Wave intensity analysis in the human coronary circulation in health and disease

Abstract

Coronary artery hemodynamics are very different to that of the systemic arteries; unlike the systemic circulation, in the coronary circulation pressure is generated from both the proximal and distal end of the artery - due to the effect of contraction and relaxation of the myocardium on the microvasculature. As a result, the systemic artery hemodynamic model cannot be used to explain the pressure-flow relationship in the coronaries. Wave intensity analysis is an investigative tool that is able to distinguish simultaneous proximal and distal influences on coronary blood flow and is therefore uniquely suitable for the study of coronary haemodynamics. This review discusses the concept behind wave intensity analysis and evaluates how it has been used to characterise and provide new insights on coronary haemodynamics in health and disease.

Fig. (1)

Difference between the pressure-flow relationship in systemic and coronary arteries. It can be seen that in a systemic artery changes in pressure and flow closely follow each other. In systole they both increase whilst in diastole they both decrease (upper panel). In the coronary circulation (lower panel) this is not the case. After an initial period in systole when pressure and flow increase together, flow velocity plateaus whilst pressure continues to increase. In early diastole it can be seen that pressure falls rapidly whilst flow velocity increases. Such a paradoxical relationship in early diastole cannot be explained by the traditional haemodynamic model of the systemic vessel.

Fig. (2)

Coronary wave intensity analysis and intra-coronary pressure and flow velocity relationship. (Upper panel): Waves originate from both the proximal (positive values) and distal circulation (negative values). Waves from both ends of the arteries are present in both systole and diastole. The 4 predominant waves of the cardiac cycle are highlighted. The predominant determinant of coronary flow is the myocardial originating decompression wave (or backward expansion wave), which has a suction-like effect. (Lower Panel): Wave intensity can be used to explain the pressure-flow relationship in the coronary artery. For example it can be seen that the paradoxical fall in pressure and yet increase in flow velocity during early diastole is actually due to the myocardial originating decompression wave sucking blood into the coronary. This wave originates in the distal circulation due to relaxation of the myocardium and decompression of the microvasculature.

Fig. (3)

Coronary wave-intensity and severe aortic stenosis. Coronary physiological reserve varied considerably pre and post TAVR. Physiological reserve was assessed by measuring the microcirculatory decompression (suction) wave at rest and then by pacing at 90 and 120 bpm. Before TAVR, the myocardial decompression (suction) wave decreased with increasing heart rate. After TAVR, the reverse was observed, and the myocardial decompression (suction) wave increased with increasing heart rate.

Fig. (4)

Schematic of behaviour of coronary physiological reserve with progression of aortic stenosis over time. At present surgery is advocated in aortic stenosis in patients that have symptoms at which point prognosis is very poor. However, wave intensity analysis suggests that as aortic stenosis progresses, the ability of the myocardial expansion wave to increase will eventually progressively diminish. The decrease in this wave could potentially provide a tool to more accurately monitor the physiological impact and therefore severity of aortic stenosis. A more physiological approach may help identify patients that require valve replacement earlier when they are more likely to be a better substrate for surgical/ percutaneous intervention.

Fig. (5)

The diastolic wave-free period – a window in the cardiac cycle free of accelerative and decelerative forces. A period in diastole free of accelerative and decelerative forces has been demonstrated to be the most appropriate time in the cardiac cycle to assess the fluid dynamics of an upstream coronary stenosis. Whilst previously identified using manual analysis of pressure and flow, wave intensity permits identification of such a period (termed the diastolic wave-free period) using automated algorithms. This period has been demonstrated to be associated with a more stable and lower microvascular resistance than the rest of the cardiac cycle. The instant wave-free ratio (iFR) is a pressure only index of stenosis severity that is calculated over this window.