To illustrate some
common design principles for pain experimentation we will examine
a model system designed to induce and sustain deep jaw pain
of predetermined intensity, defined by upper and lower boundaries.
This is achieved by a computer-controlled pump that is used
to infuse a pain-producing 5% infusion-grade saline solution
into the target tissue at a rate necessary to achieve the stated
goal. Pump infusion rates are based on the subject's initial
response to the presentation of a standardized bolus, such as
pain rise and fall time constants, peak pain intensity, and
the pain intensities reported during a subsequent 90 second
constant infusion. A suitable infusion rate match for the 90
second constant infusion is chosen by comparing the subject's
reaction to the response of 65 subjects previously tested and
picking the closest. Once the subject's response characteristics
are modeled, the controller ensures preset pain intensities
by adjusting the infusion rate according to regular feedback
of the actual pain intensity received from the subject via an
electronic visual analog scale (VAS). The VAS zero score corresponds
to "no pain" and the upper VAS endpoint (100) is defined
as "pain as intense as it can be". Isotonic
(0.9%) saline is used as the non-painful control. Every 15 sec
throughout the pain and saline-control trials, the computer-controlled
VAS bar display begins to rise, prompting subjects to press
the button they are holding when the level corresponds to their
current pain intensity. Model parameters and feedback received
from subjects are stored on hard disk for subsequent analyses.
| Figure
7.1: System to Induce and Sustain Deep Jaw Pain
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| The
subject observes the VAS and holds a switch
to indicate the actual pain intensity whenever
prompted. Note the infusion needle inserted
into the jaw muscle. To familiarize themselves
with the apparatus and to assess each subject's
reporting accuracy and frequency of errors,
subjects are asked to indicate arbitrarily selected
points on the VAS every 15 sec. Training will
occur until it is clear that the subject is
capable of reliably using the VAS. |
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