When the wire sensor is located at the distal end of the normal coronary artery, the Pd value may exceed the Pa value by a few mmHg which is related to the difference of atmospheric pressure. In this case, the FFR value is displayed in excess of 1.00. In fact, this is not a true error but rather a phenomenon in which the pressure wire sensor is located at the distal point of the coronary artery, especially distal part of right coronary artery or circumflex artery, which is a lower height than the aorta level (Fig. 23.2). Usually this difference is so small that it does not confuse the interpretation of the FFR value and neither affects the clinical interpretation [2].

Any size catheter can be attempted to measure FFR. However, because of the high level of friction in the inner coat of diagnostic catheter or smaller guiding catheter than 5 Fr, those catheters are not usually recommended in FFR measurement, or at least inspective measurement should be warranted. As the size of catheter increases, the coronary artery opening is more wedged which impede coronary blood flow (Figs. 21.2 and 21.3 in Chapter 21) [3]. This impeded flow can be noticed by ventricularization of Pa waveform especially in hyperemia. In this condition, the FFR value is incorrectly high, which underestimates the degree of coronary artery stenosis. Thus, the operator should pay attention to the change of wave morphology such as ventricularization or damping. Therefore, disengagement of guiding catheter from the ostium before hyperemia can be a good way to prevent this issue.

Contrast material in the catheter can subtly subside the aortic waveform especially in smaller catheters and can be easily corrected by flushing the guiding catheter with saline. Ideally, the dicrotic notch in pressure wave should be discernable on the aortic waveform to verify an adequate pressure tracing.

Other important issue related to catheter is usage of side-hole-guiding catheter . Because the coronary pressure measured through the side-hole catheter can be affected by the pressure interference through the side-hole-like pseudo-stenosis, it is not recommended for FFR measurement usually. However, in some situation such as concomitant proximal left main disease, the operator has to use the side-hole catheter. In that case, removing catheter from the ostium of coronary artery before FFR measurement and intravenous continuous hyperemia should be performed to get an adequate value.

To advance the FFR wire through the guide catheter, the introducer is inserted into the Y-connector, and the wire is manipulated together. Due to the volume occupied by the introducer, the Y-connector is partially opened, and consequently the aortic pressure can be measured low. In this state, the FFR measurement result will be different from the values obtained without the introducer (Fig. 23.3). This change can be amplified by hyperemia. Even if the difference is negligible, the meaning may change if the FFR value corresponds to a borderline zone. Therefore, the operator makes sure that the introducer is removed before measuring the coronary pressure.

Another forgetful step is the equalization of pressure wire. This pressure sensor is located at the 3 cm proximal end of opaque wire tip, at the junction between radiopaque and non-radiopaque portion. Pressure equalization between arterial pressure of coronary ostium and pressure sensor is performed by placing pressure sensor in front of catheter tip. When the guiding catheter is unstably engaged, some stenosis is observed in ostium of coronary artery, or significant catheter moving is observed, placing of additional guide wire can help the stabilization of pressure wire.

The sensor is positioned to the distal of the stenosis for FFR measurement, usually as far distal of the coronary artery as possible to determine the total degree of ischemia of the targeted coronary artery. It is also recommended to place the sensor at least 2–3 cm distal to the stenotic lesion to avoid the turbulence flow influence caused by the front stenosis lesion.