Arterial pulse wave velocity in coronary arteries. 
Pulse wave velocity is related to arterial stiffness.  Pulse wave velocity changes with age and disease and is a useful indicator of cardiovascular disease.  Different methods are used for evaluating pulse wave velocity in systemic vessels, but none is applicable to coronary arteries.  In this study we first compare values of wave speed (c) calculated from measurements of pressure (P) and velocity (U) using different analytical methods: PU-loop, beta stiffness parameter, characteristic impedance, foot-to-foot method, and the sum of squares (Sigma(2)), a novel way of calculating the wave speed (calculated from the square root of the sum of the ratio of the dP(2) and dU(2) over a complete cardiac cycle).  Results from human measurements using Doppler ultrasound on carotid arteries show good correlation between the PU-loop method, beta stiffness parameter and Sigma(2).  Characteristic impedance calculations show the greatest variation of all methods.  The Sigma(2) method was further assessed in vitro for use in coronary vessels.  Pressure and velocity measurements were obtained from human coronary arteries following angiographic studies.  The measurements were made invasively by co-locating two wires with pressure and velocity transducers.  Pressure and velocity data in the left anterior descending, circumflex, left main stem and right coronary arteries were acquired simultaneously along with the ECG signal.  Wave speed was calculated using Sigma(2).  Wave intensity analysis was used to determine forward and backward traveling waves at different times in different locations, for which wave speed, approximate distance and timings between waves need to be known.