Aortic distensibility and hypercholesterolaemia

Elastic properties and Windkessel function of the human aorta

An understanding of the role of the aortic elastic properties indicates their relevance at several sites of cardiovascular function. Acting as an elastic buffering chamber behind the heart (the Windkessel function), the aorta and some of the proximal large vessels store about 50% of the left ventricular stroke volume during systole. In diastole, the elastic forces of the aortic wall forward this 50% of the volume to the peripheral circulation, thus creating a nearly continuous peripheral blood flow. This systolic-diastolic interplay represents the Windkessel function, which has an influence not only on the peripheral circulation but also on the heart, resulting in a reduction of left ventricular afterload and improvement in coronary blood flow and left ventricular relaxation. The elastic resistance (or stiffness), which the aorta sets against its systolic distention, increases with aging, with an increase in blood pressure, and with pathological changes such as atherosclerosis. This increased stiffness leads to an increase in systolic blood pressure and a decrease in diastolic blood pressure at any given mean pressure, an increase in systolic blood velocity, an increase in left ventricular afterload, and a decrease in subendocardial blood supply during diastole, and must be considered a major pathophysiological factor, for example, in systolic hypertension. The elastic properties of the aortic Windkessel can be assessed in vivo in humans in several ways, most easily by measuring the pulse wave velocity along the aorta. The higher this velocity, the higher the elastic resistance, that is, the stiffness. Other methods depend on assessment of the ratio between pulse pressure and aortic volume changes (delata P/delta V), which can be assessed noninvasively by ultrasonic or tomographic methods. All assessments of vessel stiffness have to take into account the direct effect of current blood pressure, and thus judgements about influences of interventions rely on an unchanged blood pressure. Alternatively, to derive the "intrinsic" stiffness of the aortic wall one has to correct for the effect of the blood pressure present. Recently reports about pharmacologic influences on the elastic properties of the aorta have emerged in the literature.(ABSTRACT TRUNCATED AT 400 WORDS)

Noninvasive determination of local wavespeed and distensibility of the femoral artery by comb-excited Fourier velocity-encoded magnetic resonance imaging: measurements on athletic and nonathletic human subjects

The local distensibility of arteries is of interest because distensibility varies from artery to artery, may be altered by disease to different extents in different arteries, and may be modified by physiological or pharmacological means. Using magnetic resonance imaging (MRI) we have measured local arterial wavespeed in the femoral artery in healthy human subjects and calculated local arterial distensibility. We acquired 2-D coronal and sagittal MR phase contrast angiograms of the femoral artery. We used a novel imaging technique, comb-excited Fourier velocity-encoded MRI, to obtain simultaneous measurements of arterial blood velocity at two stations 14 cm apart on the femoral artery. The separation of the two stations divided by the delay between the onset of forward flow at the two stations was used to calculate the wavespeed. The measurements were made on 16 healthy men (8 athletes, 8 non-athletes) in the age range 20-30 years, who were scanned with the use of ECG gating and an extremity coil in a 1.5 Tesla scanner (GE Medical Systems, Milwaukee, WI). By systematically altering the delay between the R-wave and data acquisition, a temporal resolution of 2-4 ms was achieved. The onset of forward flow at each station was determined from a least-squares fit to the data for 30% of the maximum velocity during the cardiac cycle. Average femoral artery wavespeed was 7.7 m/s +/- 1.2 in the athletes and 11.5 m/s +/- 1.1 in the non-athletes (P < 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)

Aortic compliance measured by non-invasive Doppler ultrasound: application of a personal computer based MkII system and its repeatability

A non-invasive pulse-wave-velocity Doppler ultrasound technique for the assessment of aortic compliance is described. A computational approach for correcting for the effect of non-chronic changes in blood pressure is considered and applied to compliance measurements performed on an age-select cohort of 70 normotensive, normal healthy volunteers. In order to permit the wider availability of the pulse-wave-velocity Doppler ultrasound technique, the authors have developed a MkII system based on a standard 80486/33 MHz IBM compatible WINDOWS based personal computer; real-time spectral analysis being achieved using a relatively inexpensive but fast analogue to digital signal processing card. An overview of the new apparatus is provided and verification work to compare the repeatability of the MkI and MkII systems is described. Medical disorders such as atherosclerosis, diabetes mellitus, familial hypercholesterolaemia, growth hormone deficiency, and Ehlers-Danlos and Marfan's syndromes have all been shown to affect arterial wall compliance. We suggest that the in vivo clinical measurement of blood pressure corrected aortic distensibility using the MkII system may be a useful, reproducible, non-invasive tool for assessing such patients' susceptibility to atheromatous arterial disease as well as for monitoring their response to therapeutic interventions. Measurements in the aorta may be especially pertinent since the natural history of fatty streaks there tends to parallel that in the coronary vasculature thereby potentially affording a convenient surrogate estimate of coronary heart disease.

Relation of aortic stiffness to factors modifying the risk of atherosclerosis in healthy people

To identify factors predicting aortic stiffness, we studied the modulus of elasticity of the thoracic aorta in relation to sex, obesity, blood pressure, physical activity, smoking, ethanol consumption, salt intake, and serum lipid and insulin levels in 55 healthy people born in 1954. A transverse cine magnetic resonance image of the thoracic aorta was made, and the modulus of elasticity was determined as brachial artery cuff pulse pressure/aortic strain, where strain was determined as the ratio of pulsatile aortic luminal area change to the diastolic luminal area. The average of measurements made in the ascending and descending aorta was used as the elastic modulus of the thoracic aorta. Habitual physical activity, smoking, and alcohol use were quantified by 2-month prospective daily recording and salt intake by 7-day food records. The aortic elastic modulus ranged from 100 to 2091 10(3) dyne/cm2 (median, 390 10(3) dyne/cm2). In multiple regression analyses, log10 aortic elastic modulus was related directly to mean blood pressure (standardized coefficient [beta] = .37, P = .002), serum high-density lipoprotein cholesterol (beta = .36, P = .012), square root of daily energy expenditure in physical activity (beta = .33, P = .005), and log10 serum insulin (beta = .27, P = .047) and inversely to serum low-density lipoprotein cholesterol (beta = -.26, P = .035). A relation to salt intake was also observed, but the regression slope was dependent on mean blood pressure (P = .005 for interaction). These data suggest that many modifiable constitutional and lifestyle characteristics may contribute to the stiffness of the thoracic aorta.

Validation and reproducibility of pressure-corrected aortic distensibility measurements using pulse-wave-velocity Doppler ultrasound

A non-invasive Doppler ultrasound technique is described for the assessment of aortic compliance based on the in vivo measurement of pulse wave velocity along the thoraco-abdominal aortic pathway. A structured protocol, which has been developed to improve the reproducibility of the technique, is validated. A method of correcting for the effect of non-chronic changes in blood pressure on arterial elasticity is considered and applied to compliance measurements performed on 66 normal, healthy volunteers. The results of a study to ascertain the overall reproducibility of the method are provided and problems associated with the technique are discussed. Medical disorders such as atherosclerosis, diabetes mellitus, familial hypercholesterolaemia and growth hormone deficiency have all been shown to affect arterial wall compliance. It is suggested that the in vivo measurement of pressure-corrected aortic distensibility may be a useful, non-invasive tool for assessing such patients' susceptibility to atheromatous arterial disease and for monitoring their response to therapy. Measurements in the aorta may be especially pertinent since the natural history of fatty streaks there tends to parallel that in coronary arteries thereby potentially affording a convenient surrogate estimate of coronary heart disease.

A blood pressure independent index of aortic distensibility

A non-invasive Doppler ultrasound technique for the assessment of aortic compliance based on the in vivo measurement of pulse wave velocity along the thoraco-abdominal aortic pathway is described. An approach for correcting for the effect of blood pressure on aortic compliance is considered. The derivation of an index of distensibility, Cp, which is independent of blood pressure is provided and applied to data collected from 58 normal, healthy volunteers. Medical disorders such as atherosclerosis, diabetes mellitus and familial hypercholesterolaemia have all been shown to affect arterial distensibility. We suggest that the clinical measurement of Cp may be a useful, non-invasive tool for assessing such patients' susceptibility to atheromatous arterial disease as well as for monitoring their response to therapy.

Aortic compliance measurements using Doppler ultrasound: in vivo biochemical correlates

A noninvasive Doppler ultrasound technique for the assessment of aortic compliance based on the in vivo measurement of pulse wave velocity along the thoraco-abdominal aortic pathway is described. An approach for correcting for the effect of blood pressure on aortic distensibility is considered. The derivation of an index of intrinsic distensibility, Cp, which is independent of blood pressure, is provided and applied to data collected from normal, healthy volunteers. Overviews are provided of studies utilising the technique to determine aortic compliance in medical disorders, which are known to predispose to premature cardiovascular disease, such as diabetes mellitus, familial hypercholesterolaemia and growth hormone deficiency. The significance of correlations between in vivo aortic compliance measurements and plasma concentrations of total cholesterol, low-density-lipoprotein cholesterol, high-density-lipoprotein cholesterol and insulin-like growth factor-I are discussed. It is proposed that the measurement of aortic compliance in normal, healthy individuals may potentially be a useful in vivo research tool for investigating the effects of biochemical factors on the biophysical properties of the aortic wall. Furthermore, we believe that the routine measurement of blood pressure-corrected aortic distensibility may prove a useful, noninvasive clinical tool for assessing patients' susceptibility to atherosclerosis, as well as for monitoring their response to therapeutic interventions.