Forearm Blood Flow

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

• This discussion only mentions equipment that we distribute in Australia and New Zealand; if you are in another country, please follow the provided links to the original equipment manufacturers. Of course there may be alternative manufacturers - but you can find those yourself!

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 ...

• Don't be tempted into thinking that there is a unique value for forearm blood flow, and all you have to do is measure it! The reality is that blood flow varies continuously in response to various physiological effects, including thermoregulation, psychological state, exercise (coming up the stairs to the lab) and so on. A rested subject in a stable environment is likely to give the most consistent results, but there is still likely to be some more-or-less cyclical variation over periods of minutes to tens of minutes.

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:

• A strain gauge. While you can make your own, it's probably not worthwhile; we supply ready-made gauges at moderate cost from Hokanson Inc. Gauges need to be selected according to the limb circumference (so you may need a small set of gauges).

• Some electronics to measure the stretching of the gauge. From Hokanson we supply the EC-6 Plethysmograph, which gives a calibrated output for any gauge between 4 and 50 cm in length. Alternatively, gauges can be directly connected to the Biopac Data Acquisition System below.

• A means of recording the plethysmograph output. We supply and recommend a versatile Data Acquisition System from Biopac Inc. This system is available for Windows or Macintosh and provides easy recording and analysis of the SPG signal (as well as being ideal for a host of other laboratory recording tasks). The system will accept the output of the Hokanson EC-6, or alternatively gauges can be directly connected to Biopac's DA100A Differential Amplifier.

• An occluding cuff. We supply suitable arm cuffs from Hokanson Inc., in several different widths. Some users also prefer to exclude the hand tissue from the measurement by fitting a tourniquet cuff at the wrist.

• The means to inflate the occlusion cuff. While a simple sphygmomanometer might be adequate for occasional measurement, hand cuff inflation gets laborious for many repeated measurements. In any case, rapid inflation is desirable in order to measure the initial slope of the inflow curve. An automatic cuff inflator is therefore recommended, and an ideal unit is the Hokanson E-20 Rapid Cuff Inflator (with optional AG-101 air source). The E-20 also has a cycle-timer option, which makes repeated measurements on the same subject very easy (for example with the cuff repeatedly inflated for 10 seconds and deflated for 15 seconds).

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:

   

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:

• Venous capacitance. As remarked above, the inflow curve plateaus because eventually the venous pressure rises sufficiently to force blood past the occluding cuff. The increased volume at this point thus represents the capacity of the venous system to store blood and is termed the venous capacitance.

• Venous compliance. If the maximal (plateau) limb swelling is recorded, and the measurement repeated at different occluding pressures, the venous capacitance has been measured as a function of pressure. The slope of this relationship measures the venous compliance.

• Venous outflow. If the occluding cuff be rapidly deflated after the plateau has been reached, the rate of reduction of swelling will be determined by the flow resistance of the venous system in the limb. This measure of venous outflow - or in practice a combination of it and venous capacitance - is well documented as an indicator of deep venous thrombosis (DVT), and is used as a sensitive diagnostic test, particularly in the lower limb.