The diagnosis of epicardial coronary artery disease (CAD) during left heart catheterization is made through the angiographic interpretation of percent narrowing of the arterial lumen. The severe stenotic lesions typically have an appearance of at least 70 percent diameter reduction. However, there is often inter-observer variability in the assessment of intermediate lesions (40% to 70% stenosis). These lesions may have functional impairment of flow distally to cause myocardial ischemia and eventually left ventricular contractile dysfunction. The physiologic assessment of such lesions is paramount in the cardiac catheterization lab.
The purpose of angiography is to visualize the coronary arteries, major branches, or anomalies. Angiography also studies plaque burden, calcification, thrombus, or aneurysms. The major epicardial coronary arteries are left anterior descending (LAD), left circumflex (LCx), and the right coronary artery (RCA). The LAD and LCx arise from the left main coronary artery (LM). The LM and RCA typically originate from the left and right aortic sinus of Valsalva, respectively. The branches of LAD are called diagonals and septals. The branches of the LCx are called obtuse marginals. The RCA usually bifurcates into the right posterior descending artery (RPDA) and right posterolateral artery (RPL). RCA also gives acute marginal branches. These are visualized during an angiogram, and pressure measurement can be performed in these. The cardiac microvasculature is not discreetly visualized during cardiac catheterization.
The coronary arteries fill during diastole as the vigorous contraction of the heart during systole allows for little driving pressure. Using the principle of Ohm’s law, V = IR (V is voltage difference, I is current, R is resistance), the coronary pressure and flow are directly related, assuming minimal resistance. The pressure gradient in the coronaries is the difference between a distal coronary bed and aortic root pressure. The pressure gradient could be derived by measuring trans-stenotic gradient at rest or hyperemia, a direct coronary flow reserve (CFR) measurement, or the slope of the relationship between mean gradient and coronary flow. The two most commonly used physiological tools are fractional flow reserve (FFR) and instantaneous wave-free ratio (iFR).
Functional assessment of a lesion with fractional flow reserve (FFR) or instantaneous wave-free ratio (iFR) is appropriate in case of intermediate stenosis (40% to 70% diameter reduction angiographically), multivessel disease, serial coronary stenoses, nonculprit lesion in acute coronary syndrome, saphenous vein graft disease severity and post-intervention assessment. The various recommendations by major society guidelines are as below:
There are no specific contraindications to the measurement of IFR/FFR. Adenosine infusion to measure FFR should be avoided in patients with active bronchial asthma.
The miniaturized pressure sensor, which is embedded into a coronary wire is able to carry out pressure measurements with high fidelity. The pressure sensor contains piezo-electric sensors and is made by St. Jude and Philip Volcano. The sensor is typically located at the proximal end of the radiopaque wire tip. The disadvantage of these wires is the potential for signal interference at connecting points. These wires typically have a signal drift of < 7 mmHg/hr. FFR is a ratio of pressure distal to stenosis (Pd) relative to proximal pressure (Pa) during maximal hyperemia or vasodilation . On the other hand, iFR equipment relies on the cardiac mechanics of wave generation. A wave-free period (WFP) exists in diastole, and during this period, the intracoronary pressure and flow decrease in a linear fashion, and the microvascular resistance are minimal. Therefore iFR relies on this WFP to derive coronary pressure assessment without inducing hyperemia.
The personnel performing coronary arterial pressure measurements are interventional cardiologists with the assistance of a cardiology fellow or a technician and a nurse.
The preparation involves the availability of the equipment and installed software.
After obtaining arterial access via the radial or common femoral artery, the index vessel is engaged with a guiding catheter. The pressure wire product packaging is opened under sterile conditions, flushed with saline, and then zeroed. The wire tip is typically shaped per standard practice based on angulation and tortuosity of the vessel. The wire is then advanced through an introducer and guiding catheter into the coronary artery. Normalization of the waveform is obtained, at the tip of the guide catheter. Pressure wire is then parked, just distal to the stenosis. An iFR measurement is recorded. An iFR ≤ 0.89 may prompt the interventionalist to perform revascularization.
Similarly, during FFR measurement, the pressure wire is placed distal to the stenosis, and a hyperemic agent such as adenosine is administered (15 to 30 mcg bolus in RCA and 20-40 µg bolus in the LM or 140 mcg/kg/min for 3 min via IV infusion). FFR calculation is then obtained. FFR values of less than 0.80 indicate significant stenosis.
The complications are similar to any PCI and include vessel dissection, perforation, embolism, and coronary spasm. Other complications of a diagnostic catheterization include vascular complications, contrast induced nephropathy, stroke, myocardial infarction, arrhythmias and death.
Large randomized controlled trials are now available, which has studied the primary endpoint of all-cause mortality, myocardial infarction (MI), or revascularization in 12 to 24 months in patients treated with iFR/FFR guided PCI. The DEFER trial enrolled 325 patients and showed that patients with FFR values greater than 0.75 did not benefit from percutaneous coronary intervention (PCI); however, patients with FFR less than 0.75 had symptomatic improvement. The FAME trial randomized 1005 patients to either FFR guided PCI or PCI by angiography alone. The primary endpoint of death, MI, or repeat revascularization at 12 months was lower in the FFR guided group and was sustained at 24 months. Similarly, the FAME-2 trial enrolled 1220 patients with a FFR value of ≤0.80 who were randomized to either PCI and optimal medical therapy (OMT) versus OMT alone. The trial was prematurely terminated due to an increased incidence of urgent revascularization in the OMT only group.
The DEFINE-FLAIR and IFR SWEDEHEART trials showed the noninferiority of iFR compared to FFR with the primary outcome of major adverse cardiovascular events at 1 year. A pooled patient-level meta-analysis of these 2 trials revealed that iFR based deferral of revascularization was greater than FFR with similar outcomes in both groups.
After identification of an intermediate severity lesion during angiography, coronary artery pressure measurement with FFR or iFR should be done to determine the use of further intervention, which has shown to improve clinical outcomes.
Coronary artery pressure measurement is an invasive procedure performed in the cath lab with an interprofessional team approach. While an intervnetional cardiologist perform the procedure with a tech or fellow, the nursing staff assists with continuous monitoring of patients clinical status, delivering medications and handing all equipment.
The interprofessional team of nurses, radiation technologists, fellow, and the interventional cardiologist closely monitors vital signs, technical aspects, sterility of the procedure along with the safety of the staff and patient from radiation exposure.
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