Like the catheter-based blood pressure system , the pressure transducer should be carefully calibrated before the pressure wire is inserted. Before connecting the pressure wire to the interface connector cable, the proximal tip of the wire where three flat electrodes are located should be gently wiped out with dry gauge to remove blood, contrast medium, and damp on the wire. Then, the proximal end of the wire is plugged into the interface connector cable (Fig. 21.1b). When it is connected successfully, the signal of the pressure wire will appear on the monitor. The wire and the sensor should be flushed with heparinized saline inside the plastic housing. Finally, located on the flat angiography table at the patient’s heart level, the pressure transducer is zeroed and calibrated manometrically.

After calibration of the microchip transducer of the pressure guidewire for distal coronary pressure, the pressure wire is steered into the proximal part of the artery to be interrogated. It is essential to ensure the pressures by the guiding catheter and the pressure sensor equalized before advancing the wire over the coronary lesions. The radiopaque tip of the pressure wire should be visible outside the catheter, with the sensor positioned 1–2 mm distal to the guiding catheter. Catheter-induced pressure damping or ventricularization (Fig. 21.2) should be monitored which is caused by engagement of the catheter in significant ostial lesions, misalignment of the guides, during coronary spasm, or when a larger Fr guiding catheter is engaged in a smaller coronary artery (Fig. 21.3). When ventricularization or pressure damping appears on the arterial pressure tracings, the guiding catheter should be properly disengaged from the ostium of the coronary artery, and then the pressure sensor should be repositioned in the aorta outside the coronary artery. The guiding catheters with side holes are generally not recommended for FFR measurements because the pressure signal attained through these catheters reflects a mix between the coronary pressure (through the distal end) and the Pa (through the side holes). Indeed, FFR values are underestimated (i.e. lesion severity is overestimated) in side-hole catheters when FFR measurements are performed with engaged guide catheters [3]. If side-hole catheters are clinically indicated due to significant ostial stenosis, IV continuous infusion of adenosine (not IC injection) is mandatory for adequate hyperemia induction, and the catheter should be disengaged from the coronary artery. After positioning the pressure wire at the tip of the catheter, the guiding catheter is properly flushed with saline to remove any residual contrast. The introducer is removed from the Y connector to prevent the pressure leak. Finally, the two measured pressures are equalized electronically using that function of the console.

Before advancing the pressure sensor, it is advisable to wait for 10–30 s to avoid spontaneous pressure signal drift. If it does occur, two pressures should be re-equalized. The pressure sensor is positioned as distally as possible in the coronary artery [4], at least 2–3 cm distal to the stenosis to be assessed. The exact position of the sensor should then be documented using side branches, stenoses, or coronary stent deployed as fiducial and recorded by fluoroscopy.

For maximal vasodilation of the epicardial artery, IC bolus injection of nitroglycerine of 200 μg should precede hyperemic stimulus administration. In addition to vasodilators, anticoagulant should be given intravenously or subcutaneously as per local protocol. Before administering a hyperemic agent, the stable baseline Pd/Pa should be recorded for several heartbeats to measure resting coronary flow.