Reservoir/excess pressure analysis of coronary haemodynamics (2)

Before looking at the results of the reservoir/excess pressure separation in the coronary arteries, it is useful to remind ourselves of the results of the analysis in the aorta.

wave separation in the distal aorta

This figure shows the application of wave intensity analysis to measurements of pressure and velocity in the distal aorta of man. The left side of the figure shows results using the measured pressure to calculate the wave properties. The right side shows how the results are modified when the excess pressure is used in the calculation of the wave properties.

The top row shows the separated pressures. On the left, the measured pressure (black), the forward pressure waveform (blue) and the backward pressure waveform (red) are shown with the diastolic pressure subtracted to enable comparison of the waveforms without an offset pressure. On the right, the curves are the same measured pressure (solid black), the reservoir pressure (green), the excess pressure (dashed black), the forward excess pressure waveform (blue) and the backward excess pressure waveform (red).

The middle row shows the simultaneously measured velocity (black), the forward velocity waveform (blue) and the backward velocity waveform (red). The separated velocity waveforms are different in the two cases because they are calculated using the measured pressure (left) and the excess pressure (right)

The bottom row shows the wave intensity calculated from the pressure and velocity. In both cases the net wave intensity (black), the forward wave intensity (blue) and the backward wave intensity (red) are shown. To the left, the wave intensity is based on the measured pressure (the solid black line in the plot above). To the right, it is based on the excess pressure (the dashed black line in the plot above).

wave separation in the left anterior descending coronary artery

This figure shows data measured in the LAD coronary artery of the same subject in the same format as the above figure.

The top row shows the separated pressures. In the top left graph, the measured pressure (black), the forward pressure waveform (blue) and the backward pressure waveform (red) are shown with the diastolic pressure subtracted to enable comparison of the waveforms without an offset pressure. In the top right graph the curves are the same measured pressure (solid black), the calculated reservoir pressure (green), the excess pressure (dashed black), the forward excess pressure waveform (blue) and the backward excess pressure waveform (red). The measured pressure is very similar to that measured in the aorta, but the separated pressure waveforms are very different because of the differences in the velocity waveform.

The middle row shows the simultaneously measured velocity (black), the forward velocity (blue) and the backward velocity (red). Note how different the velocity waveform is from the velocity waveform measured in the aorta.

The bottom row shows the wave intensity calculated from the measured pressure and velocity. In both cases the net wave intensity (black), the forward wave intensity (blue) and the backward wave intensity (red) are shown. To the left, the wave intensity is based on the measured pressure (the solid black line in the plot above. To the right, it is based on the excess pressure (the dashed black line in the plot above). As expected, the pattern of wave intensity is very different from that measured in the aorta. In particular note the large backward compression wave at the very start of systole and the large backward decompression wave at the end of systole.

The question of the usefulness of the reservoir/excess pressure separation is still an open question, but it is through comparisons such as these that the answer will be found. In the aorta we see that the separation of the measured pressure into the reservoir and excess pressures gives more satisfying results during diastole; the self-cancelling waves seen in the figure to the right are gone. The magnitude of the reflected waveform is significantly decreased which results in a very different assessment about the size and influence of reflected waves in the aorta from that obtained when the pressure is not separated. Note, however, that the effect of the reservoir pressure on the wave intensity is limited to a small change in the magnitude of the wave intensity; there is no change in the morphology of the curves.

In the case of the coronary artery, the patterns are much more complex. The separated pressures using the measured pressure show large, self-cancelling forward and backward waves during systole with only a small backward during diastole. Using the excess pressure, this pattern changes significantly. The self-cancelling waves during systole are greatly reduced but now we see reinforcing waves during diastole. These arise because the excess pressure is nearly zero during diastole while there is a large flow. Unlike the non-coronary arteries, this is not unreasonable because the coronary circulation is still being acted upon by the relaxing myocardium. The forward compression wave and the backward decompression wave both combine to accelerate the flow, resulting in the large diastolic velocity.

Although we have no clear answers yet about coronary haemodynamics, is seems certain that wave intensity analysis certainly and reservoir/excess pressure most probably will help us uncover the details of the complex and highly important subject.