A.P.G. Hoeks

Flow velocity patterns in and distensibility of the carotid artery bulb in subjects of various ages.

Velocity patterns in and distensibility of the carotid artery bulb were studied in younger (20 to 30 years; n = 11) and older (50 to 60 years; n = 9) volunteers without detectable lesions of the cervical carotid arteries by means of a high-resolution, multigate, pulsed Doppler system coupled to a B mode imager. In the bulb the axial velocities were highest on the side of the flow divider, while regions of flow separation and recirculation were observed on the side opposite to this divider. Flow separation and recirculation were less pronounced and less common in the older subjects. Flow separation was not continuously present throughout the cardiac cycle. The distensibility of the carotid arteries was significantly diminished in older subjects, especially in the bulb. The velocity patterns in the carotid artery bulb of younger subjects corroborate the patterns observed in models. The diminished flow separation and recirculation in the older subjects might result from alterations in distensibility at the transition from common to internal carotid artery with increasing age.

Inhomogeneities in arterial wall properties under normal and pathological conditions

Aim To determine whether there are inhomogeneities in arterial wall extensibility with age and hypertension. Background It has been known for some years that arteries lose their elasticity with increasing age and that they are less distensible in subjects with established and borderline hypertension than in age-matched controls. Results of data analysis In the carotid artery changes in arterial wall properties are not homogeneously distributed along the bifurcation. Both in older subjects and in those with borderline hypertension the loss of distensibility is most pronounced in the carotid artery bulb, while in borderline hypertensives the proximal part of the bulb, where the baroceptors are predominantly located, is more affected than other parts of the bulb. The loss of distensibility in the bulb may explain the diminished baroreceptor sensitivity observed in older subjects. The pronounced involvement of the proximal part of the bulb in the decreased distensibility in borderline hypertensives suggests that local stiffening, probably leading to disturbed baroreceptor sensitivity, may contribute to the development of hypertension in these patients. The pattern of inhomogeneities in wall distensibility in borderline hypertensives at a relatively young age (average 38 years) is similar to that seen in older normotensive subjects (average age 57 years), supporting the idea that arteries age faster in hypertensives than in normotensives. Conclusion In management of the borderline hypertensive patient, consideration should be given to preventing early ageing of the arterial wall rather than following arterial blood pressure.

Noninvasive Assessment of Arterial Stiffness Should Discriminate Between Systolic and Diastolic Pressure Ranges

Arterial stiffening plays an important role in the development of hypertension and cardiovascular diseases. The intrinsically nonlinear (ie, pressure-dependent) elastic behavior of arteries may have serious consequences for the accuracy and interpretation of arterial stiffness measurements and, ultimately, for individual patient management. We determined aortic pressure and common carotid artery diameter waveforms in 21 patients undergoing cardiac catheterization. The individual pressure-area curves were described using a dual exponential analytic model facilitating noise-free calculation of incremental pulse wave velocity. In addition, compliance coefficients were calculated separately in the diastolic and systolic pressure ranges, only using diastolic, dicrotic notch, and systolic data points, which can be determined noninvasively. Pulse wave velocity at systolic pressure exhibited a much stronger positive correlation with pulse pressure (P<0.001) and age (P=0.012) than pulse wave velocity at diastolic pressure. Patients with an elevated systolic blood pressure (>140 mm Hg) had a 2.5-times lower compliance coefficient in the systolic pressure range than patients with systolic blood pressures <140 mm Hg (P=0.002). Most importantly, some individuals, with comparable age or pulse pressure, had similar diastolic but discriminately different systolic pulse wave velocities and compliance coefficients. We conclude that noninvasive assessment of arterial stiffness could and should discriminate between systolic and diastolic pressure ranges to more precisely characterize arterial function in individual patients.

Accurate noninvasive method to diagnose minor atherosclerotic lesions in carotid artery bulb.

In a prospective study using a multigate pulsed Doppler system, minor (less than 30% diameter reduction) carotid artery lesions were diagnosed by detecting not only abnormalities in the blood flow pattern, but also local changes in artery wall distensibility. For the diagnosis of more severe lesions, additional information was obtained from disturbances in the Doppler audio spectrum. Biplane arteriography was used as a reference. The observed agreement, sensitivity, and specificity were 86.6%, 90.3%, and 88.6%, respectively, for all lesions and 85.7%, 82.1%, and 88.6%, respectively, when only minor lesions were considered. kappa (a chance-corrected measure of agreement) was 81.7%. If only blood flow abnormalities were used to detect minor lesions, 43.5% would be missed. Our results indicate that minor carotid artery lesions can be diagnosed noninvasively more accurately when both local blood flow irregularities and local changes in vessel wall distensibility are taken into account.

Noninvasive Assessment of Artery Wall Properties in Children Aged 4-19 Years

ABSTRACT: The vessel wall properties of the common carotid artery were noninvasively studied in 53 normotensive, presumed normal boys of various ages (4-19 yr) with the use of a multigate pulsed Doppler system. This device allows the on-line recording of velocity profiles and the relative changes in carotid artery diameter during the cardiac cycle. From the width of these profiles, the internal diameter of the carotid artery can be determined. With the use of internal carotid artery diameter, relative changes in carotid artery diameter during the cardiac cycle, and pulse pressure the pulse pressure, as measured in the brachial artery, the distensibility coefficient and the cross-sectional compliance were calculated. The children were allotted to three different age groups: group I (4-10 yr), group II (11- 14 yr), and group III (15-19 yr). In the older children (group III) the systolic arterial pressure and the pulse pressure were significantly higher than in groups I and II. The carotid artery diameter was significantly larger and the distensibility coefficient was significantly smaller in group III than in groups I and II. The cross-sectional compliance was not significantly different in the age groups, which can be explained by the larger artery diameter in the older age group. The findings indicate that the carotid artery wall is less distensible in adolescents than in younger children. The subjects in group III showed similar carotid artery wall properties as the young adults (aged 20-34 yr) in a previous study. Therefore, it is likely that in adolescents the vascular system can be considered as mature as far as the arterial wall properties are concerned.

A Control Systems Approach to Quantify Wall Shear Stress Normalization by Flow-Mediated Dilation in the Brachial Artery

Flow-mediated dilation is aimed at normalization of local wall shear stress under varying blood flow conditions. Blood flow velocity and vessel diameter are continuous and opposing influences that modulate wall shear stress. We derived an index FMDv to quantify wall shear stress normalization performance by flow-mediated dilation in the brachial artery. In 22 fasting presumed healthy men, we first assessed intra- and inter-session reproducibilities of two indices pFMDv and mFMDv, which consider the relative peak and relative mean hyperemic change in flow velocity, respectively. Second, utilizing oral glucose loading, we evaluated the tracking performance of both FMDv indices, in comparison with existing indices [i.e., the relative peak diameter increase (%FMD), the peak to baseline diameter ratio (Dpeak/Dbase), and the relative peak diameter increase normalized to the full area under the curve of blood flow velocity with hyperemia (FMD/shearAUC) or with area integrated to peak hyperemia (FMD/shearAUC_peak)]. Inter-session and intra-session reproducibilities for pFMDv, mFMDv and %FMD were comparable (intra-class correlation coefficients within 0.521–0.677 range). Both pFMDv and mFMDv showed more clearly a reduction after glucose loading (reduction of ~45%, p≤0.001) than the other indices (% given are relative reductions): %FMD (~11%, p≥0.074); Dpeak/Dbase (~11%, p≥0.074); FMD/shearAUC_peak (~20%, p≥0.016) and FMD/shearAUC (~38%, p≤0.038). Further analysis indicated that wall shear stress normalization under normal (fasting) conditions is already far from ideal (FMDv << 1), which (therefore) does not materially change with glucose loading. Our approach might be useful in intervention studies to detect intrinsic changes in shear stress normalization performance in conduit arteries.

Continuous wave and pulsed Doppler flowmeters - a general introduction

In the past decades several techniques have been developed to determine cardiac output and blood flow to organs. Most of these techniques are indirect and do not measure blood flow as an instantaneous function of time. This information can be obtained by means of techniques based upon Faraday’s law of electromagnetic induction 1)–5), or techniques which make use of ultrasonic waves, e.g. Doppler flowmeters 6)–11). Although very different in principle, their basic objective is similar. Both systems operate by coupling an external field through a vessel wall into the bloodstream and measure, without opening the vessel, a physical parameter which is proportional to blood flow velocity. Doppler flowmeters have some major advantages over electromagnetic flowmeters. Not only because these instruments can be used transcutaneously and in radio-telemetry, but also because some of the difficulties in electromagnetic flowmeters are not met in Doppler systems. On the other hand, quantitative information is still difficult to obtain with Doppler flowmeters. In spite of this disadvantage Doppler instruments have demonstrated their value in the clinical diagnosis of cardiovascular diseases. The aim of the present survey is to summarize the principle, advantages, limitations and applications of Doppler flowmeters.

The Performance of an Adaptive Rf-Domain Clutter Removal Filter

Ultrasound systems are widely used to visualize in real-time internal structures and blood flow velocity distributions. The latter are estimated from the received ultrasound radio frequency (RF) signals backscattered from moving red blood cells, using a mean frequency estimator. Since RF-signals contain not only scattering, but also reflections, reverberations and noise it is necessary to suppress the power of the reflections and/or reverberations (clutter removal) to estimate the temporal mean frequency of the signal component induced by scattering. Normally this is done with a static high-pass filter acting in the temporal direction with a fixed cut-off frequency. However, by using such a filter the time dependent aspect of the reflections is ignored. A more selective way is to use a band stop filter which adapts its rejection range to the mean frequency of the clutter. A great advantage of this adaptive filter (ADP-f) method is that the rejection range can be kept small.

Non-Invasive Assessment of Velocity Patterns in and Wall Properties of the Carotid Artery Bifurcation in Man by Means of a Multi-Gate Pulsed Doppler System

Multi-gate pulsed Doppler systems have been developed which allow the non-invasive on-line recording of instantaneous velocities, simultaneously at various sites along the ultrasound beam, and, hence, of velocity profiles at discrete time intervals during the cardiac cycle. With these systems one can also record on-line the relative arterial diameter changes during the cardiac cycle, providing insight into arterial wall properties in general and local differences in particular. Although the technique is not ideal yet, with multi-gate pulsed Doppler systems valuable information can be obtained about flow patterns in arteries and arterial wall properties under normal and pathological circumstances in man.