Wave intensity analysis and its application to the coronary circulation

Abstract

Wave intensity analysis (WIA) is a technique developed from the field of gas dynamics that is now being applied to assess cardiovascular physiology. It allows quantification of the forces acting to alter flow and pressure within a fluid system, and as such it is highly insightful in ascribing cause to dynamic blood pressure or velocity changes. When co-incident waves arrive at the same spatial location they exert either counteracting or summative effects on flow and pressure. WIA however allows waves of different origins to be measured uninfluenced by other simultaneously arriving waves. It therefore has found particular applicability within the coronary circulation where both proximal (aortic) and distal (myocardial) ends of the coronary artery can markedly influence blood flow. Using these concepts, a repeating pattern of 6 waves has been consistently identified within the coronary arteries, 3 originating proximally and 3 distally. Each has been associated with a particular part of the cardiac cycle. The most clinically relevant wave to date is the backward decompression wave, which causes the marked increase in coronary flow velocity observed at the start of the diastole. It has been proposed that this wave is generated by the elastic re-expansion of the intra-myocardial blood vessels that are compressed during systolic contraction. Particularly by quantifying this wave, WIA has been used to provide mechanistic and prognostic insight into a number of conditions including aortic stenosis, left ventricular hypertrophy, coronary artery disease and heart failure. It has proven itself to be highly sensitive and as such a number of novel research directions are encouraged where further insights would be beneficial.

Keywords: Wave intensity analysis; coronary vessels; diagnostic techniques; microcirculation; physiology.

Figure 1.. Pressure, flow and separated wave-intensity analysis obtained from the systemic (carotid artery) and coronary (left anterior descending artery) circulation.

In the systemic circulation the majority of the influence from the wave-intensity profile is from the proximal end of the artery with the majority of the distal-originating waves created by ‘passive’ reflection of the proximal waves. However, in the coronary circulation there is a significant proportion of the wave-intensity profile originating distally from the ‘active’ myocardium.

Figure 2.. The practical construction of wave-intensity anaylsis.

Following normalisation, the dual-tipped pressure and flow sensor wire is positioned appropriately (left). Further minor manipulations may be necessary to optimise the flow signal before pressure and flow are recorded (centre). This data is digitized and collated off-line to produce intra-coronary pressure and flow signals (right) which are processed to produce wave-intensity analysis as in Figure 1B.

Figure 3.. Separated and net coronary wave intensity constructed from an unobstructed left anterior descending artery.

Three ways of expressive wave-intensity of the backward decompression wave are highlighted: peak, cumulative and fraction. Given the active role the myocardium plays within then coronary wave-intensity profile, the benefit of separated wave-intensity analysis is highlighted.

Figure 4.. Pressure, flow and separated wave intensity analysis from the left main stem (left) and right coronary artery (right).

Six waves are evident per cardiac cycle, 3 from the aortic end (1–3)and 3 from the myocardial end (4–6)of the coronary artery. All six waves are evident in both left and right coronary arteries but with differing magnitudes depending on the ventricle that is subtended. The two dominant waves are the forward compression wave (FCW –(1)) and the backward decompression wave (BDW –(6)).

Figure 5.. Hypothesized schematic representation of the magnitude of the backward decompression wave represented by the width of the grey region following the progression and treatment options in aortic stenosis.

As the aortic valve lesion worsens the backward decompression wave increases to supra-normal levels. If intervention is performed at an appropriate time the backward decompression wave decreases to a sub-normal level as the dominant influence is from the residual left ventricular hypertrophy. Over time, this regresses and the backward decompression wave recovers. Alternatively, if the opportunity to intervene is missed, the ventricle ultimately begins to fail resulting in a steady decline in the backward decompression wave.