Aortic compliance measurements using Doppler ultrasound: in vivo biochemical correlates

Reproducibility of pulse wave velocity and augmentation index measured by pulse wave analysis

OBJECTIVE

The aim of this study was to determine the reproducibility of pulse wave velocity (PWV) and augmentation index (AIx) measured using pulse wave analysis (PWA), prior to its use in large-scale clinical trials.

METHODS

Arterial pressure waveforms were recorded and analysed using an established technique (Sphygmocor). Subjects with and without a range of recognized cardiovascular risk factors were studied to provide a wide range of values. Measurements were made after a brief introduction to the technique in a clinical setting. Two observers recorded aortic and brachial PWV in 24 subjects, each on two occasions, in a random order. In a separate study, two different observers used PWA to determine AIx in 33 subjects, each on two occasions, in a random order. Data were analysed using Bland-Altman plots and presented as mean +/- SD.

RESULTS

Brachial PWV was 8.65+/-1.58 m/s (range 6.16-10.95 m/s) and aortic PWV was 8.15+/-3.01 m/s (5.01-17.97 m/s). Within-observer variability was 0.14+/-0.82 m/s for brachial PWV and 0.07+/-1.17 m/s for aortic PWV. Corresponding between-observer values were -0.44+/-1.09 m/s and -0.30+/-1.25 m/s. AIx ranged from -15.0 to +45.0%, with a group mean of +19.6+/-12.0%. The within-observer difference was 0.49+/-5.37% and between-observer difference 0.23+/-3.80%.

CONCLUSION

PWA is a simple and reproducible technique with which to measure PWV and AIx. Reproducibility accords with that reported by other workers using different methodologies. PWA may, therefore, be suitable for large-scale population and intervention studies investigating the clinical relevance of vascular stiffness.

Relation between number of cardiovascular risk factors/events and noninvasive Doppler ultrasound assessments of aortic compliance

The aim of this study was to establish the relation between noninvasive Doppler ultrasound assessments of aortic compliance, based on "foot-to-foot" aortic pulse wave velocity measurements, and presumed atherosclerotic load in patients with vascular disease and/or diabetes mellitus. One hundred ten patients with vascular disease and/or diabetes mellitus (arteriopaths) underwent measurement of in vivo aortic compliance using Doppler ultrasound. Demographic data on these subjects were recorded along with details of cardiovascular risk factors and events. Aortic compliance values were compared with data from 51 age-matched healthy, asymptomatic subjects putatively free of vascular disease (controls). Data are expressed as mean+/-SD. Arteriopaths were aged 64.1+/-8.4 years and had total cholesterol levels of 5.9+/-1.1 mmol/L and aortic compliance of 0.78+/-0.42%/10 mm Hg [1.33 kPa]. Most arteriopaths had 2 or more cardiovascular risk factors and events: diabetes (n=41), hypertension (n=45), smoking (n=86), cerebrovascular/transient ischemic event (n=13), myocardial infarction (n=44), angina (n=51), and/or peripheral vascular disease (n=33). Controls were aged 64.3+/-12.1 years with total cholesterol of 6.1+/-1.1 mmol/L and aortic compliance of 1.14+/-0.46%/10 mm Hg [1.33 kPa] (P<0.002 versus arteriopaths). Subset analysis revealed that patients with the greatest number of cardiovascular risk factors and events (n=5) had the stiffest aortas (aortic compliance, 0.58+/-0.15%/10 mm Hg [1.33 kPa]) compared with those patients with the median and mean (n=2) number of risk factors and events (aortic compliance, 0.80+/-0.50%/10 mm Hg [1.33 kPa]; P<0.02). The data suggest that a significant inverse relation exists between presumed atherosclerotic load (as assessed by the number of cardiovascular risk factors and events) and aortic compliance determined noninvasively based on aortic pulse wave velocity measurements. If these findings are confirmed by prospective, longitudinal follow-up studies, such measurements may prove useful as a noninvasive marker of vascular risk.

Relationship between abdominal aortic aneurysm wall compliance and clinical outcome: a preliminary analysis

BACKGROUND

Aortic compliance, as measured by the pressure-strain elastic modulus (Ep) and stiffness (B), may allow a more precise estimate of abdominal aortic aneurysm rupture risk than size alone.

AIM

To determine the relationships between AAA compliance, size, growth, and clinical outcome.

METHODS

One-hundred and twelve patients with initially non-operated AAA (86 men, 26 women, mean age 73 years), recruited from five centres, underwent baseline compliance measurements and were then followed for a median of 7 (range 2-18) months; 85 patients underwent repeated measurements (median 3, range 2-5) 3-6-monthly over a median of 12 (range 3-18 months).

RESULTS

Seven patients have ruptured and 16 have undergone repair of non-ruptured AAA. AAA that ruptured had significantly lower Ep and B (more compliant). In AAA that ruptured or required repair there was an inverse relationship between diameter and Ep and B. In those undergoing repeated measurements AAA expansion was only associated with a significant increase in Ep and B in non-operated patients.

CONCLUSIONS

Baseline AAA compliance was significantly related to rupture and the future requirement for operative repair. Failure of compliance to increase with size may be a marker for rapid growth, developmental symptoms and rupture.

Developments in cardiovascular ultrasound. Part 2: Arterial applications

Many of the changes resulting from arterial disease can be measured, using Doppler ultrasound for measurement of blood velocity and B-scan imaging for measurement of tissue structure and composition. Wall thickness, the degree of arterial narrowing and plaque volume can be measured using B-scan imaging, and 3D ultrasound can be used to improve the accuracy of measurements of plaque volume and for improved visualisation of complex arterial geometries. Measurement of the dynamic properties of the arterial wall permits estimation of wall elasticity and plaque motion. From the Doppler signal, measurements of blood velocity are used to estimate the degree of arterial narrowing and volumetric flow, although measurement errors can be large. Wall shear stress can be estimated by measuring the velocity gradient at the vessel wall. The problems of inadequate spatial resolution and interference from overlying tissue are largely removed when intravascular systems are used, and these have superior capability in the assessment of arterial structure and tissue composition. However, measurement of quantities relating to blood flow is more difficult using the intravascular approach, as the indwelling cather disturbs the blood flow pattern, and currently, assessment of flow and vessel cross-section are not performed at the same site.

Doppler studies of the vascular system

Ultrasonic Doppler methods are routinely used to study the vascular system. Flow and tissue motion information can be obtained by frequency and time domain processing. Instruments range in complexity from simple continuous wave devices without imaging capability through to advanced real-time 2D colour flow scanners and intravascular devices. 3D display is also possible. Contrast agents can be used to increase the detectability of blood flow signals. The properties of the tissue impose an envelope on achievable ultrasonic imaging. Doppler studies can provide information about flow velocity profile, vessel compliance, wall shear rate, pressure gradient, perfusion, tumour blood flow and the presence of emboli. These capabilities can be integrated in an holistic picture of ultrasonic vascular studies.

Beat-to-beat estimation of windkessel model parameters in conscious rats

A windkessel model was applied on a beat-to-beat basis to evaluate the arterial mechanical characteristics in seven conscious rats. Ascending aortic arterial pressure (AP) and blood flow were recorded during steady-state in basal conditions, during infusions of isoprenaline, sodium nitroprusside, and phenylephrine, and after intravenous atenolol injection. For each cardiac cycle the exponential decay time constant (tau) was estimated from the aortic AP curve, peripheral resistances (R) were taken as the ratio of mean AP to cardiac output, and systemic arterial compliance (C) was calculated as tau/R. In all conditions, mean correlation coefficients of the exponential regression and approximately 70% of values in each rat were > 0.99, demonstrating the model validity. In all conditions tau and C exhibited a large spontaneous variability over time, and beat-to-beat correlations were high between tau and C (0.83 +/- 0.03). C was increased by sodium nitroprusside, decreased by isoprenaline, but not significantly decreased by phenylephrine [5.1 +/- 0.2, 3.2 +/- 0.3, and 3.9 +/- 0.2 microliters/mmHg, respectively, vs. 4.2 +/- 0.3 microliters/mmHg (baseline)]. In conclusion, the windkessel model enables tau and C to be reliably estimated in conscious rats during spontaneous and drug-induced hemodynamic variations.

Vasodilators, aortic elasticity, and ventricular end-systolic stress in nonanesthetized unrestrained rats

We evaluated the effect of different vasodilators on ventricular end-systolic stress by investigating the impact of sodium nitroprusside, nifedipine, and hydralazine on blood pressure, aortic stiffness, and wave reflection during drug-induced hypotension (to 80 mm Hg mean blood pressure) in normotensive (central aortic mean blood pressure, 116 to 119 mm Hg; systolic pressure, 133 to 137 mm Hg), nonanesthetized, unrestrained rats. Aortic stiffness was evaluated from the slope of the linear regression relating pulse wave velocity (PWV) to central aortic mean or pulse pressure. The fall in central aortic systolic blood pressure was less than the fall in mean pressure, especially after hydralazine (122+/-4 mm Hg; sodium nitroprusside, 107+/-2; and nifedipine, 112+/-3 mm Hg; P<.05). The PWV/mean pressure slope was linear, positive, and similar in all three groups (hydralazine, 3.3+/-0.2; sodium nitroprusside, 3.8+/-0.3; and nifedipine, 3.9+/-0.3 cm x s[-1]x mm Hg[-1]; P>.05). The PWV/pulse pressure slope was linear, negative, and less steep in the case of hydralazine (-4.9+/-0.6; sodium nitroprusside, -15.5+/-3.7; and nifedipine, -13.5+/-2.9 cm x s[-1] x mm Hg[-1]; P<.05). The travel time and augmentation index of the reflected wave were similar in all groups. In conclusion, sodium nitroprusside and nifedipine had a more beneficial effect on end-systolic stress than did hydralazine. This does not appear to be related to any specific effect on wave reflection or the "static" relationship between PWV and aortic mean blood pressure; it may be related to the effects of these drugs on the "dynamic" relationship between PWV and pulse pressure.

Sodium, blood pressure, and arterial distensibility in insulin-dependent diabetes mellitus

We investigated 24-hour ambulatory blood pressure and arterial distensibility, a marker of biophysical vessel wall properties, in 32 normoalbuminuric type I diabetic patients and 32 healthy control subjects on diets containing 50 mmol and 200 mmol sodium per day. The increase in daytime diastolic blood pressure from 50 to 200 mmol sodium was significantly higher in the diabetic patients than in the control subjects (2.3+/-4.9 versus 0.2+/-3.7 mm Hg, P<.05). On a high sodium regimen, femoral artery distensibility was decreased in the diabetic patients compared with the control subjects (19.2+/-7.6 versus 24.1+/-9.3 10[-3]/kPa, P<.05). Angiotensin-converting enzyme inhibition in the diabetic patients on a high sodium diet decreased daytime diastolic blood pressure and increased femoral artery distensibility. The blood pressure decrease in response to angiotensin-converting enzyme inhibition correlated significantly with the blood pressure increase to sodium (for 24-hour systolic and diastolic blood pressure, r=.72, P<.001 and r=.76, P<.001). In addition, we found that in the diabetic patients on a high sodium diet, the renal blood flow response to exogenous angiotensin II was not bimodally distributed, as is the case in essential hypertension, in which a subgroup of the patients are characterized by sodium sensitivity of the blood pressure and an abnormal renal blood flow response to exogenous angiotensin II ("nonmodulator phenotype"). These results show that blood pressure in insulindependent diabetes mellitus is sodium sensitive, but that this is not related to the nonmodulator phenotype, and suggest that in IDDM a relatively high sodium intake may be a factor that predisposes to the development of diabetic vascular disease.

Protective effect of chronic garlic intake on elastic properties of aorta in the elderly

BACKGROUND

Epidemiological studies have suggested that garlic may have protective effects against cardiovascular diseases. We undertook this cross-sectional observational study to test the hypothesis that regular garlic intake would delay the stiffening of the aorta relating to aging.

METHODS AND RESULTS

We studied healthy adults (n=101; age, 50 to 80 years) who were taking > or = 300 mg/d of standardized garlic powder for > or = 2 years and 101 age- and sex-matched control subjects. Pulse wave velocity (PWV) and pressure-standardized elastic vascular resistance (EVR) were used to measure the elastic properties of the aorta. Blood pressures, heart rate, and plasma lipid levels were similar in the two groups. PWV (8.3+/-1.46 versus 9.8+/-2.45 m/s; P<.0001) and EVR (0.63+/-0.21 versus 0.9+/-0.44 m2 x s(-2) x mm Hg(-1); P<.0001) were lower in the garlic group than in the control group. PWV showed significant positive correlation with age (garlic group, r=.44; control group, r=.52) and systolic blood pressure (SBP) (garlic group, r=.48; control group, r=.54). With any degree of increase in age or SBP, PWV increased less in the garlic group than in the control group (P<.0001). ANCOVA and multiple regression analyses demonstrated that age and SBP were the most important determinants of PWV and that the effect of garlic on PWV was independent of confounding factors.

CONCLUSIONS

Chronic garlic powder intake attenuated age-related increases in aortic stiffness. These data strongly support the hypothesis that garlic intake had a protective effect on the elastic properties of the aorta related to aging in humans.

Variation of common carotid artery elasticity with intimal-medial thickness: the ARIC Study. Atherosclerosis Risk in Communities

The Atherosclerosis Risk in Communities (ARIC) study is a prospective investigation of the etiology and natural history of atherosclerosis and cardiovascular disease in four U.S. communities. The purpose of this work is to investigate the relationship between common carotid artery elasticity and intimal-medial thickness (IMT) in the four race-gender groups represented in the ARIC cohort. Noninvasive ultrasonic methods were used to measure IMT and the [systolic minus diastolic] diameter change (DC) of the left common carotid artery in 10,920 black and white, men and women between the ages of 45 and 64 y. The relationship between DC and IMT and IMT2 was examined after adjustment of DC for age, height, diastolic diameter, diastolic blood pressure and linear and quadratic terms for pulse pressure. This adjusted value of DC was used as an index of elasticity of the common carotid artery in the ARIC cohort with larger values of adjusted DC implying a more elastic vessel. The general behavior of adjusted DC with increasing IMT was observed to be qualitatively similar in all four race-gender groups. Adjusted DC remained nearly constant or increased slightly for values of IMT between approximately 0.4 and 0.8 mm, up to approximately the 90th percentile of IMT, and then decreased above the 90th percentile of IMT. Common carotid artery elasticity, defined as adjusted DC, varies with increasing IMT in the ARIC cohort in a manner consistent with results from previous studies in animals and human subjects addressing the variation of several elasticity indices with atherosclerotic involvement and risk factor exposure in the aorta, and brachial and radial arteries. Our results suggest that thicker common carotid artery walls in middle-aged U.S. populations are no stiffer than thinner walls, except for the thickest 10% of arteries. Since the distal common carotid artery frequently contains atheromatous plaques in this population, the lack of change in stiffness, indeed, the reduction in stiffness per unit thickness, may reflect the various stages of early common carotid atherosclerosis most often found in this population. These are characterized more by destruction of arterial wall structural elements than by changes such as widespread or circumferential sclerosis, which would strengthen and stiffen the artery.

MR measurement of time-dependent blood pressure variations

An MR imaging method for measuring intravascular pressure variations is introduced. The technique is based on estimates of vascular compliance and vessel distension, which are obtained from a correlation of spatial and temporal velocity derivatives and measurements of the velocity gradient in the direction of flow, respectively. The accuracy of the technique was determined in vitro through a comparison of MR and transducer pressure measurements obtained in distensible vessel phantoms undergoing pulsatile flow. Results indicated that a root-mean-square error of 4-12% can be expected in phantoms covering a physiological range of compliance. In vivo feasibility was demonstrated by thoracic aorta pressure measurements, which produced pressure waveforms with an expected shape and magnitude.

A velocity correlation method for measuring vascular compliance using MR imaging

A method for estimating vascular compliance using MR velocity imaging is presented. The technique combines an analysis of pulse propagation, based on spatially averaged equations of continuity and momentum, together with phase-contrast velocity measurements to estimate the compliance from a correlation of second-order spatial and temporal velocity derivatives. The technique can be applied in the presence of reflected flow waves and uses velocity data acquired throughout the entire cardiac cycle. The accuracy of the technique was assessed in distensible vessel phantoms spanning a physiological range of compliance through a comparison with compliance estimates obtained using high-resolution MR imaging and pressure transducers. The mean error of all measurements was found to be 0.04 +/- 0.02% per mm Hg, with the relative errors ranging from 1.2% to 46%. Error was found to decrease as the temporal sampling rate and/or image plane separation were increased. This suggests that an accurate hemodynamic evaluation of a vessel's elastic properties is feasible with MR velocity imaging techniques.

Biophysical principles of vascular diagnosis

Visual inspection of the spectral composition of the Doppler signal as a function of time (sonogram) has been very helpful in detecting the presence of stenoses with substantial lumen narrowing causing abnormal flow patterns. Attempts to grade a stenosis based on the spectral width at peak systole were less successful because of the obscuring effects of the ultrasound beam width with respect to lumen diameter, dimensions of the sample volume, angle of observation, and spectral broadening due to vessel branching and bends. The introduction of color flow imaging has put emphasis on the width of the velocity distribution and the consistency of flow patterns within the region of interest. This technique requires a high resolution in space, velocity, and time necessitating the development of new velocity estimation algorithms. The observed flow patterns can be related to the echogenicity and local wall thickness of peripheral vessels. In addition, the displacement behavior of arterial walls over time provides information about the elasticity of the wall. Knowing the instantaneous velocity of arterial walls, it becomes possible to suppress selectively and adaptively the arterial wall contribution, allowing for the assessment of low blood flow velocities close to the wall and, hence, of wall shear rate. The latter development enables the study of the interaction of blood velocities and the metabolism and structure of the walls, providing possible clues for atherogenesis.

Aortic distensibility in post-menopausal women receiving Tibolone

Both oestrogen and androgens have been shown to affect the structural composition and biophysical properties of the arterial wall in vitro. A non-invasive Doppler ultrasound technique, based on the measurement of pulse wave velocity along the thoraco-abdominal aortic pathway, was used to assess aortic compliance C in vivo in 49 normotensive, normal, healthy post-menopausal female subjects. 23 of the women had been receiving Tibolone, a synthetic steroid structurally related to norethisterone, for at least 3 years. Since C varies with non-chronic changes in blood pressure, an index of intrinsic aortic distensibility, Cp, normally independent of blood pressure, was also calculated. No significant difference was found between the control group and those subjects receiving Tibolone for age, body mass index, systolic blood pressure, diastolic blood pressure, C or Cp. As one would expect the control group Cp values did not show a significant relationship with blood pressure (r = -0.1, not significant). However, in the Tibolone group some dependence of Cp values on pressure still remained (r = -0.58, p < 0.004), suggesting that oral administration of Tibolone had altered the dynamic relationship between structural and functional biophysical properties of the aortic wall in vivo.

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.