Angina is chest pain caused by insufficient blood supply to the heart (myocardial ischaemia). Stable angina is brought on by physical activity or emotional stress and goes away with rest. It is the key symptom of coronary artery disease, which is one of the main causes of morbidity and mortality in economically developed countries.
Options for managing stable angina include lifestyle advice, drug treatment and revascularisation using percutaneous (stent placement during percutaneous coronary intervention) or surgical techniques (such as coronary artery bypass surgery). Choosing the appropriate management option relies on correctly detecting and characterising coronary stenosis. Therefore, the diagnostic pathway for stable angina:
confirms a diagnosis of stable angina
defines the severity of coronary stenosis, which provides prognostic information and identifies people who are likely to benefit from myocardial revascularisation, in addition to optimal medical therapy.
The NICE guideline on assessment and diagnosis of chest pain of recent onset recommends diagnostic testing for people in whom stable angina cannot be excluded by clinical assessment alone. It recommends offering 64‑slice (or above) CT coronary angiography as the first-line diagnostic test when:
clinical assessment indicates typical or atypical angina or
clinical assessment indicates non-anginal chest pain but 12-lead resting ECG has been done and indicates ST-T changes or Q waves.
For people in whom 64-slice (or above) CT coronary angiography has shown coronary artery disease of uncertain functional significance, or is non-diagnostic, the guideline recommends offering non-invasive functional imaging for myocardial ischaemia. This could be:
myocardial perfusion scintigraphy with single-photon emission CT (MPS with SPECT) or
stress echocardiography or
first-pass contrast-enhanced magnetic resonance (MR) perfusion or
MR imaging for stress-induced wall motion abnormalities.
If the results of non-invasive functional imaging are inconclusive, invasive coronary angiography is recommended. Invasive coronary angiography shows whether the arteries are blocked or narrowed, and the degree of stenosis. It is usually used as a third-line investigation for stable angina or during the initial stages of percutaneous coronary intervention. However, it is difficult to differentiate between functionally significant and non-significant (not substantially affecting blood supply) coronary stenosis using visual assessment of invasive coronary angiograms.
If it is necessary to more accurately understand the functional significance of a stenosis, fractional flow reserve (FFR) or instantaneous wave-free ratio (iFR) measurements can be done during invasive coronary angiography. These invasive techniques use a pressure wire with or without a vasodilator drug, such as adenosine, and can only be done in interventional catheter laboratories.
QAngio XA 3D quantitative flow ratio (QAngio QFR) and CAAS vessel FFR (CAAS vFFR) are analytical software that can be used during invasive coronary angiography to assess the functional significance of coronary stenosis. By avoiding unnecessary invasive measurement of FFR or iFR, these technologies could help avoid the risks associated with passing the pressure wire to the coronary arteries, and with adenosine infusion.
Both tests included in the assessment are CE marked and available to the NHS.
The CAAS vFFR software (Pie Medical Imaging) works by building a 3D reconstruction of a coronary artery as well as assessing the pressure drop across the stenosis and calculating a vFFR value. Therefore, it gives both anatomical and functional assessments of the stenosis. It uses 2 standard X-ray angiograms, and is compatible with most X-ray systems (that is, it is vendor independent). The company claims that the total analysis time is about 2 minutes per coronary artery. Thresholds for interpretation of vFFR are not provided in the instructions for use document.
The QAngio software (Medis Medical Imaging) uses X-ray angiographic images taken during invasive coronary angiography. Two images are needed, which have to be taken with at least 25 degrees difference in viewing angle and with a frame speed of at least 12.5 frames per second. High image quality is crucial for appropriate results. The QAngio software creates a 3D anatomical model of a coronary artery from these 2 images, and then estimates QFR from the 3D vessel anatomy and flow velocity. The company claims that the total analysis time is about 4 to 5 minutes per coronary artery. The analysis time may decrease with routine use of the software. The QFR represents an assessment of the pressure drop over the artery, with a value of 1 representing a normally functioning artery with no pressure drop. A 20% or more drop in blood pressure (QFR value of 0.80 or less) is usually considered a significant obstruction, where revascularisation should be considered.
The QAngio software offers 2 different flow models to calculate QFR:
fixed-flow QFR (fQFR), using fixed-flow velocity and
contrast QFR (cQFR), using contrast frame count in an angiogram without hyperaemia.
Fixed-flow QFR is faster to compute, but may be less accurate than contrast QFR.
The comparator is clinical decision making based on the visual interpretation of the images from invasive coronary angiography, alongside clinical judgement. The reference standard for assessing diagnostic accuracy is FFR or iFR.