Assessment of arterial stiffness in clinical practice

There has been much recent interest in the relationship between arterial stiffness and cardiovascular disease. Pulse pressure and pulse wave velocity, surrogate measures of arterial stiffness, indicate that arterial stiffness increases both with age and in certain disease states that are themselves associated with increased cardiovascular risk, including hypertension, diabetes mellitus, hypercholesterolaemia and end‐stage renal failure.1 As changes can be detected before the appearance of clinically apparent vascular disease, arterial stiffness may act either as a marker for the development of future atherosclerotic disease, or may be more directly involved in the process of atherosclerosis. Arterial stiffening has been particularly implicated in the development of isolated systolic hypertension, a disease mainly affecting the elderly population and associated with considerable excess morbidity and mortality. Arterial stiffness may be measured using a variety of different techniques, although at present the majority of measurements are made for experimental and physiological studies rather than in clinical practice. However, it is likely that over the next few years measurement of arterial stiffness will become an increasingly important part of the process of risk assessment, and may possibly also improve the monitoring of therapy in patients with conditions such as isolated systolic hypertension. Therefore, it will become necessary for physicians both in primary care and hospital practice to understand the importance of arterial stiffness and the techniques available for its clinical assessment, which will be the focus of this review. Assessment of the arterial pulse has always been an important part of clinical examination, and the ancients recognized that changes in the character of the pulse indicated the presence of disease. However, the ‘golden era’ of the pulse was the 19th century. With the development of the sphygmograph by Marey2 and its refinement over the next few years by Mahomed,3 Broadbent4 and Mackenzie,5 the art of interpreting the shape of the arterial waveform, or sphygmocardiography, began. Mahomed noted important differences in the shape of the waveform with age and the pre‐albuminuric stage of Bright's disease (now thought to be essential hypertension), with a late systolic peak and little or no diastolic fluctuation. However, stiffening of the arteries was thought to be an inevitable consequence of ageing, and its clinical significance was not fully appreciated by many. Nevertheless, Roy commented as early as 1880 that with increasing age the stiffness of the arteries changed, and that this might have significance for the health of the individual: ‘Only in the case of young children do we find that the elasticity of the arteries is so perfectly adapted to the requirements of the organism as it is in the case of the lower animals.’6 However, when the modern mercury sphygmomanometer was developed by Riva‐Rocci in 1896,7 clinicians began to concentrate more on the absolute values of systolic and diastolic blood pressure rather than on the shape of the waveform, thereby disregarding important qualitative information in favour of information covering only the extremes of pressure. Indeed, as Lord Kelvin suggested, sphygmomanometry was considered far more scientific: ‘When you can measure what you are speaking about, and can express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind’ (Lord Kelvin, 1891). Thus, by the early 20th century, sphygmocardiography was in rapid decline and the sphygmomanometer has dominated ever since. Only recently have clinicians re‐discovered the importance of the arterial waveform and arterial stiffness, hence spawning a variety of different measurement techniques. The terminology used in the field of arterial stiffness can be confusing. Terms are often used interchangeably when in fact they have slightly different meanings. Therefore, the generic term ‘arterial stiffness’ is used in the present review to avoid confusion. In simple terms, arterial stiffness describes the rigidity of the arterial walls. Other terms used to describe the properties of vessel walls are defined in Table 1.8 Important indices include compliance, elasticity (or elastic modulus), distensibility and vascular impedance. However, interpretation of these indices can be complicated, especially because many are dependent on blood pressure. In this respect, the stiffness index (β) may be more useful because it is less dependent on blood pressure. Vascular impedance9 relies on consideration of the circulation as an electrical circuit. Impedance describes the relation of forces acting in the circulation to the motion of blood. However, in order to calculate impedance, it is necessary to measure pressure and flow simultaneously at one point in the circulation, which has limited its widespread usage. Different types of impedance values can be calculated. Characteristic impedance is probably the most useful, and describes the ratio of pressure to flow, assuming no reflected waves are present. In contrast, input impedance describes the impedance in the whole of the vascular bed distal to the catheter, and thus is influenced by wave reflection. Terminal impedance approximates to peripheral vascular resistance, and longitudinal impedance is the ratio of the pressure gradient to flow and describes the impedance only in the local segment of artery being studied. Indices of arterial stiffness P, pressure; D, diameter; V, volume; h, wall thickness; t, time; v, velocity; s, systolic; d, diastolic. *Most common method of measurement; **Also requires pressure measurements. Indices of arterial stiffness P, pressure; D, diameter; V, volume; h, wall thickness; t, time; v, velocity; s, systolic; d, diastolic. *Most common method of measurement; **Also requires pressure measure