Simultaneous assessment of diameter and pressure waveforms in the carotid artery

Simultaneous assessment of diameter and pressure waveforms allows the calculation of the incremental compliance, distensibility, pulse wave velocity and elastic modulus as function of the distending pressure. However, the waveforms must be obtained at the same position and acquired and processed with the same filter characteristics to circumvent possible temporal and spatial changes in amplitude and phase. In this paper, arterial diameter waveforms are assessed by means of ultrasound (US) and converted to pressure using an empirically derived exponential relationship between pressure and arterial cross-section. The derived pressure waveform is calibrated to brachial end diastolic and mean arterial pressure by iteratively changing the wall rigidity coefficient (i.e., the exponential power). Because pressure is derived directly from arterial cross-section, no phase delay is introduced due to spatial separation or different filter characteristics. The method was evaluated for the left common carotid artery of 51 healthy subjects ranging in age from 22 to 75 years old. In healthy subjects, the carotid pulse pressure is 29% lower than the brachial pulse pressure. Continuous assessment of arterial properties confirms that pulse-wave velocity and incremental elastic modulus increase, whereas distensibility and compliance decrease, as function of increasing distending blood pressure.     

Differences in carotid artery wall properties between presumed-healthy men and women

The vessel wall properties of the common carotid artery were noninvasively studied in 109 normotensive, presumed normal male and female volunteers of various ages (20-69 y) with the use of a multi-gate pulsed Doppler system. This system allows the on-line recording of velocity profiles and the relative changes in carotid artery diameter during the cardiac cycle (delta d/d). From the width of these profiles the systolic internal diameter (d) of the carotid artery can be determined. With the use of d, the maximal increase in delta d/d during systole and the pulse pressure, as measured in the brachial artery (delta p), the distensibility coefficient and the cross-sectional compliance were calculated. Both distensibility and cross-sectional compliance were significantly lower in females than in males, irrespective of age. This likely results from a stiffer arterial wall behavior in females, because the relative increase in vessel diameter during the cardiac cycle is lower in females than in males, despite the fact that their pulse pressure values are not significantly different. The carotid artery diameters were similar in men and women. The findings in the present study indicate that the common carotid artery is stiffer in females than in males of comparable age. Therefore, sex-dependent differences have to be considered in the interpretation of arterial distensibility and cross-sectional compliance.     

Transcutaneous detection of relative changes in artery diameter

The extent of the excursions of the arterial walls during the cardiac cycle depends on both the compliance of the vessel wall and the local pressure fluctuations. Simultaneous assessment of the relative change in artery diameter in combination with the velocity distribution along the vessel cross section can reveal the cause of loss of distensibility. As will be demonstrated, a multigate pulsed Doppler system with a high spatial resolution can perform simultaneously both functions. The relative change in diameter during the cardiac cycle is obtained by taking the ratio of the distension and the diameter of the artery as observed along the ultrasound beam. It can be shown that this ratio will be angle independent. Statistical and experimental evaluations demonstrate that the system allows the assessment of the relative change in diameter of major peripheral arteries as a continuous function of time with an accuracy of about 0.5%.     

高频超声对脑梗死患者颈动脉结构与功能改变的评价

目的应用高频超声研究脑梗死患者颈动脉血管结构与功能的改变。方法应用高频超声对46例脑梗死患者和40例健康老年人颈动脉行二维超声检查,观察并记录颈动脉内-中膜有无增厚、有无斑块、斑块数目和回声情况,测量舒张期内-中膜厚度(IMT);M-型超声记录颈总动脉前后壁在收缩期和舒张期运动幅度变化,测量舒张期和收缩期的内径,计算内-中膜横截面积(IMSCA)、僵硬度β指数、扩张性(distensibility)和顺应性(compliance)等参数。结果脑梗死患者比健康老年人斑块数目及低回声斑块检出率增加,最大IMT和平均IMT增厚,内-中膜横截面积、收缩期内径和舒张期内径、僵硬度指数增大,扩张性和顺应性降低(P<0.05)。IMT与僵硬度指数、扩张性和顺应性无明显相关性。IM-SCA、僵硬度指数、扩张性和顺应性与脉压相关(P<0.01),而与收缩压、舒张压等无明显相关性(P>0.05)。结论高频超声是无创诊断颈动脉早期动脉硬化的简便有效的方法,颈动脉IMT结合僵硬度、扩张性和顺应性等指标可反映脑梗死患者颈动脉血管结构和功能变化的特征,可提供更全面的信息。     

Feasibility of Dual Doppler Velocity Measurements to Estimate Volume Pulsations of an Arterial Segment

If volume flow was measured at each end of an arterial segment with no branches, any instantaneous differences would indicate that volume was increasing or decreasing transiently within the segment. This concept could provide an alternative method to assess the mechanical properties or distensibility of an artery noninvasively using ultrasound. The goal of this study was to determine the feasibility of using Doppler measurements of pulsatile velocity (opposed to flow) at two sites to estimate the volume pulsations of the intervening arterial segment. To test the concept over a wide range of dimensions, we made simultaneous measurements of velocity in a short 5 mm segment of a mouse common carotid artery and in a longer 20 cm segment of a human brachial-radial artery using a two-channel 20 MHz pulsed Doppler and calculated the waveforms and magnitudes of the volume pulsations during the cardiac cycle. We also estimated pulse wave velocity from the velocity upstroke arrival times and measured artery wall motion using tissue Doppler methods for comparison of magnitudes and waveforms. Volume pulsations estimated from Doppler velocity measurements were 16% for the mouse carotid artery and 4% for the human brachial artery. These values are consistent with the measured pulse wave velocities of 4.2 m/s and 10 m/s, respectively, and with the mouse carotid diameter pulsation. In addition, the segmental volume waveforms resemble diameter and pressure waveforms as expected. We conclude that with proper application and further validation, dual Doppler velocity measurements can be used to estimate the magnitude and waveform of volume pulsations of an arterial segment and to provide an alternative noninvasive index of arterial mechanical properties. (E-mail: cjhartley@ieee.org)     

Carotid artery wall properties in normotensive and borderline hypertensive subjects of various ages

The vessel wall properties of the common carotid artery were studied noninvasively in normotensive and borderline hypertensive male volunteers of various ages with the use of a multi-gate pulsed Doppler system. In the younger age group (20-35 y) both distensibility and cross-sectional compliance were significantly less in the borderline hypertensive group. In the older borderline hypertensive subjects (50-69 y) distensibility (p less than 0.05) and cross-sectional compliance (p = 0.06) were also less than in the control subjects. The reduced distensibility and cross-sectional compliance likely result from a decrease in arterial wall elasticity because the relative increase in common carotid artery diameter during systole is diminished in borderline hypertensives, despite the fact that their pulse pressure is similar to or higher than that in control subjects. The less pronounced differences in vessel wall properties between borderline hypertensive and normotensive volunteers in the older age group may be a consequence of the changes in these properties with age, partly masking the alterations due to borderline hypertension. Since the stiffer behavior of the common carotid artery in borderline hypertensives is associated with relatively slight changes in blood pressure, the question can be raised whether the alterations in arterial wall properties are really a result of the elevated arterial blood pressure; these alterations might develop independent of the blood pressure elevation.     

应用组织速度成像技术对颈总动脉壁弹性特征的研究

目的　应用组织速度成像技术对正常人和原发性高血压患者颈总动脉壁运动特点和弹性特征进行研究。方法　研究对象为 2 4例正常人和 2 4例原发性高血压合并右侧颈动脉硬化患者 ,应用 M-型超声和组织速度成像技术记录颈总动脉内径、计算可扩张度和顺应性 ,经颈总动脉内径和脉压矫正后的前壁峰值速度 (Vimax)和加速度 ACi;经心率和脉压矫正后的加速时间 (ATi)和减速时间 (DTi)。结果　正常对照组中年龄与 Vimax和 ACi呈负相关 ,Vimax、 ACi与颈总动脉壁弹性可扩张度 (DC)、颈总动脉弹性顺应性(CC)呈正相关。原发性高血压颈动脉硬化组 Vimax、 ACi与正常对照组相比明显减低 ,ATi和 DTi与正常对照组相比明显缩短。经统计学检验具有显著性差异 (P<0 .0 1)。结论　本研究显示组织速度成像技术可用来评价正常人和原发性高血压患者颈总动脉壁运动特点与弹性特征。     

Opposite changes in carotid versus aortic stiffness during healthy human pregnancy

Systemic arterial compliance has been known to increase during healthy pregnancy, whereas, recently, the carotid artery has been reported to stiffen. To clarify this controversy, we simultaneously measured aortic PWV (pulse wave velocity) and carotid artery elastic parameters in a cohort of pregnant women. Twelve normotensive pregnant women were studied longitudinally during the three trimesters of pregnancy (T1, T2 and T3 respectively) and 12 weeks PP (postpartum). Carotid artery diastolic diameter and pulsatile distension was measured by an echo-wall tracking method and carotid pulse pressure by applanation tonometry. Carotid strain, compliance, distensibility coefficient, stiffness index beta, Einc (incremental elastic modulus) and augmentation index were calculated. Aortic PWV was determined to estimate aortic distensibility. All carotid artery elastic parameters indicated significant stiffening from T1 to T3 (1.8+/-0.2 versus 2.9+/-0.3 mmHg for Einc), which was reversed after delivery (2.3+/-0.2 mmHg). Aortic PWV decreased during pregnancy (6.2+/-0.2 versus 5.4+/-0.2 m/s) and increased in the PP period (6.7+/-0.2 m/s). No correlation was found between changes in carotid artery elastic parameters and changes in aortic PWV either from T1 to T3 or from T3 to PP. The carotid artery exhibits regionally specific stiffening during pregnancy, which appears to represent a qualitatively different change in arterial elastic behaviour.     

Fetal Aortic Distensibility,Compliance and Pulse Pressure Assessment during the Second Half of Pregnancy

The objective of the study was to measure fetal aortic pulse wave velocity and lumen diameter waveforms and subsequently calculate local distensibility, compliance and pulse pressure. A dedicated algorithm for optimizing lumen diameter assessment from radiofrequency ultrasound data is described. Biplane raw data were obtained from a matrix array transducer. We evaluated 83 confirmed, normally developing pregnancies at 22–38 wk. Fetal aortic pulse wave velocity (PWV, m/s) = 0.047 × gestational age (wk) + 1.241, and the distensibility coefficient (1/kPa) = 1/(1.04 × PWV²). The logarithm of the local compliance index (mm²/kPa) and the pulse pressure (kPa) were both linearly related to gestational age as 0.022 × GA (wk) – 0.343 and 0.012 × GA (wk) + 0.931, respectively. In conclusion, fetal aortic elastic properties can be derived from phase-sensitive radiofrequency data and multiline diameter assessment. Future studies may shed further light on the developmental origins of vascular health and disease.     

In patients with pseudoxanthoma elasticum a thicker and more elastic carotid artery is associated with elastin fragmentation and proteoglycans accumulation

Skin biopsies in patients with pseudoxanthoma elasticum (PXE) show elastic fiber fragmentation and calcium and proteoglycans accumulation. Assuming such changes to be present in the artery wall as well, we studied the influence of such alterations on function and structure of the human common carotid artery (CCA). Indeed, elastin fragmentation and increased calcium and proteoglycans content were present in the arteries of the two PXE patients examined. Internal diameter, distension and intima-media thickness (IMT) in the CCA of PXE patients (n = 19) and controls (n = 39) were determined by ultrasound (US). Pulse pressure was assessed in the brachial artery. The distensibility and compliance coefficients as well as the Young's modulus were calculated. Diameter and pulse pressure were not significantly different in PXE patients and controls. The distensibility and compliance coefficients were significantly greater in older PXE patients than in older controls. The distensibility coefficient decreased with age in both PXE patients and in controls. Unlike in controls, the compliance coefficient did not decrease and the Young's modulus barely increased with age in PXE patients. IMT was significantly greater at both younger and older ages and the Young's modulus was significantly smaller at older ages in PXE patients than in controls. The carotid artery is thicker and more elastic in PXE patients than in control subjects; differences are most pronounced at older ages. These alterations might be explained by the elastin fragmentation and proteoglycans accumulation as observed in these patients.     

Age-related changes in carotid artery wall properties in men

Changes in distensibility and cross-sectional compliance of the common carotid artery with age were studied in 80 presumed healthy volunteers, varying in age between 20 and 69 y. The distensibility was assessed from the relative increase in arterial diameter during systole normalized with respect to the arterial pulse pressure. The cross-sectional compliance was obtained by multiplying the distensibility by the arterial diameter. The relative diameter changes of the common carotid artery during the cardiac cycle were recorded on-line with a high resolution multigate pulsed Doppler system. The arterial diameter was assessed from the width of the velocity profiles which can also be recorded on-line with this system. Arterial pulse pressure was determined from brachial artery cuff blood pressure measurements. Both distensibility and cross-sectional compliance of the common carotid artery decreased linearly with age, starting in the third age decade. The reduction in the latter parameter was less pronounced, probably as a consequence of an increase in arterial diameter with age.     

Brachial artery pulse pressure and common carotid artery diameter: mutually independent associations with mortality in subjects with a recent history of impaired glucose tolerance

BACKGROUND: Decreased large artery function, as reflected by increased brachial artery pulse pressure and increased carotid artery diameter and stiffness, may contribute to the increased mortality risk that is observed in subjects with impaired glucose tolerance. We therefore investigated the association between brachial artery pulse pressure and carotid artery diameter and stiffness, which are estimates of central artery stiffness and arterial remodelling, respectively, and mortality in subjects with a recent history of impaired glucose tolerance. DESIGN: A prospective, population-based cohort study. We measured brachial artery pulse pressure by oscillometric blood pressure measurements, and common carotid artery diameter and distensibility and compliance coefficients by ultrasound in 140 subjects with a recent history of impaired glucose tolerance. During a median 6.6-year follow-up, 16 subjects died. RESULTS: Brachial artery pulse pressure and common carotid artery diameter were positively related to all-cause mortality [hazard ratios per standard deviation, 1.7 (1.2-2.5) and 2.1 (1.3-3.3), respectively]. Results were similar after adjustment for gender, age, waist-to-hip ratio, body mass index, total cholesterol concentration, pre-existent cardiovascular disease, and hypertension, and after additional mutual adjustment. Common carotid artery distensibility and compliance coefficients were not statistically significantly associated with mortality. CONCLUSIONS: Among subjects with a recent history of impaired glucose tolerance, brachial artery pulse pressure and common carotid artery diameter are independently associated with mortality risk. Stiffness of the central arteries may explain the association between pulse pressure and mortality risk. The association between carotid diameter and mortality risk is more likely to reflect arterial remodelling in response to atherosclerosis than that in response to increased local stiffness.     

Validation of a newly developed B-mode image-processing technique versus wall-tracking ultrasound for the study of wall mechanics in small-calibre arteries

Purpose: To validate a newly developed image‐processing technique for the assessment of arterial wall compliance and distensibility from non‐invasive B‐mode ultrasound compared with the invasive wall‐tracking technique. Materials and methods: Arterial wall compliance and distensibility coefficient were measured invasively by wall‐tracking with an ultrasonic transducer implanted on the vessel wall, and non‐invasively by automatic processing of B‐mode ultrasound images, with a dedicated workstation and software (IO^ 3·1, IO^DP, Paris). Measurements were performed in the normal aorta of five animals, and upstream, at the stent level, and downstream from the stent in eight other animals (immediately after stenting in six, and 3 months later in four), for a total of 35 paired measurements. Results: There was no significant difference between the two techniques for compliance but there was a significant difference in diameter (P<0·005) and distensibility (P<0·05) as external ultrasound measured the inner diameter, while wall‐tracking measured the outer diameter. Agreement between the two methods as assessed by the Bland–Altman approach was acceptable for aortic diameter, compliance and distensibility. Conclusion: Automatic processing of B‐mode ultrasound images is a reliable non‐invasive technique to assess the compliance of small‐calibre arteries.     

Influence of diuretics on brachial artery diameter and distensibility in hypertensive patients

Blood pressure, brachial artery diameter and pulse wave velocity were determined before and after diuretic treatment in 2 groups of hypertensive patients treated either by indapamide (2.5 mg per d) or by canreonate (50 mg per d). Brachial artery diameter, measured from pulsed Doppler flowmetry, and pulse wave velocity, evaluated from mechanography, did not alter significantly despite a significant blood pressure reduction. The study indicates that, in hypertensive patients of middle age, diuretics did not change brachial artery diameter and distensibility, whether the drug caused an increase or a decrease in plasma potassium levels.     

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.     

Atherosclerosis measured by whole body magnetic resonance angiography and carotid artery ultrasound is related to arterial compliance,but not to endothelium‐dependent vasodilation – the Prospective Investigation of the Vasculature in Uppsala Seniors (PIVUS) study

Background: Arterial compliance and endothelium‐dependent vasodilation are two characteristics of the vessel wall. In the Prospective Investigation of the Vasculature in Uppsala Seniors (PIVUS) study, we studied the relationships between arterial compliance and endothelium‐dependent vasodilation versus atherosclerosis as measured with two imaging modalities. Methods: In the population‐based PIVUS study (1016 subjects aged 70), arterial compliance was determined by ultrasound in the carotid artery and the stroke volume to pulse pressure ratio by echocardiography, while endothelium‐dependent vasodilation was assessed by the invasive forearm technique with acetylcholine and brachial artery ultrasound. Intima‐media thickness was evaluated by ultrasound in the carotid artery (n = 954). Stenosis in the carotid, aorta, renal, upper and lower leg arteries were determined by magnetic resonance angiography in a random subsample of 306 subjects. Results: After adjustments for gender, Framingham risk score, obesity, myocardial infarction and stroke, distensibility in the carotid artery and the stroke volume to pulse pressure ratio were both significantly related to a weighted index of stenosis in the five arterial territories evaluated by magnetic resonance angiography (p<0·02 for both). Distensibility in the carotid artery (P = 0·021), but not the stroke volume to pulse pressure ratio (P = 0·08), was also significantly related to intima‐media thickness. Conclusion: In the elderly population, atherosclerosis is mainly related to arterial compliance, but not to endothelium‐dependent vasodilation in peripheral conduit or resistance vessels.     

Arteriosclerosis and pulse wave velocity

Impairment of the arterial compliance or loss of Windkessel effect of elastic arteries causes increased afterload to the heart and increased pulsatile flow to the peripheral vasculatures. The former induces left ventricular hypertrophy or dysfunction and the latter induces small vessel damage or end organ dysfunction. Thus, the arterial compliance plays important roles in the course of hypertension. Therefore; it is worthwhile to measure the elastic properties of aortoarterial system in patient with hypertension. The velocity of the pressure wave along an arterial system, known as pulse wave velocity(PWV), is related to the average stiffness of an arterial segment between measurement sites. The measurement of PWV is inversely related to arterial wall distensibility, which offers a simple and potential approach. There are numerous reports which PWV is a forceful marker and predictor of the cardiovascular risk in hypertensive or other arteriosclerotic disorders. Thus, PWV measurement is recommended in patients with hypertension for early detection of organ damages or estimation of the cardiovascular risk, as well as for the evaluation of the effectiveness of the treatment as a surrogate marker.     

高频超声对高血压患者颈动脉功能改变的评价

目的应用高频超声研究高血压患者的颈动脉功能的改变。方法应用高频超声对55例高血压患者和46例健康人的颈动脉进行检测，记录颈动脉中内膜的厚度（IMT）；M-型超声观测颈总动脉前后壁运动曲线，并测量舒张期和收缩期内径，计算僵硬度指数口、扩张性（distensibility）和顺应性（compliance），经心率和脉压矫正后的前壁加速度（AGi）、减速度（DCi）等参数。结果高血压患者比健康人平均IMT增厚，僵硬度指数增大，扩张性和顺应性降低（P〈0．001），加速度和减速度降低（P〈0．001）。僵硬度指数、扩张性、顺应性、加速度和减速度等指标与脉压相关（P〈0．001）。结论本研究显示高频超声所检测的多项参数能反映颈动脉血管功能变化的特征，是无创诊断动脉硬化的简便有效的方法。     

Non-invasive Assessment by B-Mode Ultrasound of Arterial Pulse Wave Intensity and Its Reduction During Ventricular Dysfunction

Arterial pulse waves contain clinically useful information about cardiac performance, arterial stiffness and vessel tone. Here we describe a novel method for non-invasively assessing wave properties, based on measuring changes in blood flow velocity and arterial wall diameter during the cardiac cycle. Velocity and diameter were determined by tracking speckles in successive B-mode images acquired with an ultrafast scanner and plane-wave transmission. Blood speckle was separated from tissue by singular value decomposition and processed to correct biases in ultrasound imaging velocimetry. Results obtained in the rabbit aorta were compared with a conventional analysis based on blood velocity and pressure, employing measurements obtained with a clinical intra-arterial catheter system. This system had a poorer frequency response and greater lags but the pattern of net forward-traveling and backward-traveling waves was consistent between the two methods. Errors in wave speed were also similar in magnitude, and comparable reductions in wave intensity and delays in wave arrival were detected during ventricular dysfunction. The non-invasive method was applied to the carotid artery of a healthy human participant and gave a wave speed and patterns of wave intensity consistent with earlier measurements. The new system may have clinical utility in screening for heart failure.     

Variability in ultrasonic measurements of arterial stiffness in the Atherosclerosis Risk in Communities study.

Arterial stiffness is emerging as an important risk marker for cardiovascular disease. Ultrasound-based measurements of arterial stiffness are in use by several large epidemiological studies. The reliability of ultrasonic measurements of arterial stiffness was assessed as part of one of these, the Atherosclerosis Risk in Communities (ARIC) study. ARIC, a prospective, four-center epidemiological study, used B-mode ultrasound with an electronic tracking device to measure arterial stiffness of the carotid artery. Oscillometric blood pressure measures were obtained before and after the arterial wall tracking. Measurement variability was estimated in 36 volunteers who were scanned at three visits conducted at 7- to 14-day intervals. Between- and within-person components of variation were estimated for arterial diameter and blood-pressure measurements. The correlation (R) between repeated measurements for pulse pressure, the percent change in arterial diameter (strain), and the percent and absolute change in the arterial area were 0.69, 0.67, 0.66 and 0.81, respectively. The R for the stress-strain elastic modulus (Ep), arterial distensibility, and arterial compliance were 0.66, 0.67, and 0.77, respectively. The R for the pressure-adjusted diameter change (i.e., diameter change adjusted for diastolic and pulse pressures) was 0.75. In summary, the ultrasonic measurements of arterial stiffness employed in the ARIC study demonstrate excellent short-term repeatability, demonstrating their utility in field settings.