Background: In the left coronary arteries, coronary flow is said to occur mainly in diastole, but this is not the case in the right coronary artery (RCA). Since the pressure being applied at the aortic end is identical for both arteries, we hypothesised that differences from the microcirculatory end account for the differences in the flow velocity waveform. We apply a new technique, wave intensity analysis, to explain the differences in these velocity waveforms.

Methods: In 10 subjects sensor-tipped intra-arterial wires were used to measure simultaneous pressure and Doppler velocity in the proximal left main stem (LMS), the left anterior descending (LAD), the left circumflex arteries (LCX) and RCA. Wave intensity analysis was applied to derive proximal- and distal-originating waves.

Results: In all three left coronary artery positions, the systolic-diastolic ratio of peak velocity was significantly lower than the corresponding ratio in the RCA (LMS 0.58 ± 0.13, LAD 0.47 ± 0.12, LCX 0.57 ± 0.15 versus RCA1.09 ± 0.48, p<0.03 for each). The cause was a lower diastolic flow velocity in the RCA than the left coronary arteries (RCA 0.37 ± 0.28m/s versus LMS 0.58 ± 0.30m/s, LAD 0.58 ± 0.44m/s p<0.05 for each, LCX 0.46 ± 0.33m/s p=NS), which is explicable by the lower distal-originating suction wave (RCA 16.6 ± 5.4% versus LMS 27.6 ± 7.6%, LAD 28.8 ± 6.3%, LCX 26.0 ± 6.6%, p<0.02).

Conclusion: The RCA has a flow velocity waveform which is evenly distributed between systole and diastole, in contrast to the diastolic dominant waveform seen in the left coronary arteries. The reason for this difference is the lower diastolic velocity of the RCA which is due to a smaller distally-originating diastolic “suck” from its microcirculation.