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\[  dP = dP_+ = \rho c dU_+ = \rho c dU \qquad \mbox{when} \qquad dP_- = dU_- = 0  \]  In practice this method of determining wave speed is somewhat subjective because of the choice of the segment of the loop that should be fitted as a straight line in never clear cut. Also, the separation of the waves into forward and backward waves, using the wave speed, generally indicate that there are small but significant backward waves at the end of diastole. This is a consequence of the falling diastolic pressure at a time when the velocity is zero. Since there is not mechanism to suddenly stop these backward waves at the start of arterial systole, it could be argued that early systole is not free of backward waves as assumed in the PU-loop method. The presence of equal and opposite forward and backward waves necessitated by a falling pressure at a time of zero velocity are not present is obviated  if we first separate the pressure into a reservoir and an excess pressure [refs] and then plot the excess PU-loop. This may be an important virtue of the reservoir/excess pressure hypothesis [ref Ashraf, Allesandro??].