Typical diameter variation of coronary arterial segment without pre-constriction was caused by cyclic transmural pressures that were generated by either intraluminal or extraluminal pressure variation. P_int, the intraluminal pressure constant at ∼90 mmHg and later cyclically varied from ∼90 to 0 mmHg; P_ext, extraluminal pressure cyclically varied from 0 to ∼90 mmHg and later maintained constant at 0 mmHg; P_int–P_ext, the cyclic transmural pressure calculated by the difference of P_int and P_ext. OD, outer diameter of the segment was automatically tracked during cyclic transmural pressure. The defined abbreviations here are applied in all subsequent figures.