Central Aortic Pressure: The Next Frontier in Blood Pressure Measurement?
First described over one hundred years ago, the non-invasive measurement of blood pressure over the brachial artery has remained in clinical practice virtually unchanged ever since. However, it has long been recognised that pressure in the brachial arter
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Bryan Williams and Peter S. Lacy
15.1
Introduction
Blood pressure (BP) has been conventionally measured over the brachial artery by sphygmomanometry for more than a century. Measured in this way, systolic BP (SBP), diastolic BP, and pulse pressures (PP) have all been shown to be predictors of cardiovascular risk [1–5]. More recently, automated blood pressure measurement devices have increased in popularity but are still designed to measure brachial blood pressure (BrBP). A key question is: how representative of aortic pressure is BrBP? This question is considered important because it is reasonable to assume that the pressure in the large conduit arteries, the so-called central aortic pressure (CAP), is more representative of the hemodynamic stress on major organs such as the heart, brain, and kidney. Furthermore, BrBP and CAP are not the same. Indeed, cardiologists undertaking cardiac catheterization will have long recognized that the directly measured pressure at the aortic root is invariably lower than that simultaneously measured over the brachial artery [6–8]. This aortic rootbrachial artery pressure difference is most noticeable for SBP and PP. In contrast, mean arterial pressure (MAP) remains relatively constant across the larger conduit arteries of the circulation [9]. The increase in SBP and PP from the aortic root to the brachial artery results from pressure wave amplification as it moves from the aortic root to the periphery of the circulation [10], an amplification ratio of 1.2–1.5 for pulse pressure being typical [11].
B. Williams (&) Institute of Cardiovascular Sciences, University College London, London, UK e-mail: [email protected] P. S. Lacy University College London, London, UK A. E. Berbari and G. Mancia (eds.), Special Issues in Hypertension, DOI: 10.1007/978-88-470-2601-8_15, Ó Springer-Verlag Italia 2012
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If the magnitude of pressure amplification remained constant in an individual patient or between patients, then BrBP measurement would always be a good surrogate for CAP and there would be no point in endeavoring to measure the latter. However, the degree of pressure amplification is highly variable and is influenced by a number of factors, including: age (usually as a surrogate for arterial stiffness), BP, heart rate, and drug therapies. Consequently, there is a rationale for considering the noninvasive measurement of CAP; i.e., that it is based on the fact that BrBP is often not a perfect surrogate for true arterial pressure. This chapter will discuss the principles and methods for the noninvasive measurement of CAP, the pathophysiological basis for the differences between CAP and BrBP, the normal ranges for CAP, and the impact of drug therapies on CAP. The chapter concludes with a look into future developments in this field and the clinical utility of CAP measurement.
15.2
Noninvasive Measurement of Central Aortic Pressure
Current methods for the noninvasive measurement of CAP are dependent on the capture of an arterial waveform which
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