BLOOD VELOCITY METER

Typical Number in Hospital: 5

Cost Bands: 4

References: 3

A continuous ultrasonic wave in the megahertz region is scattered as it passes through blood. The back-scattered part of the signal can be collected by a receiving transducer next to the transducer emitting the continuous waves, and the returning signal will be shifted in frequency in proportion to the velocity of the blood in the vessel. This doppler frequency shift can be detected and applied to a frequency meter (e.g. zero crossing detector) or frequency analyser to produce a signal proportional to blood velocity. The doppler frequency shift will normally fall inside the audible frequency range for the practical range of ultrasonic frequencies and blood velocities.

Unfortunately blood within a vessel is not all travelling at the same velocity. Blood at the centre may be moving at a different rate and even in a different direction from the blood near the vessel walls. To overcome this problem the velocity may be sampled at several points across the vessel and the average velocity computed, although in other cases the shape of the flow profile is sufficient information for the diagnosis required. Alternatively a more crude average of velocity may be discovered using a frequency meter, usually counting the number of zero crossings occurring each second. A major improvement in accuracy may be achieved by resolving the forward and reverse flow separately by identifying those doppler shifted components which lie above the transmitted frequency (flow towards the transducer) from those below.

A useful assessment of blood flow in peripheral vessels can be made using the most simple form of ultrasonic device and listening to the doppler signals through headphones. As the pitch rises the blood is flowing faster, as it would at a constriction in the vessel. This type of device uses a probe containing both transmitting and receiving transducer elements which is placed on the skin lubricated by a drop of coupling jelly. The instrument contains an oscillator to generate the ultrasonic frequency (typically 10 MHz), a receiving amplifier, a simple envelope or synchronous detector, and an audio amplifier. If forward and reverse flow can be separated the two signals are presented to the left and right ear separately.

More complex devices provide metered or chart records of the flow. The actual flow waveforms may be recognized as indicating different disorders such as constriction in the larger vessels or high impedance to flow in the extremities.