FAME randomized patients with multi‐vessel disease to a FFR or angiographic guided approach; stenoses with FFR>0.80 were deferred. FAME II randomized patients with a stenosis found to have an FFR ≤0.80 to PCI with optimal medical therapy (OMT) or to OMT alone.
Finally, pressure wire assessment can add utility in patients who have recurrent chest pain but angiographically unobstructed coronary arteries. Assessment of the microcirculation has proven useful in managing this traditionally difficult cohort of patients.
This chapter will discuss these matters in turn with a focus upon the practical application of coronary physiology in day‐to‐day practice. The principal focus will be on pressure‐based indices FFR and non‐hyperemic pressure ratios (NHPR), which are measured under hyperemic and resting conditions, respectively. Whilst a number of flow‐based and combined pressure and flow‐based parameters exist, their traction outside of the academic setting and in mainstream clinical practice is minimal.
Practical considerations of pressure wire measurement
When performing physiological assessment, a rigorous standardized approach is necessary. Although simple, errors can be made by even established centers [39]. Higher volumes and high quality training improve the quality of the data acquired [40,41].
Pressure recording system
A pressure transducer system continuously records aortic pressure (Pa) and it is essential that it is set at 5 cm below the sternum, estimating the position of the aortic root. Both the transducer system and the pressure wire system should be zeroed simultaneously prior to the case; this is by opening all ports to air. The entire system should be appropriately flushed afterwards.
Guiding catheter selection
Physiological assessment can be readily performed using 5 Fr and 6 Fr guiding catheters. Smaller 4 Fr catheters have been used but there is concern regards potential bailout options in the event of coronary dissection and potential for aortic waveform damping. Although the majority of pressure wire assessments are safe, wire‐related complications such as wire‐related coronary dissection or vessel occlusion occur at the rate of 0.5–1.0% [42]. The 6 Fr catheters provide a better bail‐out strategy and enable immediate percutaneous treatment without delay.
Guide catheter damping
Caution is required to ensure there is no damping of the pressure signal after coronary engagement. Catheter‐induced damping can cause ischemia but will also compromise pressure‐wire readings by artificially exaggerating a proximal stenosis. Catheters with side‐holes should be avoided; although the side‐hole can improve the appearance of the pressure trace, there remains a relative ostial obstruction which will alter the measured physiology. Furthermore, there is a risk that the pressure wire may pass out of the catheter through a side‐hole.
A better solution would be to disengage the guiding catheter, and to “normalize” or “equalize” the coronary pressure wire within the aorta. The wire can then be withdrawn into the guiding catheter before re‐engagement with the coronary ostia. Once the pressure wire is positioned distally, the guiding catheter can be disengaged to relieve the damping. An alternative approach would be to use a “buddy wire” which can be placed within a secondary vessel, such as the circumflex when interrogating the LAD. This enables the guiding catheter to be held out of the left main stem before delivering the pressure wire. A major limitation of guide disengagement is that delivery of using intracoronary vasodilators or injectants particularly challenging.
Pressure wire preparation
Pressure wire systems require preparation before they are introduced into the catheter systems. Typically, they should be positioned flat on the table, ideally at the level of the patient’s heart. The wire housing should be flushed with saline to activate the sensor prior to the pressure wire being connected to the console system. When using those with plugin connectors, care is required to avoid the connector becoming wet. Once connected, an onscreen display will indicate the wire is “zeroing”. With bluetooth systems, onscreen instructions should be followed carefully. Only once the pressure wire systems have fully activated should the wire be removed from the housing and the tip be shaped.
Pressure wire normalization or equalization
“Normalization” of the pressure wire at the vessel ostium is an essential step and must be performed before the wire is passed into any vessel of interest. The process ensures that intracoronary pressure measurements are made in comparison to the aortic pressure. It provides the operator an opportunity to review the aortic pressure traces and ensure they are not damped in appearance; both anacrotic and dicrotic notch should be present (Figure 7.5). Normalization can be performed with the pressure‐sensor placed at the ostium of the vessel. It is recommended that normalization is formally documented under fluoroscopy to demonstrate wire position during this key step. Normalization should be performed with the introducer needle removed, since this can create an offset of 1–2 mmHg which may be important for borderline stenoses. Active normalization will not only ensure the pressure ratios are 1.0, but also ensure there is no time delay between proximal and distal pressure tracings. This is pertinent for phasic pressure analysis such as iFR, since time offsets will generate incorrect calculation. Operators should visually confirm normalization by assessing the pressure traces, and if a residual issue remains, the normalization should be repeated.
Figure 7.5 Active and Phasic Normalization. (a) The pressure wire sensor should be normalized at the ostium of the vessel. For most pressure wires the sensor is at the junction of the radio‐opaque marker. (b) There is a phase delay between proximal aortic pressure (Pa, red) and distal wire pressure (Pd, yellow); while the Pd/Pa ratio over the whole cycle is 1.0, during the wave‐free period in diastole, the ratio is an erroneous 1.08 at the ostium. Therefore, any measurements taken would be erroneous. (c) After the formal normalization process, the Pd and Pa are now aligned.
Essential pharmacology
1 Anticoagulation: all invasive coronary physiological assessment requires unfractionated heparin to prevent thrombosis upon the wire; 50–100 IU/kg must be administered prior to any intracoronary wires being placed.
2 Epicardial Artery Stabilization: Adequate doses of intracoronary nitrates are essential prior to physiological measurement and should be given immediately after the guiding catheter has engaged and before the passage of the pressure wire. Doses can vary according to patient factors and blood pressure but 200–300 mcg aliquots is common; some operators use 1 mg. The purpose is to stabilize epicardial resistance and minimize micro‐vasospastic changes of the epicardial vessel which are triggered by the passage of the intracoronary wire. Such spam can artificially escalate the significance of a given stenosis. Prolonged pressure wire cases should have repeated doses of intracoronary nitrates. This is facilitated by using a manifold with ports readily available for intracoronary injections. Nitrates should also be given again for any physiological measurements made after PCI.
Nitrates do not cause significant or long‐lived hyperemia. Shortly after injection, it will be noted that translational gradients will increase because there is a temporary increase in flow, however, the flow returns to baseline state within 30 seconds. To mitigate against confusion this may cause, it is best practice
