Strain Gauge Plethysmography

Overview:

Venous occlusion plethysmography is a non-invasive method used to study human vascular physiology and pharmacology in vivo. The technique is typically combined with intra-arterial drug administration, usually into the forearm vascular bed to assess changes in forearm blood flow. This is an ideal method to assess the local effect of drugs and hormones on peripheral resistance vessels without invoking systemic effects.

Changes in forearm blood flow or volume are measured by a plethysmograph, such as mercury-in- rubber (or silastic) strain gauge or indium-gallium gauge. Venous occlusion plethysmography can also be applied to the lower limb, such as the calf.

 

Method:

A strain gauge plethysmograph is made up of a stretchable tube containing a liquid metal, such as mercury or an indium-gallium alloy. When performing forearm venous occlusion plethysmography, the strain gauge is placed around the widest part of the forearm and acts as a resistor connected to a Wheatstone bridge. Changes in forearm volume result in a corresponding change in arm circumference and thus strain gauge length, which can be detected as an alteration in electrical resistance of the gauge, and thus potential difference. If the gauge length is made equal to the resting circumference of the forearm, then changes in forearm volume are directly proportional to the changes in resistance. This can be monitored by a suitable meter, which can output an analog voltage signal direct to a PowerLab system.

Blood flow can be halted (venous occlusion) and restarted by an inflatable cuff placed on the upper limb. Forearm blood flow is measured by occluding venous flow out of the limb at an instant while not changing the rate of arterial inflow. This is done by inflating a venous occlusion cuff placed above the elbow to a pressure of 40-50mmHg (sufficient to shut the veins but not the arteries). As blood flows into the limb, it causes the limb to swell and the rate of swelling is a measure of the arterial flow rate at that instant. Flow to the hand is quiet variable and is usually stopped by inflating a wrist cuff above systolic pressure.

Suitable strain gauges and meters, as well as cuffs and inflators, are manufactured by D.E Hokanson, Inc.

Studies are conducted in a quiet, temperature-controlled room, with the subject resting in a comfortable supine or sitting position with the arm level and relaxed. Often a number of other physiological signals are also measured in these experiments, such a skin or core body Temperature, ECG, heart rate, Pulse Oximetry, finger pulse etc.

Note: If continuous intravascular or non-invasive blood pressure measurements are also required then see the following:

Blood Pressure – Invasive
Blood Pressure – Non Invasive

Refer to the Blood Flow application page for details relating to transit-time ultrasound and laser Doppler techniques for measuring blood flow and volume.

Software:

The LabChart Advantage:

(may require additional Modules and Extensions)

• LabChart data files can be marked with events using the Comments feature
• Maximum, minimum and average signal amplitudes can be calculated using the Data Pad
• The Units Conversion feature can be used to convert signals into relevant units e.g. ml/min
• The Zoom View can be used to overlay blood flow signals for comparison (control arm versus infused arm)
• Macros can automate many tedious and repetitive analysis tasks
• Data can be easily transferred into popular spreadsheet and graphing programs for further analysis and reporting e.g. Excel, Matlab

LabChart

LabChart software (for Windows and Macintosh) together with a PowerLab data acquisition system offers up to 32 channels of real-time data acquisition, data integrity, easy selection of hardware settings, powerful online and offline analysis, procedure automation, seamless extraction of experimental data and flexible display options. Additional acquisition and analysis functionality is provided with the use of specialized LabChart Extensions and LabChart Modules. Modules are available as part of LabChart Pro while Extensions are free for download from the website for existing LabChart users.

GLP and 21 CFR Part 11

For researchers working within a laboratory requiring GLP and 21 CFR Part 11 compliance, the GLP Client and GLP Server are available for use with LabChart (Windows only) and PowerLab data acquisition systems. For more information, visit the Good Laboratory Practice application page or contact your nearest ADInstruments representative.

Hardware:

PowerLab Data Acquisition Systems

The PowerLab is a high-performance data acquisition unit capable of recording at speeds of up to 400,000 samples per second continuously to disk (aggregate). PowerLab units are compatible with instruments, signal conditioners and transducers supplied by ADInstruments, as well as many other third-party companies. In addition to standard single-ended BNC inputs, 4 differential Pod ports are also available for direct connection of Pod signal conditioners and appropriate transducers.

Strain gauges (including cuffs and inflators) and suitable meters which can output an analog voltage signal can be connected directly to a PowerLab system. Research PowerLab units include:

• PL3504 PowerLab 4/35 - 4 Channels
• PL3508 PowerLab 8/35 – 8 Channels
• PL3516 PowerLab 16/35 – 16 Channels

The effects of obesity and non-pharmacological weight loss on vascular and ventricular function and structure
M. R. Skilton, D. P. Sieveking, J. A. Harmer, J. Franklin, G. Loughnan, S. Nakhla, D. R. Sullivan, I. D. Caterson and D. S. Celermajer., Diabetes, Obesity and Metabolism, 874-884, 2008

Chewing and taste increase blood velocity in the celiac but not the superior mesenteric arteries
Someya N, Hayashi N, American Journal of Physiology: Regulatory, Integrative and Comparative Physiology, R1921–R1925, 2008

Enhancement of Vascular Endothelial Function by Recombinant Human Thyrotropin
Napoli R, Biondi B, Guardasole V, D’Anna C, De Sena A, Pirozzi C, Terracciano D, Mazzarella C, Matarazzo M, and Sacca L, Journal of Clinical Endocrinology and Metabolism, 1959–1963, 2008