Structure (Epicardial Stenosis) and Function (Microvascular Dysfunction) That Influence Coronary Fractional Flow Reserve Estimation

Background. The treatment of coronary stenosis is decided by performing high risk invasive surgery to generate the fractional flow reserve diagnostics index, a ratio of distal to proximal pressures in respect of coronary atherosclerotic plaques. Non-invasive methods are a need of the times that necessitate the use of mathematical models of coronary hemodynamic physiology. This study proposes an extensible mathematical description of the coronary vasculature that provides an estimate of coronary fractional flow reserve. Methods. By adapting an existing computational model of human coronary blood flow, the effects of large vessel stenosis and microvascular disease on fractional flow reserve were quantified. Several simulations generated flow and pressure information, which was used to compute fractional flow reserve under several conditions including focal stenosis, diffuse stenosis, and microvascular disease. Sensitivity analysis was used to uncover the influence of model parameters on fractional flow reserve. The model was simulated as coupled non-linear ordinary differential equations and numerically solved using our implicit higher order method. Results. Large vessel stenosis affected fractional flow reserve. The model predicts that the presence, rather than severity, of microvascular disease affects coronary flow deleteriously. Conclusions. The model provides a computationally inexpensive instrument for future in silico coronary blood flow investigations as well as clinical-imaging decision making. A combination of focal and diffuse stenosis appears to be essential to limit coronary flow. In addition to pressure measurements in the large epicardial vessels, diagnosis of microvascular disease is essential. The independence of the index with respect to heart rate suggests that computationally inexpensive steady state simulations may provide sufficient information to reliably compute the index.

Estimation of coronary stenosis severity based on flow distribution ratios

We suggest improving minimally-invasive stenosis severity estimation, using a combination of existing geometry-based methods with Transluminal Attenuation Gradient measurements. Instead of local flow values, the method uses flow distribution ratios along the entire tree. The tree geometry is used to derive a lumped model and predict the 'theoretical' ratios in each bifurcation, while attenuation measurements are used for extracting 'actual' ratios. The discrepancies between the measured and the theoretical values are utilized to assess a functional degree of stenosis. Our experimental and numerical analyses show that the quantitative value of discrepancy is proportional to stenosis severity, regardless of boundary conditions.

Can Functional Testing for Ischemia and Viability Guide Revascularization?

Cardiac imaging procedures are a cornerstone of the diagnosis and management of patients with cardiac disease. The optimal management of the patient with stable ischemic heart disease or ischemic heart failure often rests on the totality of symptom burden, patient risk, and disease severity, whether assessed anatomically or functionally. Recent trials have demonstrated the power of flow measurements to direct revascularization as well as the strengths and limitations of ischemia and viability/hibernation imaging as markers of risk to direct interventions. They have also highlighted the challenges in evaluating imaging or functional testing to direct therapies, because imaging does not directly affect outcome itself, rather it affects the management decisions that may result in a positive outcome. Ongoing studies with randomized designs, such as FAME 3 (Fractional Flow Reserve versus Angiography for Multivessel Evaluation), ISCHEMIA (International Study of Comparative Health Effectiveness with Medical and Invasive Approaches), and AIMI-HF (Alternative Imaging Modalities in Ischemic Heart Failure) (IMAGE-HF [Imaging Modalities to Assist with Guiding Therapy in The Evaluation of Patients with Heart Failure]), will provide the highest level of evidence to support practice changes that may further clarify the role of cardiac imaging in the evaluation of these patients and result in improved patient outcomes.

A Test in Context: Fractional Flow Reserve: Accuracy, Prognostic Implications, and Limitations

Fractional flow reserve (FFR) is an invasive procedure used during coronary angiography to determine the functional significance of coronary stenoses. Its use is particularly helpful in intermediate or angiographically ambiguous lesions in the absence of noninvasive functional studies. Randomized clinical trials have reported improved clinical outcomes with the use of FFR to guide coronary revascularization, including a reduction in cardiac death or myocardial infarction, as well as costs, with an FFR-based strategy compared with a conventional angiography-based approach. Current societal guidelines provide a Class II, Level of Evidence: A recommendation to perform FFR in angiographically intermediate stenoses in the absence of stress testing or in the presence of discordant stress test results and angiographic findings. However, despite the relative ease of use of FFR, multiple technical factors can impair its accuracy, and attention to detail is critical when performing the test. This review focuses on the fundamental basics of FFR testing, clinical evidence, and limitations.