HEAT PROBE CIRCUITRY

Comparator and Power Amp

Copper disc

Copper Copper

Constantum _Copper

Thermocouple

Hold Ice Junction for thermocouple

Digital Panel Meter Display

Reset

sprung handle, probe skin pressure was constant during skin contact (45g). The apparatus incorporated a subject-operated ‘ofT switch. Closure of the switch interrupted the current to the heating element and reset the heat ramp, whilst the VDU displayed the probe temperature at the point of switch activation. Thus the subject could either stop the rise in temperature of the copper disc at the instant of heat pain threshold, or at any time during the experiment, by closing the switch, or by withdrawing his foot from the heat probe. The accuracy of the apparatus was checked at the beginning of each e^qDerimental session, by recording the probe temperature, at 3 points: in contact with melting ice, in water at 35®C, and 55®C.

The apparatus was set to operate within defined temperature ranges. Thinner areas of skin such as the dorsum and the non-weight bearing areas of the foot were heated through a maximum of 20®C, from 35-55®C. Areas of more heavily keratinised skin, such as the plantar skin overlying the 1st metatarsophalangeal joint, were heated through a maximum of 22®C, from 35-57®C, as these tend to show higher pain thresholds (Stoll et al, 1981; Hardy, Woolf and Goodall, 1952). Skin injury, at the upper temperatures was not anticipated, as the time of application of high temperatures was of very short duration (Fruhstorfer et al, 1976). Even though epidermis is irreversibly damaged if it is maintained at over 56®C for more than 9 secs, and C-PMN unit activity can be suppressed if the skin is held at over 53®C for 30 secs (Beitel and Dubner, 1976), skin, or nerve ending damage, was unlikely. The threshold heat pain stimulus was maintained for no more than 1 or 2 seconds at any site, and probably less, i.e.: the reaction time

of the subject to operate the hand switch and report threshold pain, and / or the operator to remove the heat probe from skin contact. The longest (theoretical) time that the probe would be in skin contact during a single stimulus was approximately 32secs, when the medium heating ramp (0.7®Csec'*) was used, over the greater temperature range (22®C) at weight bearing skin. In this example, the probe temperature would only exceed 55®C during the last 3 secs of heating. Thus, if the reaction time is added to this figure, it is apparent that the maximum duration of the potentially damaging stimulus was less than 50% of the time it takes to cause injury.

Thus the selected method was considered to be very safe, and unlikely to cause any skin damage. C-PMN hypersensitivity was unlikely to occur (Severin et al, 1985; LaMotte et al,

1982), as each skin site was heated to pain threshold only twice, and at least 10 mins was allowed to elapse between each stimulus at the same site. The small size of the heated copper disc (7mm diameter), ensured that spatial and ten^oral summation were minimal (Price et al, 1989). As there was a minimum of 15mm from centre to centre of each adjacent stimulus zone, nerve endings beyond the probe surface during heating were unlikely to be stimulated to firing threshold, or sensitised by adjacent stimuli.

Application of the heat probe:

Subject Familiarisation Process

Each subject went through two 6 miharisation processes. The first was to give basic

information on the working of the apparatus, and the second was a ‘rehearsal’ of the experimental technique.

1 ; Prior to heat pain threshold e?q)eriment, the equipment was demonstrated to the subject. This was to allow him to become femihar with the apparatus, and the operation of the ‘o ff switch. He was able to determine his personal criterion of heat pain threshold, and assuage any possible feelings of apprehension about the test. The tip of the left index finger was used for this familiarisation exercise. The subject was seated on a comfortable chair, with the left arm extended, palm upwards, along a cushioned bench, at just above waist height. The subject held the ‘o ff switch, so that it could be easily operated by his left thumb, with the heat probe, held in his right hand. He applied the heat probe to the tip of the extended left index finger. The tester set the heat ranq) to 1°C, and switched on the apparatus. The subject noted that the probe began to warm the skin sur&ce under the copper disc, and closed the switch to stop the heat stimulus as soon as the incoming sensation changed from one of warmth, through hot, to the pricking sensation of heat pain threshold (Hardy, Woolf and Goodall, 1952;). Subjects were advised that the ejqjeriment required them to identify threshold, NOT tolerance. This frmiliarisation exercise was repeated, as desired, until the subject was confident that he had established his personal criterion of heat pain threshold, and was comfortable and confident with the action of the ‘o ff switch, and aware of the rapidity that the skin cooled after the heat stimulus was removed. Subject knowledge, and therefore co-operation, is essential to good test outcome (Fowler et al, 1987).

2: A second 6 miliarisation exercise was carried out, to ensure that data generated during the

experiment would reflect true threshold, rather than an inability to perform the test properly (Smith et al, 1991). Heat pain threshold test(s) were made, this time to the skin of the subject’s left palm. The subject was seated as above, holding the ‘o ff switch in the right hand, but this time with the heat probe operated by the tester. The subject was asked to keep his eyes closed, and to concentrate on the incoming stimulus. All distractions were kept to an absolute

minimum. All test stimuli began with skin sites pre-warmed to the same temperature (35®C) (Bush et al, 1993), as the initial skin temperature has been shown to contribute to the response of C fibre polymodal nociceptors (Beitel and Dubner, 1976). The pre-warming was achieved

by holding the probe in contact with the skin test area for a minimum of 10 secs before starting the test run. The probe was positioned with the tensator spring depressed by 50% to ensure constancy of pressure (45g) throughout all apphcations. The heat ranq) was selected, and the apparatus switched on by the tester. The subject closed the switch at the point that he decided that his heat pain threshold had been reached, whilst giving a simultaneous verbal report of threshold. The tester removed the probe away from the skin surfrce immediately this occurred. This exercise was used to increase subject familiarity with the test procedure, and to ensure his confidence that the probe would not be kept in contact with his skin past the instant of

threshold pain. Thus, any tendency to activate the ‘ofiT switch prematurely would be minimised. As in the first familiarisation exercise, this process was repeated as often as the subject wished. Once he was sure of his personal criterion o f heat pam threshold, and felt confident and comfortable with the methodology, the ejqjeriment began.

Application of the heat stimulus:

The subject was seated in the same manner as in the touch-pressure and pin prick pain

thresholds: i.e., on a comfortable upholstered chair, with the leg extended, and supported to hip height on cushions, yet free to move away from the test stimulus. The test areas of plantar skin, and the visual display of the heat probe temperature, were obscured from the subject’s view. A record sheet was completed at each test time (see Diagram 21)

The skin sites to be tested were marked on the foot with a felt tip pen, to ensure that all repeat tests were made to the same points, as fer fewer test sites were used, in conq>arison to the touch-pressure and pin prick threshold tests. Subjects were advised to keep eyes closed during the test, and all extraneous disturbances were kept to a minimum, to maximise subject

concentration on the test. Two heat ramps (0.7®C sec'\ and 1®C sec'^) delivered in random order (see below) were used in these e?q)eriments.

The thermode, maintained at 35®C, was placed in full contact with the skin surfrce at the test point, so that the tensator spring was depressed by 50%, to ensure the same loading force (45g) at each apphcation of the probe (Diagram 29). Care was taken to ensure that there was good contact between the thermode and the skin (Bessou and Perl, 1969). The probe was held in position at the test site for 10 secs, prior to starting the heat ranq), to ensured that all tests were made from the same initial skin tenq)erature (Fowler et al, 1987; Jamal et al, 1985). The subject was alerted that the test was about to start, by the experimenter saying ‘now’, and the heat ramp switched on in accordance with the randomisation tables (Tables 15 and 16). Two basic methods were used:

Diagram 29: THE HEAT PROBE

The Heat Probe - Note the unladen probe shaft, tensator spring non-activated

r

The Applied Heat Probe - Note the 50% loading of probe shaft = 45g load, via tensator spring

In the first.each of 8 skin test points along the medial border of the non-weight bearing skin of

the medial longitudinal arch (MLA) of the left foot were heated to threshold pain, twice, from a start tenqjerature of 35°C, following the schema below (Table 15). The second heat stimulus to each test point was given a minimum of 10 mins after the first. Although the subject was alerted that each heat stimulus was about to begin, he was unaware that the heat ramp would be varied during the experiment. Heat pain thresholds were assessed at the MLA using a mix of fast (1®C secT) and medium (0.7®C secT) ramp stimuli. The two heat ramps were alternated throughout the e5q)eriment (i.e.; Stimulus 1 - Fast [F], Stimulus 2 - Medium [M], Stimulus 3 - Fast [F], and

so on) to reduce subject anticipation of pain threshold. The second stimulus to each zone used the alternative heat ramp to that given first, so that Wiere the first was at a heating rate of 1®C secT, the second was given at was given at 0.7®C secT (and vice versa). Thus, each test zone was heated to threshold twice, once at 1®C secT, and once at 0.7®C secT. Half of the subjects were tested in the order am / noon / pm, and the remainder p m /am /n o o n , to partly balance out order effects, and stimuli were delivered in identical ‘random’ pattern, outlined above, to all subjects. Thus, the opportunity to anticipate the stimulus was minimised by varying the heat ramp, the test site, and the test order, for each subject (Miron et al, 1989).

Table 15: O rder o f Heat Pain Stimuli to the M LA

Test O rder of Application o f Heat Stimuli to M LA

Proxim al Distal

1 IF* 2M* 3F 5F 7F 4M 6M 8F

2 IM 2F 3M 5M 7M 4F 6F 8M

*M = Medium Heat Ranq), 0.7®C sec ’

In the second. 4 non weight bearing, (medial longitudinal arch: MLA), 4 weight bearing (medial area of the ball of the foot: 1st Ray) plantar skin sites, and a dorsal skin site (the dorsum of the great toe joint: 1 MTPJ) on the left foot were heated to threshold pain, twice. All tests were made in the same manner as the first experiment, from a start temperature of 35®C, following the schema below (Table 15). The second heat stimulus to each test point was given a minimum of 10 mins after the first. Heat stimuli were alternated as in the first experiment, and delivered in a similar ‘random’ manner. Tests were made in the morning, and repeated in the late afternoon, with half of the sanq)le tested in the order morning / afternoon, and the remainder afternoon / morning (i.e.: am=>pm, or pm=>am) to partly balance out order effect. In all other respects, the second experiment was carried out as the first.

Table 16:

H eat pain stimuli to w eight bearing and non-weightbearing plantar skin sites, and the dorsum of the foot

Skin Test Area

Plantar Proximal Plantar Distal Dorsum

M LA 1st Ray IM T P J

T e s t l IF* 2M* 3F 5F 7F 4M 6M 8M 9F

Test 2 IM 2F 3M 5M 7M 4F 6F 8F 9M

*F = Fast Heat Ramp, 1 C sec *M = Medium

RESULTS