Measurement of Blood Flow

Friday 3rd June 2005 V F Murphy

Continuous wave ultrasound

Continuous wave ultrasound probe techniques involve an ultrasound signal being transmitted into the skin, and the difference in frequency reflected being measured. Along with the spectral broadening, this provides an indication of blood flow in terms of turbulence and presence. It however is difficult to get a qualitative measure, to discriminate between venous, capillary, or arterial vessels, is sensitive to the angle to flow, and lacks depth resolution. On the positive side it is non-invasive and by modulating the response using a carrier signal an audible signal can be produced which sounds either dirty (turbulent flow), or clean (laminar flow), giving an indication of underlying problems. The signal is continuous which allows for monitoring during surgical applications and detection of pulse.

Venous Occlusion Plethysmography

Venous Plethysmography is a technique which looks at the performance of the venous system in a limb. The limb is enclosed within an airtight chamber, and a cuff applied at about 40mmHg such that the venous return is stopped, but arterial flow may still pass. The chamber is fluid filled, and after due calibration the rate of volume increase is indicative of the rate of arterial flow into the limb. It is a consequence of venous occlusion that the venous pressure within the limb will increase. This takes place until it reaches the 40mmHg of the cuff, and volume will not increase much further. Once the cuff is removed the rate at which the volume leaves is indicative of the efficiency of venous return, a slow return showing a low venous flow, and indicative of underlying problems. The spatial resolution is much worse than the ultrasonic method, and the depth resolution is nonexistent. The results however are quantitative, and differentiation between arterial and venous flow is a valuable feature. The apparatus is large and cumbersome to use, which limits the use of this method. Further, the measurement is not continuous, which is a disadvantage compared to the ultrasonic method.

Blood flow electromagnetic

Blood flow electromagnetic methods make use of the principle that movement of a charged particle through an electromagnetic field produces an emf. Essentially two electrodes are attached along the length of a vessel, and an electromagnetic field is applied at approximately 90o to the flow. The emf between the two electrodes can be measured and gives a continuous result proportional to the flow velocity. The response is governed by uBL, where u is the velocity (typically 10mm/s), B the magnetic field strength (typically 0.1T), and L the length between electrodes (10mm). Smaller vessels will have a lower u, and will be unable to have such a great L, reducing the signal. Typical voltage signals are in the region of 0.01mV, which is in the region of the electrode contact potential (which poses a great problem to the S/N ratio), and interferes with the ECG signal. These problems however can be reduced by using a 400MHz A.C. signal. The spatial and depth resolution are better than both previous measurements, and once the angle of field to flow has been allowed for, it is potentially an accurate and continuous measurement. The disadvantage of this is that it is by no means non-invasive, unlike the previous two methods[quote]The disadvantage of this is that it is by no means noninvasive, unlike the previous two methods:MedicalEngineer.co.uk[/quote], needing exposure of a given length of vessel for electrode fitting. This however makes it suitable for measurements during surgical procedures, especially coronary bypass or plastic cosmetic surgery.