Background: The coronary flow velocity profile is strikingly different from that of the proximal aorta, even though they are only a few centimetres apart and have almost identical pressure waveforms. We use wave intensity analysis to help explain this phenomenon, and to explore the importance of the coronary microcirculation in the regulation of coronary blood flow.

Method and Results: In 18 subjects (mean age 54 years, 12 female) we measured simultaneous pressure and Doppler velocity using intra-arterial wires in the proximal left main stem, left anterior descending, circumflex artery and proximal aorta. Wave intensity analysis was used to separate the pressure waveform into its proximal- and distal-originating components. In the aorta the flow velocity waveform follows the aortic pressure waveform reasonably closely, although the peak velocity occurs before the peak pressure. Using wave intensity analysis we found that more than 70% (47.3 versus 19.7 mmHg, p < 0.001) of the increase in the aortic pressure waveform was from proximally-originating pressure. In contrast, in the coronary arteries, only 48% of the increase in pressure came from a proximal origin and the remainder from a distal (microcirculatory) origin (31.3 ±11.5 versus 33.7 ±8.4mm Hg, p = 0.47). Distal-originating pressure rises prior to proximal-originating pressure (41±28 ms versus 104 ±25 ms, p < 0.001). This excess distal-originating pressure attenuates the rise of coronary flow velocity (0.2±0.23 m/s), which is only reversed during cardiac relaxation when distal-originating pressure falls rapidly, and coronary flow velocity peaks (0.58±0.49 m/s).

Conclusion: Aortic flow velocity is largely driven by the proximallyoriginating aortic pressure. In contrast, coronary blood flow velocity is heavily regulated by the coronary microcirculation. During cardiac contraction distal coronary pressure exceeds proximal-originating pressure – restricting blood flow. Only after cardiac relaxation begins does distal pressure fall, allowing coronary flow velocity to rise rapidly.