1. Basic Concept. In the normal case, arterial and venous flow in any limb is equal on the average. Venous occlusion plethysmography measures arterial inflow by abruptly stopping venous outflow with a proximal cuff inflated to above venous pressure, but below arterial pressure (typically about 50 mmHg). This causes the volume of the limb to increase due to arterial inflow.

A mercury strain gauge plethysmograph is used to measure the volume change in the limb. The rate of change of the volume, in percent per minute, is the arterial flow rate at the moment of venous occlusion. Often the flow rate is reported as being cc’s/100 cc’s/minute. This is equivalent to percent per minute.

2. Instrumentation. The NIVP3 software is installed on a personal computer (PC) running the Windows® operating system. The strain gauge plethysmograph can be an EC6 Plethysmograph, which has been fitted with an RS232 interface. An E20 Rapid Cuff Inflator is required to abruptly occlude the proximal venous cuff to the preset pressure of 50 mmHg. A source of clean, pressurized air is required for the E20; the AG101 Air Source is ideal. An Interface Box and cables are required to hook all the instruments to the PC.

3. Interconnection. Connect the Interface Box (IB) to a serial port ( Com Port ) of the PC using the short cable which is permanently connected to the IB. The NIVP3 software will need to be configured using the Tools/System Configuration menu to match the Com Port that is used. A cable with a round 5-pin DIN connector is connected to the back of the plethysmograph (EC6, EC5R, or TL400). (If the plethysmograph instrument has a switch to select the RS232 output on the back, select it.) The cable has a DB9 connector on the other end which should be connected to RS232 jack on the IB. A cable with a ¼” stereo phone plug on each end (similar to the plug on stereo headphones) is connected from the back of the IB to the Foot Switch jack on the rear of the E20 Rapid Cuff Inflator.

Finally the DC power adapter needs to be connected to IB.

4. Subject Setup. Position and protocols may vary depending on the experiment. For forearm studies, an SC10D or SC12D cuff is placed above the elbow and connected to the Y-connector of the E20. Connection to the Y-connector is done by simply pushing the large tube from the cuff directly into one of the tapered holes of the Y-connector. The other half of the Y-connector should be plugged with a short piece of tube which is plugged by a ball.

Flow to the hand is quite variable and will affect flow measurements if it is not eliminated during the time of measurement. Flow to the hand is usually eliminated by placing a TMC7 cuff on the wrist and inflating it with a sphygmomanometer to above systolic pressure during the course of the measurements. The elbow and wrist should be supported so that the strain gauge, which is placed around the largest part of the forearm, does not touch anything other than the arm. It is a good idea to fix the gauge to the skin by putting a piece of masking tape over the head of the gauge and another over the cable a couple of inches away. This is to prevent the gauge from moving if the cable is disturbed.

Flow studies on the leg are similar except that the cuffs are larger. A contoured thigh cuff (either CC17 or CC22) is used on the upper thigh and an SC10 is used to occlude the foot.

5. Instrument Setup. Start the NIVP3 program and select Inflow. Relieve the pressure in the AG101 Air Source tank if it has been running recently. This can be accomplished by presetting the pressure on the E20 to any value above zero and then pressing the Mode switch on the E20 to switch to Cuff Mode. This will exhaust any pressure in the AG101 tank. Now start the AG101.

With the E20 in Preset Mode, set the preset pressure to 50 mmHg. Set the plethysmograph Mode to DC or Vein. Be sure that the Balance setting is NOT set for Auto; if it is, the plethysmograph will reset the baseline whenever the signal goes off scale and you could upset your readings. Set the plethysmograph Range Gain to 0.5%/cm and verify the Range on the PC.

It is important to be sure that the Range Gain settings on the PC and on the Plethysmograph are the same. This setting will allow you to measure about 1.5% volume change during the measurement period. After making a couple of measurements you may want to adjust the Range Gain for better resolution. The ideal sensitivity will give an inflow slope of approximately 45 degrees.

Select the Reading Mode in the NIVP3 software, either Constant/Manual stop or Table/Auto Stop. Set the cuff inflation time and the time between measurements on the PC screen or alternatively, enter the times for making measurements in the Table. Press F1 for Help from the Inflow screen for instructions on editing the Table or setting the Inflate and Interval for measurements. An inflate period of 4 or 5 seconds is adequate to determine the flow rate and the interval between measurements can be varied according to the user’s wish.

6. Making Measurements. Test that everything is working correctly by selecting Instrument Check on the PC. Verify that the trace (after balancing the plethysmograph) is moving across the screen near the bottom and that the settings, Range, etc. agree with the plethysmograph. Escape to get back to the main Inflow screen. When you are ready to start making measurements, select the Start Readings button. The PC will control the instruments and automatically begin taking readings according to the Reading Mode you selected.

The sequence of events is: 

1. Balance the plethysmograph (reset the baseline),

2. Inflate the venous occlusion cuff to 50 mmHg. 3. Hold the inflation for (4) seconds. 4.

Deflate the venous occlusion cuff. 5. Wait the selected period and repeat the sequence.

Select Done with Readings when finished.

7. Editing Readings . The arterial inflow slopes chosen by the NIVP3 program are not perfect and need to be edited. When you get done taking readings the data is ready to be edited. The cursor at the left of the Readings Table shows which reading will be edited. Select the first reading and the recorded data will appear. The vertical lines intersect the volume flow curve and a slope is drawn between the intersection points. 

The lines can be moved with the Mouse, or once a line is selected with the mouse, it can be moved with the arrow keys. Some judgement is required in placing the slope. The left hand point should be chosen to coincide with the peak of the first “good” pulse after the “cuff artifact”. The “cuff artifact” is the abrupt rise in limb volume that sometimes occurs due to reflux of blood when the cuff is inflated. This volume increase is not due to arterial inflow and should be ignored.

After the venous occlusion cuff is inflated the volume of the limb increases due to blood flow. Depending on the flow rate, the increase in venous pressure caused by the arterial inflow and venous occlusion, will cause the rate of arterial inflow to diminish. It is good to choose the right-hand intercept to be about two or three pulse-peaks to the right of the starting point.

This gives sufficient accuracy in determining the slope before a noticeable drop off in the slope occurs. Depending on the flow rate and appearance of the recorded data, it will be necessary to exercise judgement in setting the end points. The program very accurately sets the slope according to the chosen intercept points and displays the flow rate as the slope is edited.

One way of verifying that you have selected the initial slope between two pulse peaks, is to move the cursor bars with the left/right arrow keys after you have set them coincident with the peaks of the chosen pulses. If you have the left cursor on a peak, the slope (flow rate) will increase when the cursor is moved to the right or left. Conversely, when the right cursor is on the absolute peak, the flow rate will diminish in both directions.

When you are satisfied with the settings you have made, pressing the Spacebar will return you to the main Inflow screen and the next reading will be highlighted. Pressing the Spacebar again will bring up the next waveform for editing. When all readings have been edited you can select Graph to show the results on a chart of Flow versus Time.