APPENDIX
37360 Instruction Manual (Rev. 0)
14 APPENDIX
Foot Withdrawal Response to Noxious Radiant Heat in the Rat
In the living animal, the temperature at the target site, to which the infrared (IR) beam is applied, is definitely
an elusive datum.
Physiological and anatomical features significantly affect “temperature”. Heat transfer constantly occurs via
vascular flow, dissipating the heat from the site where the IR stimulus is applied. Obviously, the extent of
vascularization of the target site affects the heat transfer from the stimulated site. And although vasculariza-
tion of tissue is generally comparable between animals, there also may be a measurable degree of variation
among test animals within a group.
Pigmentation strongly affects the rate of absorption of heat; it is well known that darker skin will “heat” more
quickly than lighter areas of the epidermis.
Further still, temperature measurements (using subdermal thermocouples or thermistor probes, etc.) are af-
fected by variation between experimental animals, due to small or great differences in the orientation of ex-
ternal probes or sensors, due to variation in depth of subcutaneous implantation of subdermal probes, and
perhaps even more seriously, due to tissue damage when positioning subcutaneous probes or sensors.
There is a relevant and logical solution to these questions about temperature; one should measure a param-
eter that is able to be quantified, and is not affected by the physiological and anatomical problems which af-
fect “temperature”, per se.
An objective way to quantify the intensity of the IR stimulus is to calibrate its power. Power may be quickly
and definitively be measured in mW per square centimetre, by a suitable radiometer (see our Cat. No.
37300).
To quantify power of the IR stimulus is to measure something certain. Any measure of power is a metric not
subject to the above physiological and anatomical variation lacking in definiteness and precision.
The power of the IR stimulus, in physical sense, is the basic parameter. We know by experience with this
type of experiment, that the animal reacts to the stimulus after a certain time (latency) at a certain IR power.
Power multiplied by time is equal to energy, which is a parameter that we can measure in Watts per second
or Joule.
The experiment therefore delivers a certain quantity of energy. Classical experiments, (see paragraph 13)
show that the threshold of pain takes place at epidermis temperature of 45°C.
Heat transfer and absorption may be affected by treatment, whether anti-inflammatory drug treatment, nerve
blocking drug CIA, induced inflammation, nerve injury, etc.
The pathophysiology of any injured site is quite different from a non-treated site, and the temperature fluctua-
tion via vascular heat transfer becomes a relative and moot term, further complicating comparison of heat per
se.
It is obvious that the instruments Plantar Test and Tail Flick are not lesion making devices; they are not
meant to be. The threshold of irreversible tissue damage should not be reached. In an uninjured paw or tail,
experimentation should be a sequence of algesia tests carried out on consistently sound tissues.
