across the stenosis or the Pd / Pa. We can express Pd / Pa as a ratio which equates to flow called a fractional flow reserve or FFR. This should obviously be
[00:03:00] one if there is no stenosis and no difference in the two recorded values. We can then give a drug to dilate the microcirculation. We conventionally use adenosine which can be given intravenously or intracoronary. This causes maximal blood flow or hyperemia very quickly. It will increase any gradient across a stenosis as the myocardial blood flow increases mimicking exercise. This will generate
[00:03:30] a new Pd / Pa or fractional flow reserve which we then use to determine if a stenosis is physiologically important. As mentioned earlier, if there was no stenosis, then the fractional reserve or FFR would be one and the blood flow in the artery would be normal with stress or hyperemia. Any stenosis or narrowing in the coronary artery will reduce this value. But when does this become significant
[00:04:00] for the patient? In order to determine this, a number of landmark trials have been performed to investigate what level of reduced FFR impacts on cardiovascular outcome. The landmark trial for this technology was the defer trial which determined that at an FFR greater than 0.75, it was safe to defer intervention or revascularization and treat the patient medically with tablets as PCI did not improve the outcome.
[00:04:30] These results were sustained on longer term follow up. Subsequent studies have looked at adopting this technology in multivessel coronary artery disease in large numbers of patients. The FAME one and FAME two trials concluded that determining revascularization strategies based on FFR, in other words, medical management of those with FFR greater than 0.8 and intervention for vessels with an FFR less than 0.8 was both safe and improved
[00:05:00] long term outcomes for patients with stable coronary artery disease. New concepts are emerging in this arena all the time and this lesson is just a very basic overview. Groups have now looked at using different parts of the pressure wave to assess flow that does not require the patient to be hyperemic and therefore, obviates the need for adenosine. This is the so-called way free ratio used to generate an instantaneous free ratio or iFR.
[00:05:30] This has potential benefits in not adding time and cost to the procedure in terms of giving adenosine but generates slightly different numbers. It's not as well established yet as FFR globally but is rapidly becoming more widely adopted due to its ease of use. Generally, for an iFR greater than 0.94, we would recommend medical therapy and less than 0.86, we would recommend revascularization. grey areas.
[00:06:00] With both FFR and iFR, there are gray areas where we are unsure what to recommend, that's outside the scope of this basic overview. For iFR this gray zone is 0.86 to 0.94. So now you understand some of the technology and terminology, we'll have a look at how we go about performing a pressure wire study in the cath lab in the next lesson.