Coronary Microcirculation in Aortic Stenosis: Pathophysiology, Invasive Assessment, and Future Directions

Figure 3

Myocardial flow versus coronary pressure relationships: during hyperemia, a linear relationship exists between absolute myocardial blood flow and coronary pressure (basically equal to aortic pressure in the absence of a stenosis). This so-called myocardial “load line” has both slope (how much extra flow for an increase in driving pressure) and offset (often referred to as the zero-flow or wedge pressure depending on how it is measured). The slope of the myocardial load line corresponds to the myocardial resistance which can be calculated through the formula R = (Pc − Pzf)/Q, where R is the resistance, Pc is the coronary pressure, Pzf is the zero-flow, and Q is the flow. Under resting conditions (horizontal dashed line), the myocardium is capable of autoregulation to maintain a roughly constant flow over a wide range of perfusion pressures reflected by a constant nonhyperemic pressure ratio (NHPR). A fixed coronary stenosis produces both friction (“f ”) and separation (“s”) components to net pressure loss as can be deduced from the well-known coronary stenosis formula ΔP = f ∗ Q + s ∗ Q², where is the pressure loss in mmHg and Q is the coronary flow in mL/min [67]. Its intersection with the myocardial load line represents the observations of FFR and maximum flow at peak hyperemia. Potential changes in the myocardial load line have been shown before versus after transcatheter aortic valve implantation (TAVI), although the relative magnitude and time course of a left shift (due to a fall in left ventricular filling pressures) and counterclockwise rotation (corresponding to more flow for the same driving pressure) have not yet been quantified (reprinted from the figure of recent 2020 editorial [68]).