Thermographic assessment of skin temperature during a cold provocation test.

A description was made of the development and application of infrared thermography and fingertip thermometry for the assessment of finger skin temperature during a cold provocation test. Three hundred and seventeen lumberjacks, grinders, metalworkers, stone cutters, and students were examined in laboratory and field investigations. Under laboratory conditions, the influence of water temperature and exposure duration was studied, and skin temperature measurements were made on the volar and dorsal side of the fingers. In the field occupational health examination, infrared thermography and fingertip thermometry were simultaneously applied during a cold provocation test. The following three types of temperature reaction could be recognized: normal rewarming and moderate delay and strong delay of rewarming. The temperature distribution along the finger length was analyzed. The following conclusions were drawn for practical application of the thermometric methods. Skin temperature must be measured on all 10 fingertips, either on the volar or on the dorsal surface. Fingertip thermometry may be used in occupational health examinations. Infrared thermography may yield more information on the development of disturbances in peripheral circulation along the finger length and may be used in special clinical work.

Since 1976 in Germany, vibration-induced white finger (VWF) has been included in the official list of occupational diseases, but there are yet no guidelines for the diagnostic methods to be used for this disorder. The Ministry of Research and Technology and the Central Association of the Industrial Injuries Insurance Institutes have financed research with the purpose of establishing diagnostic methods developed under laboratory conditions and tested in the field. However, for several reasons, there has been no attempt to obtain epidemiologically valid data. The results of this project will be used for the standardization of screening tests in occupational health examinations of vibrationexposed workers. The part of the project described in the present report concerns fingertip thermometry and infrared thermography on subjects with and without Raynaud's phenomenon.

Subjects and methods
The investigation comprised a total of 317 subjects. Seventyfive of them were students [mean age 25. 2  In addition to other methods (general health examination, occupational history, anamneses on peripheral circulation and nervous function disturbances, nerve conduction velocity, vibration perception threshold, and temporary threshold shift) reported elsewhere (I), a cold provocation test was carried out, together with infrared thermography, and fingertip thermometry.
In a pilot study it was shown that water temperatures below 10°C gave rise to pain and therefore were not generally tolerated. A few subjects were excluded because of circulatory debility. At water temperatures above 10°C, a variation of 2°C did not influence the thermal reaction of the skin during the test. Doubling the time of hand immersion in the water led, however, to a prolongation of the rewarming time. For practical reasons it was desirable that the duration of the complete test not exceed 30 min. Therefore, in the field research, the conditions of the cold provocation test were standardized as follows: room temperature 22-24°C, acclimatization before the test 20 min, water temperature 15°C, immersion time I min, test duration 30 min.
The following two methods for measuring skin temperature were used simultaneously in all cases: I. The skin temperature was measured during the test from a thermosensor attached to the volar surface of each finger-tip. The temperatur e was measured every 12 s, stored, computed, and recorded on a 6-color plott er. The fingertip thermometry system is shown in figure I. 2. In order to obtain information on skin temperatur e not only at one point, but along the finger length and the hands, infrared thermography was also used. By way of temperature radiation, the system (as shown in figure 2) enables the recording of temperature images of the dorsal sides of both hands. Thus, during the test, 16 images could be recorded and stored in a computer. Special software allowed the analysis of temperature profiles along the finger and the drawing of rewarming curves for the measuring points on the fingertips (in figure 5 of the Results and Discussion section).

Results and discussion
Thus the skin temperature at th e fingertips incre ases rapidly up to about 28°C or higher (curve I in figure   3). Further increase in skin temperature becomes co nti nuo us ly sma ller and sma ller ho wever . Del ayed recover y may be characte rized by a temperature of < 28°C a fter 15 min (curves 2, 3, and 4 in figure 3).
In this way the a na lysis of temperature reco very of 2 220 fingers of 222 subj ects (lumberjacks, grinders, metalworkers, students) revealed the following three reaction type s: Normal recovery: Fingertip skin temperature of 28°C within 15 min The temperature measurements made on 90 fing ers in a pilot study show ed small differences bet ween the vo lar a nd dorsal sur faces (mean a.2°C , ma ximum 0.9°e). The results of th e IwO thermometric method s did not differ very mu ch in accuracy and always sho wed the same trend. The va lues of the finge rt ip th ermometry so metimes tended to be lower than th ose measured with the in fra red thermography , the re ason po ssibl y being the difficul ty in thoroughl y dr ying the skin of th e area around the thermosenso rs after the cold provocation .
There is no ab solute and genera lly agreed measure o f " no rmal" and " patho log ica l" react ion s to guide the choice of the rewarming criterion. In principle it is, however , clear that fast rewarming is a sig n of high dynamic activity of the muscles o f the blood vessels, whereas delayed recovery is a sympto m of weak d ynamic ac tivity . Di ffer ent criteria ma y be used to fo llow thi s principle .
The ba ckground for the cho ice m ade in the present wo rk was the following . Temperature rec overy for healthy persons mo stly showed exponential rewarming curves, ie, normally recovering peripheral vessels dilate so me minutes a fter the end of th e co ld pro vo cati on .     A mor e differen tiat ed assessment did not seem justified. Both thermomet ric methods may be used for the evaluation of cold provocatio n tests. For the initial evalua tion of the reaction type of a subject the "worst" of the 10 fingers was decisive. If all 10 fingers showed nor mal recove ry, the subject would be evaluated as norma l. If one or mo re fingers showed moderate delay or strong delay, the subject was placed in the mode rate-delay or strong -delay gro up, respective ly. Figure 4 sho ws the summarized assessment of the skin tem perature recovery of 220 subject s duri ng the cold provocation test. It may be seen tha t at 15 min the mean skin tempera ture of the worst finger increased to 32.1"C in the normal group, to 24.5°e in th e moderate-delay group , and onl y to 22.4°e in the strong-delay gro up . All gro up differences were statistically significant. At the end of the test (30th minute) the skin temperature of the normal group reached 32.T'C, tha t of the moderate-de lay gro up 31.1"C, and that of the stro ng delay group 24.7°e. The differe nces between the normal and strong-dela y gro ups and between the moderate-delay and strong -delay groups were statistically significant.
With in the no rmal gro up there was no sta tistically significant increase in ski n tempe rature between the 15th and 30th minutes , ie, the recove ry proc ess was complete already at the 15th minute. The corres pon ding increase in th e mo derate-delay gro up was statistically significant, signifying that the main recovery took  place after the 15th minute. For subjects of the strongdelay group however, there was only a very small increase in skin temperature. This finding demonstrates a very low level of peripheral circulation. On the basis of the statistical analysis made , it may be concluded that an assessment according to three different reaction types (normal, moderate delay, strong delay) is justified. In a recent study by Steeger (personal communication) on intraindividual variation in skin temperature reaction with eight repetitions of the cold provocation test on different days, a very high consistency (variation coefficient < 5 0/0) in temperature recovery was demonstrated for eight of nine healthy subjects . Variation in temperature reaction may reflect the different physiological types (acrohomoiothermal, acropoikilothermal) of Kliiken (2) (1964). Further research on intraindividual variation between VWF patients in cold provocation tests is needed.
The kind of evaluation of cold provocation tests with fingertip thermometry or infrared thermography described in this report is based on temperature measurements from reference points of ten fingertips and, therefore, allows a comparison of temperature recovery between all the finger s of a subject. This procedure proved to be very important since, in many cases of VWF , the peripheral circulation disturbance only concerns single fingers, mostly exposed to vibration and static forces. On the contrary, Raynaud 's phenomenon of constitutional origin generally shows delayed temperature recovery for all fingers. A cold provocation test of this kind therefore may be of differential diagnostic value.
Infrared thermography, however, gives additional information which may be used in a second step of assessment. With this method it is possible to evaluate circulation disturbances on the basis of spatial and temporal changes in finger skin temperature . Figure 5 shows the mean skin temperature at four points along the fingers at 14 and 30 min of the test (corresponding to the 13th and 29th minutes after the end of the cold provocation). In the 1 732 fingers assessed as normal, the temperature was high at distal points 2 and 3. In the two proximal points (I and 0), however , the temperature was 0.5 -I.O°C lower at 14 min than at 30 min. In the 257 fingers given a moderate-delay rating, the temperature distribution at 14 and 30 min was very different. At 14 min the temperature tended to be progressively lower from proximal to distal, whereas at 30 min the opposite was observed . This finding reflects the course of rewarming, which starts distally. For the 231 fingers with a strongdelay rating, there was a clear decrease in skin temperature from proximal to distal at both 14 and 30 min. The skin temperature of the fingertip was about 3°C lower than that of proximal point 0 as a result of strong peripheral vasoconstriction.
In summary, the following conclusions may be drawn: I. Skin temperature must be measured on all 10 fingers during the cold provocation test to enable a comparison of temperature recovery. Observed finger differences in rewarming rate may also be of differential diagnostic value.
2. Fingertip thermometry and infrared thermography are both suitable for the testing of finger thermoregulation.
3. The temperature may be measured either on the volar or on the dorsal surface of the fingertip.
4. The following three types of temperature reaction may be recognized: normal recovery, moderate delay in recovery, and strong delay in recovery.
5. Temperature measurements on the fingertips are sufficient for most subjects. In advanced cases, measurements along the length of the finger with infrared thermography may yield more information on the development of disturbances in peripheral circulation.
6. Considering the expense and effects of both the two methods used in this study to measure skin temperature, fingertip thermometry may be used with advantage in occupational health examinations, and infrared thermography is suitable for special clinical work.