Analysis of wave reflections in the arterial system using wave intensity: a novel method for predicting the timing and amplitude of reflected waves

The timing and amplitude of reflected arterial waves in the ascending aorta were studied by analysis of the aortic pressure waveform and were compared with those derived using wave intensity analysis. Wave intensity analysis considers aortic pressure changes to be the result of forward and backward wavelets carrying energy. Wave intensity (dI = dPdU) is calculated from changes in pressure (dP) and flow velocity (dU), and its sign indicates the direction of travel of propagating wavelets (positive for forward-traveling waves and vice versa). We measured aortic pressure and flow velocity in 14 patients, mean age 60 +/- 9 years, with three-vessel coronary artery disease at the time of surgical revascularization. The travel time of the reflected wave derived from analysis of the aortic pressure waveform (t(p)) was measured from the foot of the aortic pressure waveform to the inflection point of the aortic pressure (derived objectively from the zero of second derivative of aortic pressure). From wave intensity analysis, the travel time of the reflected wave was measured to the onset of the wave intensity of the backward-traveling wave dI_ (t(i)), and to the onset of the separated backward pressure wave (t(b)). All patients showed an aortic pressure waveform characterized by an inflection point on the rising limb of the aortic pressure, followed by a secondary rise in pressure, representing the return of reflected waves. Wave intensity analysis consistently showed a negative peak in mid systole, the timing of its onset corresponding closely to the inflection point of the aortic pressure. The travel time of the reflected wave derived from the analysis of the aortic pressure waveform (t(p)) was 121 +/- 21 ms and showed close agreement with t(i) (118 +/- 28 ms) and t(b) (115 +/- 29 ms), with mean differences of 4 and 6 ms, and 95% confidence intervals of difference (-2 to 7 ms) and (1 to 12 ms), respectively. The augmentation index, a measure of the secondary increase in aortic pressure due to reflected waves, was significantly correlated with the magnitude of dI_ (r = 0.63, P < 0.001). Wave intensity is a quantity that indicates the rate of energy flux due to wave travel and since its value is positive for forward-traveling waves and negative for backward-traveling waves, its calculation allows the timing of reflected waves to be accurately predicted. Furthermore, the magnitude of wave intensity in backward-traveling waves (dI_) is related to the augmentation index and map provide a measure of the amplitude of the reflected wave. This analysis of the arterial system is done in the time domain and therefore can be easily applied to assess temporal changes in arterial characteristics.