Tw. Koh et al., Analysis of wave reflections in the arterial system using wave intensity: a novel method for predicting the timing and amplitude of reflected waves, HEART VESS, 13(3), 1998, pp. 103-113
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 cha
nges in pressure (dP) and flow velocity (dU), and its sign indicates the di
rection of travel of propagating wavelets (positive for forward-traveling w
aves and vice versa). We measured aortic pressure and flow velocity in 14 p
atients, mean age 60 +/- 9 years, with three-vessel coronary artery disease
at the time of surgical revascularization. The travel time of the reflecte
d wave derived from analysis of the aortic pressure waveform (t(p)) was mea
sured from the foot of the aortic pressure waveform to the inflection point
of the aortic pressure (derived objectively from the zero of second deriva
tive 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 b
ackward-traveling wave dI_ (t(i)), and to the onset of the separated backwa
rd pressure wave (t(b)). All patients showed an aortic pressure waveform ch
aracterized by an inflection point on the rising limb of the aortic pressur
e, followed by a secondary rise in pressure, representing the return of ref
lected waves. Wave intensity analysis consistently showed a negative peak i
n mid systole, the timing of its onset corresponding closely to the inflect
ion point of the aortic pressure. The travel time of the reflected wave der
ived from the analysis of the aortic pressure waveform (t(p)) was 121 +/- 2
1 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 refle
cted waves, was significantly correlated with the magnitude of dI_ (r = 0.6
3, P < 0.001). Wave intensity is a quantity that indicates the rate of ener
gy flux due to wave travel and since its value is positive for forward-trav
eling waves and negative for backward-traveling waves, its calculation allo
ws the timing of reflected waves to be accurately predicted. Furthermore, t
he magnitude of wave intensity in backward-traveling waves (dI_) is related
to the augmentation index and map provide a measure of the amplitude of th
e 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 ar
terial characteristics.