Amplification and Reflection Central vs Peripheral BP

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The morphology of a pulse contour at any point along the vascular tree represents the sum of the forward and reflected pressure waves at that point. Pulse wave morphology is unique in each artery because of differences in time of arrival of the forward and reflected pressure waves. Unlike diastolic or mean BP, systolic BP is thus not constant throughout the arterial tree.

In the arm, the amplitude of the forward wave is generally much greater than the amplitude of the reflected wave, so brachial cuff systolic BP conveys information primarily about the forward wave, including PPA, but is largely blind to changes in wave reflection (fig. 3). In contrast, peak central systolic BP is the result of a different admixture of the forward and reflected pressure waves, where the pulse pressure generated by the forward pressure wave may be augmented by as much as 100% by the reflected wave. In such cases, there may be very little difference between central and peripheral peak systolic pressure [40] and thus very little apparent PPA. This does not mean that true PPA of the forward wave is absent, just that the sum of the forward and reflected waves in the aorta is equal to the degree of true PPA of the forward wave in the arm. PPA tends to be greater in young individuals with isolated systolic hypertension despite lower SVR [47].

Relations between age and wave reflection are complex in normal and hypertensive individuals. Augmentation index (AI, the fractional increment of central PP caused by the principal reflected wave) is higher in hypertension but in the general population, AI increases until middle age, then plateaus or declines [9, 48]. There has also been significant confusion in the literature about the relation between AI and arterial stiffness. AI is only minimally related to the properties of central arteries [49-52] and is more governed by the degree of arteriolar narrowing of the distal circulation [52-54]. In contrast, widening of PP with age is almost always due to increased amplitude of the forward wave due to increased Zc in the central aorta.

Because the reflected wave is not apparent in the peak systolic BP measured by arm cuff, phenomena related to altered wave reflection go unnoticed in everyday clinical medicine. Newer techniques of pulse wave analysis offer new insight into this problem, however. The most popular method utilizes a generalized transfer function to derive central systolic BP from peripheral to-nometric recordings of radial pressure. There is recurring debate regarding the reliability of the generalized transfer function in estimating central systolic BP

[55], in part due to the need for different formulas for different disease states

[56]. Nevertheless, the insensitivity of brachial cuff BP readings to detect clinically important changes in central systolic BP has been demonstrated in recent studies. For example, ^-blockers have been found to be less effective than vasodilators in reducing augmentation pressures or central systolic BP [57-59]. Such studies suggest that non-invasive assessment of central systolic BP may be useful in assessing cardiac loading characteristics and the relative benefits of different antihypertensive drug classes [60].

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