| Photoplethysmograph
(PPG) was developed in the 1960's and
1970's by psychophysiology researchers. The
instrument is used for assessment of peripheral
blood flow to the extremities. When the client is
stressed their blood vessels constrict (less blood
flow in fingers). Photoplethysmograph is based
upon the premise that all living tissue and blood
have different light-absorbing properties. PPG
works by placing an individual finger between two
parts of a transducer consisting of a light source
and a photocell (which converts light to
electrical energy). A beam of infrared light is
projected toward the photocell. The blood in the
finger scatters light in the infrared range, and
the amount of light reaching the cell is inversely
related to the amount of blood in the finger.
Hence, when blood vessels in the finger dilate,
the increased blood flow allows less light to
reach the photocell, when blood vessels constrict,
blood flow is decreased and increased light
reaches the photocell. Pulse volume measurements
are related beat-to-beat variations in the force
of blood flow. Phasic changes made on a
beat-to-beat basis, are called pulse volume
measurements and are related to beat variations in
the force of blood flow. These beat-to-beat
changes in peripheral blood flow reflect ANS
regulatory activity and can be used for the
purpose of evaluation of sympathetic activity or
stress. (“Measuring Stress” by S. Cohen, R.
Kessler and L. U. Gordon,
1995) |
