Thursday, October 28, 2010

ABC of arterial and venous disease

Noninvasive methods of arterial and venous assessment

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.

Principles of vascular ultrasonography

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.

Doppler ultrasonography

Ultrasound signals reflected off stationary surfaces retain thesame 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.
Pulsed ultrasonography

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.
Duplex scanners

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 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

By combining the pulsed Doppler system with real time B mode ultrasound imaging of vessels, it is possible to examine Doppler flow patterns in a precisely defined area within the vessel lumen. This combination of real time B mode sound imaging with pulsed Doppler ultrasonography is called duplex scanning. The addition of colour frequency mapping (so called colour duplex or triplex scanners) makes the identification of arterial stenoses even easier and reduces the scanning tim

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