Development of a simple radiant heat induced experimental pain model for evaluation of analgesics in normal healthy human volunteers.

OBJECTIVE: Human experimental pain models help to understand the mechanism of the painful conditions and can also be adopted to test analgesic efficacy of drugs. In early phases, the clinical development of new analgesics is hindered due to the lack of reliable tests for the experimental pain models. In the present study, we have developed and validated a simple radiant heat pain model which can be used for future screening of various analgesic agents.
MATERIALS AND METHODS: We have standardized the thermal pain model by recording pain threshold and pain tolerance time in seconds at three different intensities and levels in 24 healthy subjects. Reproducibility of the test procedure was evaluated by recording the pain parameters by two observers on three consecutive days. Validity of model was further tested by evaluating the analgesic effect of tramadol. RESULTS AND
CONCLUSIONS: Use of radiant heat pain model with high intensity and short level was found to produce low variability with coefficient of variation less than 5%. Interobserver and interperiod reproducibility was very good as shown by Bland - Altman plot; with most of the values within ± 2SD. Tramadol produced statistically significant increase in pain threshold time. The newly developed pain model produces a type of experimental pain which is responsive to analgesic effects of tramadol at clinically relevant doses.

Introduction

To test efficacy of drugs used in the management of pain, human experimental models are useful in understanding the mechanisms underlying clinical pain conditions. Many human models simulate clinical pain conditions similar to chemically or thermally inducing cutaneous hyperalgesia with or without an actual injury.[1]

Thermal stimulation historically has been the most widely used pain-induction method. Heat stimulation can be achieved through contact with a hot object or with hot water, but most studies employ the radiant heat. This generally involves the use of an infrared light source focused on a skin site. Other methods employ a CO2 laser and Argon-laser as the heat source. The laser-induced thermal stimulation produces a double pain sensation: an initial pinprick sensation attributable to A-delta fiber activity and a second diffuse burning sensation due to C-afferent activity. Although this method is sensitive to changes in pain experience produced by analgesic drugs, it is expensive, requires sophisticated equipment and trained personnel and also the variability in response between individuals is high that may be due reflectance, transmission, and absorption of epidermis.[2-4] Lasers and other thermal probes (for e.g., Peltier thermode) have been extensively used to assess drug efficacy, such as topical analgesia (eutectic mixture of local anesthetics), epidural and spinal administration (morphine and bupivacaine), weak analgesics (e.g., acetaminophen, ibuprofen and codeine) and intramuscular alfentanil, intravenous lidocaine, and 5-HT antagonists.[5]

In the last decade, a number of experimental pain models involving healthy volunteers for comparative evaluation of non-opioids efficacy have been reported.[6] In these experiments, detectable effects were mainly demonstrated in complicated pain models including an inflammatory component. In the present study, we have simplified the pain model and have optimized the experimental conditions in human subjects. A validation of this model was also carried-out by demonstrating the analgesic effect of tramadol.

Materials and Methods

Radiant Heat Analgesiometer

A radiant heat analgesiometer designed and developed by the Department of Clinical Pharmacology and Therapeutics, Nizam's Institute of Medical Sciences, Hyderabad, was used to deliver variable, quantifiable and reproducible radiant heat stimulus via a heating element to induce thermal pain on volar surface of forearm. This apparatus [Figures 1 and 2] is made up of two heat resistant acrylic chambers. The lower chamber (radiant heat source) is rectangular in shape (length, 31 cm; width, 26 cm; height, 23 cm) with openings on all four sides to place the radiant heat source (double heating element toaster). The upper chamber (radiant heat delivery chamber), having the dimensions of 22 cm (l) × 17 cm (w) × 20 cm (h) is placed on the center of the lower chamber. The upper chamber is fitted with an additional freely movable acrylic chamber to raise the height of the radiant heat delivery chamber to a maximum of 58 cm and has three circular apertures, each of 4 cm diameter on the two side walls to reduce the intensity of excess heat generation from the filament. There is a slot on the top portion of chamber to fix the thermometer. The height of the chamber can be adjusted with a knob at three different levels by raising the movable chamber. This chamber can be moved to three levels (short level 1 = 43 cm, medium level 2 = 50.5 cm, and high level 3 = 58 cm). At short level, the distance between the radiant heat source and the site of heat application (volar surface of forearm) is smallest, while it is maximum when chamber is raised to the high level. There is also a 7 cm × 4 cm aperture at the top of the radiant heat delivery chamber to place the volar surface of the non-dominant experimental hand. There is another 2.5 cm × 2.5 cm square aperture on top to reduce the excessive heat generation during the test procedure. A highly sensitive temperature recording sensor (Nippon Instruments (India) Pvt. Ltd) is also placed on the top portion of the chamber. Electronic digital timer is used to record the reaction time in seconds.

Figure 1

Showing radiant heat apparatus

Figure 2

Showing delivery of radiant heat to study participant

The intensity of heat generation from the radiant heat source can be regulated with a knob between 1 - 3 levels (1 low, 2 medium and 3 high).

Study Participants

Twenty-four healthy subjects (mean age 28.4 ± 4.4 yrs; mean height 169.3 ± 6.8 cm and mean weight 65.5 ± 12.3 kg) were recruited. The study protocol was approved by the Institutional Human Studies Committee; Nizam's Institute of Medical Sciences, Hyderabad, India and complied with the Declaration of Helsinki on Biomedical Research Involving Human Subjects.

Exclusion criteria were: pre-existing neurological disease, any acute or chronic drug or alcohol use, diabetes mellitus, known allergy to tested substances, inability to communicate in the local language, inability to perform the test as per protocol procedure and prior wounds or fractures on the tested extremity. Before commencement, the subjects were introduced to the study protocol and a written informed consent was obtained. To acclimatize with the test procedure, all subjects had a trial run of radiant heat induced pain testing. Only those who could perform the test as per the procedure described in the protocol were included for the study.

Test Procedure

After overnight fast, the participants were asked to sit in a quiet room with ambient temperature of 22°C ± 2 °C for half an hour before the initiation of test procedure. On the volar surface of the subjects’ non-dominant forearm, a spot was marked and the subject was instructed to place his/her non-dominant forearm exposing the surface of the spot on heat source chamber which has an oval aperture, 7 cm × 4 cm in size. Initially, the radiant heat intensity level was fixed at 1 and the delivery was done by adjusting the height of chamber to a short level of 43 cm. Subjects were instructed to raise the index finger of the other hand (dominant, non-experimental hand) as soon as he/she perceived the pain sensation (pain threshold time) and to pull his/her forearm away from the aperture when they perceived the heat to be unbearable (pain tolerance time). On each occasion, the test procedure with heat stimulus was repeated thrice to record pain threshold time and pain tolerance time with inter-stimulus interval of five minutes. The mean of the three measurements was determined.

To find out the reproducibility with minimum variation in the values, the above procedure was repeated by changing the distance between the heat source and pain site on forearm. This was done by raising the height of chamber C to medium and high levels. Similarly, the pain threshold and pain tolerance were also recorded with 1, 2, and 3 intensity levels of heat source by adjusting the knob accordingly. The order of intensity 1, 2, or 3 was selected in a randomized fashion to avoid any bias. The recording of pain parameters was done at a gap of at least two days. The cut off period of 180 seconds was kept when participants were not responding to the pain stimulus.

It was observed that data obtained was reproducible and consistent with high intensity and at short level. The participants having pain tolerance time of less than 30 sec and/or more than 180 sec with short level and high heat intensity were excluded from the study.

To evaluate the inter-observer variation, two observers simultaneously conducted the procedure independently in the same subject. To assess the interday (interperiod) variability and reproducibility of the method, pain threshold and tolerance times were recorded in the same subject on three consecutive days. Both the interobserver and interperiod studies were performed by keeping the distance at short level and the high radiant heat intensity.

Effect of Tramadol

Twelve healthy male volunteers (mean age 31.1 ± 7.3 yrs; height 167.7 ± 6.9 cm and weight 63.6 ± 10.5 kg) were included in the study. The volunteers received either one capsule of 50 mg tramadol or similar looking placebo in a randomized manner at 9:00 AM after light breakfast. In between the administration of tramadol and placebo one week washout was allowed. Treatment allocation was not known to subject and observer. On the day of experiment, procedure as described earlier to elicit pain was carried out. As there was more consistent result with short level and radiant heat intensity 3, these conditions were used throughout the experiment. Pain threshold time was recorded at baseline (0 min) and then at 30, 60, 120, and 180 min after administration of drug. Subjects were asked to report any side effect during the study.

Data Analysis

The data on pain threshold and tolerance time was recorded in seconds and presented as mean ± SD.

Bland - Altman Plotting was performed for the assessment of method validity and reproducibility.[7] The relative (positive or negative) difference between each pair of measurement was plotted against the mean of the pair to make sure that no obvious relation appeared between the estimated values of mean and difference. The Bland - Altman analysis was done to compare the values of pain threshold time and pain tolerance time obtained by two observers separately. Similarly, the comparisons were also made to confirm the reproducibility by analyzing the pain threshold time and pain tolerance time values obtained on three consecutive days (periods).

The paired student t-test was used to compare the difference within the group, while the unpaired student t-test was used for comparison among the two groups, value of P<0.05 was considered as significant. All statistical analysis was performed using the Graph pad PRISM software 4 (Graph pad software Inc. San Diego, California, USA).

Results

Out of 24 participants, 18 were males and 6 were females. Participants were of average height and weight for their age and their mean BMI was 22.7 ± 3.2 kg/m2.

The pain threshold time obtained at three distance levels namely, short, medium, and high with different radiant heat intensities like low, medium, and high is given in Table 1. With low intensity, none of the participants perceived pain until the cut off limit with any of the distance levels. Participants could perceive pain only with medium and high intensities. The mean pain threshold time with three distance levels (short, medium, and high) was found to be less with high intensity than the medium. When the pain threshold time is compared between highest distance level and shortest distance level of heat source, it was noticed that pain threshold time was higher with longer distance with both the intensities (medium and high). The mean pain threshold was 64 ± 5 sec with high intensity and short distance. The maximum pain threshold time was 113.5 ± 41 sec with medium heat intensity and with high distance level of 58 cm.

Table 1

Pain threshold time (seconds) with three radiant heat intensities at three distance levels (n = 24)

The pain tolerance time obtained at three distance levels (short, medium and high) with different radiant heat intensities is given in Table 2. Similar to pain threshold time at low intensity, pain tolerance time extended beyond 180 sec. At short distance, with high intensity the pain tolerance time was reached earlier than medium intensity. The pain tolerance was reached much earlier with the short distance. The mean pain tolerance time was found to be 73 ± 5 sec during stimulation with high intensity radiant heat applied at short distance level of 43 cm and it was found to be 131.8 ± 43 sec with medium intensity and at high level. High variability in reaching pain threshold and tolerance time was found when the participants were exposed to medium intensity radiant heat at the highest distance of 58 cm with coefficient of variance of 36.1% and 32.7% for pain threshold time and pain tolerance time, respectively.

Table 2

Pain tolerance time (seconds) with three radiant heat intensities at three distance levels (n = 24)

To study the validity of the method, the data obtained by two observers on pain threshold and tolerance time were studied. It was observed that there was very good correlation with minimum variation in the mean values of two observers with coefficient of variance below 5%. In the Bland - Altman Plot of interobserver measurement of pain threshold time, there was no significant difference in the values for reproducibility reported between the observers and the range of most of the values was within mean ± 2SD [Figure 3a]. Similarly in the measurement of pain tolerance time, there was minimal difference in the mean values and coefficient of variance between the two observer's data. There was significantly less variation in reproducibility with most of the values of pain tolerance time lying within the mean ± 2SD. To study the interperiod variability, pain threshold and tolerance times were recorded for three different days (Period I, Period II, and Period III) by the same observer. The variability was minimal in both the times and the data was highly reproducible with coefficient of variance below 5%. There was good reproducibility between the differences among three periods which is shown as Bland - Altman Plot [Figures 3b–d] for pain threshold time and same was observed with pain tolerance time showing less variation in reproducibility with most of the points lying within mean ± 2SD.

Figure 3

(a) Bland - Altman plot showing observer difference in measurement of pain threshold time, (b) Bland - Altman plot showing period I and period II difference in measurement of pain threshold time, (c) Bland - Altman plot showing period I and period III difference in measurement of pain threshold time, (d) Bland - Altman plot showing period II and period III difference in measurement of pain threshold time

To confirm the validity of radiant heat experimental model, we have used tramadol as a reference opioid analgesic in 12 healthy male individuals. Participants were of average height and weight for their age and their mean BMI was 22.5 ± 2.9 kg/m2.

As compared to placebo, tramadol significantly increased the pain threshold time at 30, 60, 120, and 180 minutes. The comparative data on pain threshold time obtained with placebo and tramadol is shown in Figure 4. Single oral administration of 50 mg tramadol produced statistically significant prolongation in pain threshold time at all intervals as compared to baseline. None of the participants reported any side effects expect mild drowsiness which disappeared after 4 hrs.

Figure 4

Effect of placebo and tramadol on thermal pain threshold. Data are expressed as mean ± SE of pain threshold at different times in comparison with baseline

Discussion

A reliable, non-invasive human experimental pain model that shares features with clinically significant analgesic efficacy is highly desired and could bridge the gap between experimental pain in animals and acute and chronic pain in humans. Several methods of experimentally induced pain have been described, but have met with variable success. Cold immersion, tourniquet tolerance, pressure pain tolerance, and thermal pain threshold techniques are not intended to sensitize central neurons and may thus be less relevant to acute or chronic pain states.[8-9] Reproducibility of the model is also an important factor in testing analgesics.[10]

In the present model, the intensity of radiant heat can be controlled at three levels and the distance between the heat source and stimulus site can also be adjusted. This allows an optimum stimulus to the participant and also ensures repeatability. To optimize a proper intensity and to elicit a consistent pain threshold and tolerance, we gradually increased the intensity from low to high and also changed the distance between the heat source and pain stimulus site. We observed that with high intensity and short distance, the pain perception by the volunteers was clearly noticeable and highly reproducible with minimum variation. It should be emphasized that pain models which evoke higher pain intensities recruit more C-fibers than intensity close to or under the detection threshold. Accordingly, supra threshold pain measures are traditionally thought to be more sensitive in drug research than the pain detection threshold.[11]

In the present study, two observers independently recorded the pain threshold and pain tolerance time which was highly reproducible with coefficient of variance (CV) less than 5% and most of the values ranged within mean ± 2SD of the Bland - Altman Plot. Reproducibility is an important factor in the testing of analgesics where it is necessary to repeat the pain stimulation several times during active and placebo treatments. Earlier studies aimed at presenting the data of heat-induced pain, assessed, and reported inter-session repeatability employing methods based on standard recognized statistical technique.[7] One well known index of accuracy of a method is the coefficient of variation (CV = ± S.D./mean) and CVs of less than 10% are considered to be the hallmark of a good assay for a subjective phenomenon.[12] To test the sensitivity, reproducibility, and predictability of our experimental model, we investigated the analgesic effect of tramadol, 50 mg in healthy volunteers. As expected, it produced a significant analgesic effect. Pain measurements were done with the radiant heat method with least possible discomfort and risk of injury to the subjects. Radiant heat has the advantage that the stimulus can be applied without contact to the skin bypassing the shortcomings of the contact heat.[4] The present model is probably ideal for pain induction as its intensity can be quantified, it produces natural pain which can be measured as pain threshold and tolerance and this appears to be highly comparable across individuals.

Since the opioid system is universal for pain modulation, most types of experimental pain are affected by the administration of exogenous opioids. However, the most sensitive models include tonic pain with stimulus intensity evoking pain above the pain detection threshold.[13-15] As opioids mainly affect dorsal horn activity produced from tonic C-fiber activation; it is likely that the analgesic effect will be exerted on a pain threshold evoked by a tonic nature of pain using radiant heat method as in the present study. However, exceptions exist and ischemic muscle pain has been tested with alfentanil, morphine, and tramadol, where only morphine decreased the pain.[16-18] In the pain model described here, tramadol induced a significant increase in the heat pain threshold, indicating that the study design was valid and capable of detecting an analgesic effect on heat pain. There are still major problems in the exact determination of the activated pathways and pain mechanisms in human experimental pain.[19] Nevertheless, the experimental human models give the possibility to obtain reproducible results in test-retest experiments and hence can be useful for drug screening.[20] The apparatus used by us can be fabricated indigenously at a low cost. The model may potentially allow analgesic effects of new compounds to be quantified in healthy volunteers, before proceeding to expensive clinical trials in acute and chronic pain sufferers.