Quantification of Wave Reflection in the Human Aorta From Pressure Alone

Abstract

Abstract-Wave reflections affect the proximal aortic pressure and flow waves and play a role in systolic hypertension. A measure of wave reflection, receiving much attention, is the augmentation index (AI), the ratio of the secondary rise in pressure and pulse pressure. AI can be limiting, because it depends not only on the magnitude of wave reflection but also on wave shapes and timing of incident and reflected waves. More accurate measures are obtainable after separation of pressure in its forward (P f ) and reflected (P b ) components. However, this calculation requires measurement of aortic flow. We explore the possibility of replacing the unknown flow by a triangular wave, with duration equal to ejection time, and peak flow at the inflection point of pressure (F tIP ) and, for a second analysis, at 30% of ejection time (F t30 ). Wave form analysis gave forward and backward pressure waves. Reflection magnitude (RM) and reflection index (RI) were defined as RMϭP b /P f and RIϭP b /(P f ϩP b ), respectively. Healthy subjects, including interventions such as exercise and Valsalva maneuvers, and patients with ischemic heart disease and failure were analyzed. RMs and RIs using F tIP and F t30 were compared with those using measured flow (F m Key Words: aorta Ⅲ blood flow Ⅲ blood flow velocity Ⅲ blood pressure Ⅲ pulse A ortic pressure, and especially pulse pressure (PP), is now recognized as an important indicator of cardiovascular risk 1-4 and can guide pharmaceutical treatment. 5,6 Wave reflections affect the pressure and flow wave in the proximal aorta, 7 and their contribution depends on their magnitude (determined by the periphery and the large arteries) and time of return (mainly determined by the large, conduit arteries). When the reflected wave arrives in systole, it augments pressure, leading to increased systolic and PP. This augmentation is greater when the heart is hypertrophied. 8 In heart failure, wave reflections affect the flow wave negatively, thereby reducing stroke volume and cardiac output. 8 -10 One way to estimate the amount of reflection is by waveform analysis in which aortic pressure is separated into its forward and backward components. 7,11,12 The ratio of the magnitudes of the backward (reflected) wave and the forward (incident) wave, the reflection magnitude (RM), allows for the estimation of the amount of reflection, but this waveform analysis requires measurement of both pressure and flow waves. A method that requires the measurement of pressure only is computation of the augmentation index (AI). 13,14 AI gives reproducible results 15,16 and is in use in clinical settings. [17][18][19][20] However, AI is determined by both the magnitude and timing of the reflected wave. This is evident from Figure 1A. In this figure, the original pressure wave is separated into its forward and backward components and then reassembled for different delays of the same backward wave. AI is clearly influenced by the time of return of the reflected wave. Figure 1B gives 2 examples in w...

Determination of Aortic Pulse Wave Velocity From Waveform Decomposition of the Central Aortic Pressure Pulse

“…In all of the data analyzed, this algorithm was able to determine a value for T 1 in all of the waves. The triangular approximation is consistent with the general aortic flow envelope obtained from Doppler ultrasound 25 or electromagnetic 19,26 flow measurements. Furthermore, the timing of peak flow corresponds with the time of early systolic inflection in the aortic pressure wave.…”

“…27,28 Input impedance (Z in ) is calculated as a function of frequency from Fourier decomposition of P m (t) and Q m (t) shown in Figure 1, and Z c is estimated from the average of the Z in modulus between harmonics 4 and 7. 19 Because Z in is a ratio of pressure and flow quantities, the product Z c .Q m (t) in equations 3 and 4 is always in units of pressure; hence, an absolute flow calibration is not required. Thus, a flow scale is assumed of 0 to 100 arbitrary units.…”

“…12,16 -18 The central aortic pressure waveform is decomposed into forward and reflected waves using an uncalibrated triangular aortic flow waveform, as shown in a recent study validating the proof of concept. 19 PTT is then estimated from the time difference between the derived forward and reflected waves after a signal processing procedure based on waveform constraints criteria. Calculated PTT was evaluated against transit time measurements determined from independent recordings of carotid and femoral pulses in 2 groups of subjects.…”

Reliability of pulse waveform separation analysis

“…The aortic forward (Pf) and backward (Pb) wave pressures were determined by assuming a triangular flow wave. [10,15] This method creates a triangular-shaped flow wave by matching the start, peak, and end of the flow wave to the timings of the foot, inflection point, and incisura of the aortic pressure wave (Figure 1). Thus, the forward and backward components of the pressure wave can be constructed using the following equations:…”

“…[4,7] The AIx, which is calculated by dividing the central augmentation pressure by the corresponding pulse pressure, is affected by the reflected wave transit time. [8,9] Alternatively, by assuming a triangular or a physiologic flow waveform, [10] the aortic wave can be separated into its forward (Pf) component and timing-independent reflected component (Pb). Two large prospective studies [4,5] suggest that wave separation analysis may be superior to AIx as a subclinical marker of cardiovascular disease, one reporting that Pb better predicts 15-year cardiovascular mortality than AIx, [5] the other that reflection magnitude (RM, Pb/Pf) better predicts cardiovascular events than AIx.…”