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\]  but the forward and backward wave intensities require knowledge of the wave speed  \[  dI_\pm = dP_\pm dU_\pm = \pm  (dP \pm \rho c dU)^2/4\rho c \]  This separation of pressure, velocity and wave intensity into their forward and backward components has proven useful in the aorta, the pulmonary artery and other large systemic arteries [refs]. In the coronary arteries, however, it is proving essential for the analysis of the complex influences introduced by the compression of the intra-myocardial blood vessels during systole. Unfortunately, these complexities make it much more difficult to estimate the local wave speed in the coronary arteries.