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Figure 1 The early part of the ascending aortic pressure wave, with amplitude (Pi – Pd), is generated by the left ventricular (LV) ejection (flow) wave. This forward-traveling pressure wave is dependent upon central (elastic) arterial stiffness and pulse wave velocity (PWV) and is not influenced by wave reflections. The later part of the pressure wave, with amplitude (Ps – Pi), is generated by the reflected wave arriving from the lower body during systole and adding to the forward pressure wave. This wave is dependent upon the elastic properties of the entire arterial tree and PWV. Pulse pressure = (Pi – Pd) + (Ps – Pi) = (Ps – Pd), and augmentation index (AIa) = (Ps – Pi)/(Ps – Pd). AIa is dependent on the elastic properties of the entire arterial tree, transmission velocity of the reflected wave, and distance to the major reflecting site.  tp is inversely related to arterial stiffness and is the round-trip travel time of the pressure wave from the heart to the periphery and back; tr is the systolic duration of the reflected wave. Wasted LV pressure energy (2.09 tr[Ps – Pi]) and tension-time-index (area under the systolic portion of the pressure wave) are measures of myocardial oxygen demand. The broken curve is the pressure or flow wave without reflections.
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