Richard Donnelly, David Hinwood, Nick J M London
Although diagnostic and therapeutic decisions in patients with vascular disease are guided primarily by the history and physical examination, the use of non-invasive investigations has increased significantly in recent years, mainly as a result of technological advances in ultrasonography. This article describes the main investigative techniques.
In the simplest form of ultrasonography, ultrasound is transmitted as a continuous beam from a probe that contains two piezoelectric crystals. The transmitting crystal produces ultrasound at a fixed frequency (set by the operator according to the depth of the vessel being examined), and the receiving crystal vibrates in response to reflected waves and produces an output voltage. Conventional B mode (brightness mode) ultrasonography records the ultrasound waves reflected from tissue interfaces, and a two dimensional picture is built up according to the reflective properties of the tissues.
Ultrasound signals reflected off stationary surfaces retain the same frequency with which they were transmitted, but the principle underlying Doppler ultrasonography is that the frequency of signals reflected from moving objects such as red blood cells shifts in proportion to the velocity of the target. The output from a continuous wave Doppler ultrasonograph is usually presented as an audible signal, so that a sound is heard whenever there is movement of blood in the vessel being examined.
Continuous wave ultrasonography provides little scope for restricting the area of tissue that is being examined because any sound waves that are intercepted by the receiving crystal will produce an output signal. The solution is to use pulsed ultrasonography. The investigator can focus on a specific tissue plane by transmitting a pulse of ultrasound and closing the receiver except when signals from a predetermined depth are returning. This allows, for example, the centre of an artery and the areas close to the vessel wall to be examined in turn.
An important advance in vascular ultrasonography has been the development of spectral analysis, which delineates the complete spectrum of frequencies (that is, blood flow velocities) found in the arterial waveform during a single cardiac cycle. The normal ("triphasic") Doppler velocity waveform is made up of three components which correspond to different phases of arterial flow: rapid antegrade flow reaching a peak during systole, transient reversal of flow during early diastole, and slow antegrade flow during late diastole.
Doppler examination of an artery distal to a stenosis will show characteristic changes in the velocity profile: the rate of rise is delayed, the amplitude decreased, and the transient flow reversal in early diastole is lost. In severe disease, the Doppler
Left: Doppler velocity waveforms: (a) triphasic waveform in normal artery; (b) biphasic waveform, with increased velocity, through a mild stenosis; (c) monophasic waveform, with greatly increased velocity, through tight stenosis; and (d) dampened monophasic waveform, with reduced velocity, recorded distal to tight stenosis. Right: Anatomical chart used to record position of stenoses, showing three stenoses with velocity increases of 7x, 4x, and 3x compared with adjacent unaffected arteries
Left: Doppler velocity waveforms: (a) triphasic waveform in normal artery; (b) biphasic waveform, with increased velocity, through a mild stenosis; (c) monophasic waveform, with greatly increased velocity, through tight stenosis; and (d) dampened monophasic waveform, with reduced velocity, recorded distal to tight stenosis. Right: Anatomical chart used to record position of stenoses, showing three stenoses with velocity increases of 7x, 4x, and 3x compared with adjacent unaffected arteries waveform flattens; in critical limb ischaemia it may be undetectable.
Examination of an arterial stenosis shows an increase in blood velocity through the area of narrowing. The site(s) of any stenotic lesions can be identified by serial placement of the Doppler probe along the extremities. The criteria used to define a stenosis vary between laboratories, but a twofold increase in peak systolic velocity compared with the velocity in an adjacent segment of the artery usually signifies a stenosis of 50% or more.
Relation between increased blood velocity and degree of stenosis
Was this article helpful?