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1 Department of Cardiology, Ruprecht-Karls Universität Heidelberg, 69115 Heidelberg; and 2 I Physiologisches Institut, Abteilung Biophysik des Keislaufs, Ruprecht-Karls Universität Heidelberg, 69120 Heidelberg, Germany
Basic Physiological Principles
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The classic definition of compliance is the change in blood volume relative to a given change in distending pressure. The direct measurement of regional aortic compliance is difficult, because there is no simple means of estimating regional changes in blood volume. However, assuming that there is mainly radial and negligible axial vessel movement during pulse pressure, compliance can be estimated as a change in radius, diameter, or cross-sectional area for a given change in pressure (11, 17, 19). This assumption of negligible axial vessel movement during pulse pressure appears to be a valid approximation when compliance is estimated from radial extension of vascular cross sections for several reasons. It should be noted that neglect of longitudinal vessel axis is accounted for in the theory of pulse wave propagation developed by O. Frank (7), assuming that volume changes occur mainly because of expansion of the vessel wall along its radius. Also, when aortic pressure and radius are analyzed in the living dog, the ratio of pressure changes to changes in vessel radius obtained at identical sites compared well with measures of impedance (19), justifying the use of simpler radius measurements for estimating dynamic elasticity of the aorta.
Direct Imaging of Pulsatile Vascular Dimensions
Although the central aorta contributes most to total compliance of the arterial tree, most studies employing noninvasive imaging modalities have focused on evaluating superficial arteries such as the carotid, brachial, and radial arteries. Recently, the mechanical behavior of the aorta in humans has been studied using two-dimensional and transesophageal echocardiography (14, 16) and magnetic resonance imaging (18).Initial studies have been published suggesting the use of intravascular ultrasound to determine aortic compliance in humans (13). Validity of intravascular ultrasound measurements has recently been proven by comparison with sonomicrometer measurements in conscious and anesthetized normal dogs (12). Intravascular ultrasound allows for mapping of viscoelastic behavior of multiple aortic segments and thus may help in investigating the mechanisms of changes in viscoelastic properties in experimental arterial hypertension and atherosclerosis models, avoiding the use of sonomicrometers. Potential limitations that need to be considered are misleading area measurements in oblique catheter positions and the need for careful calibration of ultrasound equipment (3).
Evaluation of Aortic Compliance from Pressure-Dimension Relationships
There are two approaches to evaluating compliance of large arteries from pressure-dimension relationships. The first approach is to measure instantaneous pressure-dimension relations of single pulse waves at varying levels of distending pressures, resulting in a family of hysteresis loops. Arterial compliance, representing the slope at each point of the pressure-dimension curve, is a nonlinear function of pressure (4). Higher distending pressures are accompanied by smaller changes in aortic dimensions that are determined by shifts along the same pressure-dimension curve when there is no change in arterial wall elasticity (Fig. 1). Changes in wall elasticity unrelated to acting distending pressures are eminent from shifts of pressure-dimension curves and changes in the slope of the pressure-dimension curve at a given pressure. The second approach is a simplification considering only peak systolic-to-diastolic differences in pressures and dimensions at varying blood pressure levels (Table 1) (20). However, the assessment of pressure-dimension relationships may give only an estimate of arterial elastic properties, because a distinction has to be made between distensibility of the aorta as a tube and elastic material properties of the vessel wall components. Evaluation of elastic material properties needs to account for vessel geometry (vessel diameter-to-thickness ratio) to calculate stress-strain relationships (1). In the clinical setting, noninvasive measurements of aortic wall thickness are difficult but may presently be best approached semi-invasively using high-resolution transesophageal echocardiography (16).
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Potential Clinical Importance of Estimating Aortic Compliance
Compliance of large conduit arteries has been found to be decreased as a result of aging (9), arterial hypertension (6), atherosclerosis (5), diabetes (22), and heart failure (15). Changes in the composition of the vessel wall and changes in vessel geometry accompanying these cardiovascular and metabolic disease states are the leading mechanisms explaining a decrease in vascular compliance. A decrease in aortic compliance increases cardiac and vascular load and leads to increases in systolic pressure and pulse pressure, an independent risk factor for development of cardiovascular disease (8). With recent reports suggesting that arterial wall compliance is improved by drugs acting on vascular structure or endothelial cell or smooth muscle function (2), estimation of central aortic compliance in clinical settings may become more relevant.Address for reprint requests and other correspondence: H. F. Kuecherer, Dept. of Cardiology, Ruprecht-Karls Universität Heidelberg, Bergheimstr. 58, 69115 Heidelberg, Germany (E-mail: helmut_kuecherer{at}med.uni-heidelberg.de).
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D) divided by change in pressure (