What measurements are necessary for a comprehensive evaluation of the peripheral arterial circulation?
Several methods are available to detect atherosclerotic lesions with a severe degree of stenosis (>70%), but the diagnosis of atherosclerotic lesions with no stenosis or with a minor degree of stenosis (<20%), is problematic. Hemodynamics associated with stenotic lesions are well described by the relationship of blood pressure and blood flow velocity, both as a function of time and localization (along the length and cross-section of the vessel). The use of this relationship in the clinic is difficult because no precise information is available about the geometry and branching of arteries, blood viscosity, and the velocity distribution over the cross-sectional area of the blood vessel. Besides, the invasiveness of the technique to measure arterial pressure as a function of time and localization does not allow routine application in patients. Because of these limitations, alternative methods have been developed. The degree and extensiveness of atherosclerotic disease can, for instance, be estimated from the changes in maximum blood flow velocity and in velocity profile, i.e., velocity distribution along the cross-section of the vessel. Moreover, the delay between simultaneously recorded arterial blood flow velocity tracings (pulse-wave velocity determination) is used to assess the elastic properties of the vessel. Changes in velocity profile occur at relatively slight degrees of arterial stenosis (around 20%), so that determination of these profiles along diseased arteries may contribute to the early diagnosis of atherosclerotic lesions. In man, transcutaneous information about the maximum and mean blood flow velocities over the cross-sectional area of the artery as an instantaneous function of time as well as the flow pattern can be obtained online with continuous wave Doppler flowmeters, at least when audio spectrum analysis is used as a processing technique. Velocity profiles can be determined with multichannel pulsed Doppler systems if the resolution of the system is adequate and a sufficient number of sample volumes can be obtained, limiting the interpolation between these samples. The on-line recording of velocity profiles can be facilitated by combining the pulsed Doppler device with either a velocity imaging system or a B-mode scan. In systems with a high resolution (sample distance 0.5 mm), one should be able to detect local disturbances in the velocity profile at the site of the lesion (due to local increases in shear stress) and proximal to the lesion (due to reflections), so that lesions with a minor degree of stenosis can be detected. In resistive systems (e.g., internal carotid arteries) in which the relationship between pressure and velocity changes during the cardiac cycle is relatively simple, the elasticity of the arterial wall can be determined by relating the relative diameter changes of the vessel, determined on-line with multichannel pulsed Doppler systems, to the instantaneous velocity pulse. Although the detection of atherosclerotic lesions at an early stage of the disease with sophisticated Doppler devices looks promising, further clinical evaluation is required.
Reneman
Robert S.
RS
Department of Physiology, Biomedical Center, University of Limburg, Maastricht, The Netherlands.