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Forearm Blood Flow

This note discusses non-invasive measurement of forearm blood flow in humans - a topic of interest to physiologists, endocrinologists and others.

Principles

Forearm blood flow is fairly simply measured by temporarily occluding the venous return (with a cuff inflated to say 60 mm mercury) and measuring the slight swelling of the distal portion of the limb due to continued arterial inflow. In theory, the limb swelling will plateau after a while as the venous pressure rises sufficiently to cause renewed venous outflow past the obstruction. However, the initial rate of swelling should represent arterial inflow under "normal" conditions.

A word of caution is called for here ...

Measurement Methods

Measurement of the limb swelling is called plethysmography (pleth-is-mography, from the Greek). Generally, there are 4 plethysmographic methods:

  1. The distal portion of the limb is immersed in a water-filled tank so that water is displaced as the limb swells. Accurate, but rather messy; not commonly used.

  2. Electrodes are placed around the limb and the reduction of limb impedance is recorded as the limb fills with blood. Theoretically precise but more cumbersome than the following methods.

  3. An inflated cuff is placed around the limb and the pressure rise for a small volume increment is calibrated by injecting some air.

  4. A length transducer - specifically, a fine rubber tube containing mercury or other liquid metal - is placed around the limb and the increase in circumference recorded. This method samples the swelling at one point in the length of the limb, as opposed to measuring the (possibly uneven) swelling all down the limb. Hence the absolute results should be treated with caution, but comparison between treatments and subjects is legitimate and shows reasonable consistency in practice. This technique is called strain gauge plethysmography (SPG) and is the most commonly used technique.

 

Basic Equipment for SPG

To make blood flow measurements with the SPG technique, you need:

Technique

  1. The rested subject is made comfortable with the arm supported by foam blocks or slings at the wrist and elbow.

  2. The cuff is fitted, but not yet inflated.

  3. A gauge is selected and fitted to the limb at a standardised fraction of the forearm length. The system is then electronically calibrated to give exact % stretch for that particular gauge and initial limb circumference. (The calibration method depends on the electronics used. Actually, the output is % increase in limb cross-sectional area, which is twice the % circumference increase).

  4. The cuff is inflated to a standard pressure (say 60 mm Hg) while the SPG signal is recorded. If required, this is repeated a number of times. At this pressure, the occlusion cuff is not painful.

  5. On the recording, the initial slope is measured and noted. On the BIOPAC system with AcqKnowledge software, this consists of dragging a cursor over the relevant period of the waveform and transferring the resulting least-squares slope to a journal with a single keystroke. The following window shows a typical result:

    Sample window

  6. With a calibrated system, the result will be in %/s. This can be interpreted (on the assumption of uniform swelling all along the limb) as mls blood / 100 ml tissue / second. A typical result for forearm flow is of the order of 0.1 ml/100 ml/s (or 5 ml/100 ml/minute).

Other Uses

The same technique can be used for various other measurements:


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