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Nguyen, Thach

In the continuing debate regarding the mechanism of atherosclerotic plaque formation in arteries, one question remains unanswered: While all arteries in a patient are exposed to the same systemic risk factors (hypertension, diabetes, aging, nicotine of cigarettes, etc.), why do plaques frequently present in the coronary arteries, and to a lesser extent in the arteries of the lower extremities, and still less in the carotid or renal arteries? Over the past five years, in order to find an answer to the above question, there has been a radical shift in our research strategy: The principles of fluid dynamics in industrial and domestic pipes/pipelines were employed to decipher changes in the cardiovascular system. The types of flow under investigation included laminar, entrance, turbulent, and helical flows in systole and diastole, as well as the interface between antegrade and retrograde flows resulting in water hammer shock and cavitation phenomena. We aim to highlight the similarities and differences among flow types in arteries and pipes and apply the same methodologies to study the formation, growth, and rupture of coronary plaques leading to inactive and active clinical syndromes and the beneficial mechanism of percutaneous coronary interventions (PCI). In this article, we list the questions and the answers based on the primary data of several completed studies and the preliminary results of ongoing projects from a fluid mechanics perspective. The angiographic coronary images of recirculation flow, vortex formation, collision, hammer water shock, and cavitation will be showcased in detail, and their videos in slow motion are uploaded in the addendum for further in-depth review.

Background. Patients afflicted with aortic stenosis (AS) may present chest pain (CP), shortness of breath (SOB), syncope, and no lesion in the coronary arteries. So far, no clear mechanism could convincingly elucidate the pathophysiology of symptoms in such patients. We conducted a study to clarify the mechanism of CP, SOB, and syncope in AS patients without coronary artery stenosis based on coronary flow patterns or abnormalities. Methods. One hundred and thirty two (132) patients visiting the emergency room with CP, SOB, or syncope were screened for AS. Forty four (44) patients with a clinical diagnosis of AS underwent right and left heart catheterization and a novel dynamic coronary angiographic technique. Twenty AS patients without coronary artery disease (CAD) were enrolled. All patients were divided into either: group A for severe AS or group B for mild to moderate AS. The control group consisted of Five patients with normal left ventricular function without CAD or AS (group C). The flow data included the coronary transit time duration, the presence of retrograde flow at the proximal coronary segment, and the persistence of contrast spill-out from the coronary ostium. Results There was prolonged arterial phase and retrograde flow in the proximal coronary segment, including persistent spilling of contrast into the coronary sinus (p<0.01 when compared with groups B and C) in 20 patients with severe AS (group A). In 24 patients in the mild to moderate AS group (group C), there was only a moderately prolonged arterial phase without retrograde flow nor spilling of contrast from the ostium (p=0.99 when compared with control group C).  Conclusions In patients with AS, significantly prolonged arterial coronary transit time, reversed coronary flow, and retrograde ejection of contrast into the coronary sinus correlated statistically with the severity of AS. In patients with mild to moderate AS, with only moderate prolongation of the arterial phase without reversed coronary flow nor retrograde ejection of contrast into the coronary sinus.

Computational fluid dynamics (CFD), alone or coupled with the most advanced imaging tools, allows for the assessment of blood flow patterns in cardiovascular disease to both understand their pathophysiology and anticipate the results of their surgical or interventional repair.

CFD is a mathematical technique that characterizes fluid flow using the laws of physics. Through the utilization of specific software and numerical procedures based on virtual simulation and/or patient data from computed tomography, resonance imaging, and 3D/4D ultrasound, models of circulation for most CHDs can be reconstructed. CFD can provide insight into the pathophysiology of coronary artery anomalies, interatrial shunts, coarctation of the aorta and bicuspid aortic valve, tetralogy of Fallot and univentricular heart, valvular heart disease, and aortic disease. In some cases, CFD may be able to simulate different types of surgical or interventional repairs, allowing for the tailoring of treatment accordingly.