resistance index of frostbite as a predictor of cold injury in arctic operations

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RESEARCH ARTICLE

Resistance Index of Frostbite as a Predictor of Cold

Injury in Arctic OperationsHein A. M. Daanen and Norbert R. van der Struijs

DAANEN HAM, VAN DER STRUIJS NR. Resistance index of frostbiteas a predictor of cold injury in Arctic operations. Aviat Space Envi-ron Med 2005; 76:1119–22.

Introduction: Cold-induced vasodilation (CIVD) is mentioned as amechanism that may prevent the occurrence of local cold injuries. Themagnitude of the CIVD reaction differs considerably between subjectsand there were some indications that those subjects with a fast CIVDreaction with high amplitude had a reduced risk for cold injuries. Thepurpose of this investigation was to determine the magnitude of thefinger CIVD reaction in subjects prior to operation in cold areas and torelate these scores to the occurrence of cold injuries. Methods: In orderto evaluate the magnitude of the CIVD response, 206 subjects immersedtheir left middle finger in ice water for 30 min. The Resistance Index forFrostbite (RIF) according to Yoshimura was determined on the basis of the finger skin temperature response. This index ranges from 3 (high risk)to 9 (low risk) depending on the response time and response magnitude.Later, most of the screened subjects deployed as part of a group of 1080marines for winter operations in Norway. Results: The Caucasian subjects in this study had higher RIF scores than the non-Caucasians (7.0 Ϯ1.6 vs. 6.1 Ϯ 2.1). The mean RIF was relatively high as compared witha reference group of Japanese male soldiers (6.9 Ϯ 1.7 vs. 5.7 Ϯ 1.7).Unexpectedly, smokers had a higher RIF score than non-smokers did.The RIF score was inversely related to pain during the test. There were54 marines who suffered cold injuries during training in Norway. Of

those, 11 were in the measured group of 206 marines. These subjectshad a RIF of 5.2 Ϯ 1.6, as compared with 7.0 Ϯ 1.6 for the remainingsubjects, which was significantly different. Conclusions: The RIF showsconsiderable differences between subjects. The RIF, determined in asimple lab test, may be related to the risk for cold injuries duringoperations in the field.Keywords: frostbite, cold injuries, screening, cold induced vasodilation.

DURING THE OCCUPATION of Manchuria by Japanese forces in the Second World War, it

became clear that some soldiers were more prone tolocal cold injuries than others. Some Japanese scientistsrelated the occurrence of cold-induced vasodilation(CIVD) to the cold injury risk (18). CIVD is the para-

doxical vasodilatory reaction of blood vessels to cold (Ͻ15°C), during which a considerable amount of heat isreleased that may prevent the occurrence of local coldinjuries (18). The CIVD-reaction is attributed to paraly-sis of the muscle wall around the arterio-venous anas-tomoses (4,6).

In 1950, Yoshimura and Iida developed a simple testto determine the risk for local cold injuries using theCIVD reaction (18). The left middle finger was im-mersed in ice water for 30 min. The fingertip tempera-ture was determined each minute using a small ther-mocouple at the fingertip. The water was well stirredand ambient temperature was set at about 20°C. A

“Resistance Index for Frostbite” (RIF) was calculatedfrom the CIVD response.

To our knowledge no studies are currently availablethat validate the RIF as an indicator for the risk of getting cold injuries during operations in the cold.Therefore, this study was performed, in which results

of a RIF prospective screening test were compared withthe possible occurrence of cold injuries during opera-tion in a cold area. Also, for the screening group, weinvestigated the relation between RIF-score and otherfactors such as ethnicity and smoking behavior.

METHODS

Subjects

A total of 206 subjects participated in the screeningprocedure. The subjects were recruits of the RoyalNetherlands Navy with a mean stature of 182 Ϯ 7 cm,

bodyweight 77 Ϯ 7 kg, and age 20 Ϯ 2 yr. Of the 21subjects with ‘other ethnicity,’ 5 were Moroccan, 8 In-

donesian, 4 Afro-Caribbean, and 4 had mixed ethnicity.The local ethics committee of TNO Human Factorsapproved the protocol and the subjects gave their writ-ten consent prior to the immersion test.

Protocol

The measurements were performed in January andFebruary 2000 and 2001 indoors. The mean ambienttemperature in the room was 18.7 Ϯ 1.1°C (range 16.3°Cto 20.9°C). The subjects reported in groups of no morethan 14. They were dressed in combat suits. Each sub-

ject received a long cup with ice cubes and cold water.The subjects sat on the floor or on a chair with the cup

at the level of the heart. They immersed their left mid-dle finger in ice water for 30 min after application of atype T thermocouple at the fingertip (T-T-28M, Temp-

From TNO Defence, Security and Safety, Business Unit HumanFactors, Soesterberg (H. A. M. Daanen), and the Royal NetherlandsNavy, Office of the Surgeon General (N. R. van der Struijs), TheHague, The Netherlands.

This manuscript was received for review in July 2005. It wasaccepted for publication in October 2005.

Address reprint requests to: Hein Daanen, Ph.D., TNO Defence,Security and Safety, Business Unit Human Factors, P.O. Box 23, 3769ZG Soesterberg, The Netherlands; [email protected].

Reprint & Copyright © by Aerospace Medical Association, Alexan-dria, VA.

1119 Aviation, Space, and Environmental Medicine • Vol. 76, No. 12 • December 2005



control, Voorburg, The Netherlands). The subjects wereasked to stir the ice water with their immersed finger.Ice was added to the cup as soon as the cubes becametoo small to maintain a constant water temperature of 0°C. Every 5 s the skin temperature of the fingertip wasmeasured and stored in a PC.

Since core temperature is known to influence theCIVD-response (5,7), the oral temperature was mea-

sured in a subsample of 94 subjects using a thermistorwith an RD-temp datalogger (Omega Technologies Ltd,Broughton Astley, UK). The subject put the thermistorin the sublingual pocket and was asked not to talk or to

breathe through the mouth. Every 5 min the subjectsrated their pain experience on the Borg-scale (1). Scoresare rated 0 or more: 0 (nothing at all), 0.5 (extremelyweak), 1 (very weak), 2 (weak), 3 (moderate), 5 (strong),7 (very strong), and 10 (extremely strong). The RIF wascalculated according to Yoshimura and Iida (18). Threevariables were used: the lowest finger skin temperatureprior to the CIVD response (Tmin); the onset time of CIVD (Tonset); and the mean finger skin temperaturefrom minute 5 to 30 (Tmean).

Yoshimura and Iida (18) constructed a scoring systemfor each variable: 2 points if the score was close to theaverage of their reference group of 100 Japanese sub-

jects; 1 point if the reaction was considerably worse; and3 points if the reaction was considerably better (TableI). This system is also used in the current experiment.The values are derived from each individual CIVDresponse. The onset time was determined from the tem-perature chart by two investigators and omitted if dis-agreement occurred.

Follow-Up Study

In 2002 a group of 1080 marines went to Norway fortraining in the cold. A part of this group consisted of subjects who had performed the finger immersion test.When cold injuries occurred during training, a medicaldoctor checked the subjects and the location and sever-ity of the cold injury were recorded. The RIF values of the subjects that suffered cold injuries and participatedin the immersion test were compared with the RIFvalues of the other subjects in the immersion test. Atotal of 57 cold injuries were observed in 54 marines,which was exceptionally high and probably related tothe extreme cold during the training period. The sever-ity and location of the cold injuries of the total group isshown in Table II.

Data Processing

The relation between RIF and ethnicity, body massindex, and smoking behavior was investigated usingregression analysis (15). A t-test was used to identifydifferences on the RIF-index between the subjects withcold injuries and the remaining group.

RESULTS

In 16 out of 206 subjects, the CIVD response was tooambiguous to determine the onset of CIVD. This mayhave been due to continuous heat transfer to the water(14) or because the finger was accidentally removedfrom the water in the cup. The mean RIF-score of theremaining 190 subjects was 6.9 Ϯ 1.7 SD.

Since one subject did not fill in his smoking behavior,the number of subjects for the analysis was 189. Sur-prisingly, the 106 non-smokers had an average RIFscore of 6.6Ϯ 1.7, while the 83 smokers had a higher RIFscore of 7.2 Ϯ 1.5 (t-test, t ϭ 2.6, df ϭ 187, p ϭ 0.0098).The Caucasians had a higher RIF score (7.0 Ϯ 1.6) thanthe other ethnicities (6.1 Ϯ 2.1) (t-test, t ϭ 2.2, df ϭ 188,p ϭ 0.031).

The pain score was maximal 5 min after immersionand decreased with time (Fig. 1). Pain was well corre-lated to the RIF score: the lower the RIF, the higher thepain score (p Ͻ 0.001). The correlation was strongestafter 10 min of immersion (Ϫ0.46), followed by 15 minafter immersion (Ϫ0.36). The average oral temperatureof the measured 94 subjects was 36.8Ϯ 0.2°C. There wasno significant relationship between oral temperatureand RIF score (r ϭ Ϫ0.08).

Of the 54 marines who suffered cold injuries duringtraining in Norway, 11 of them were in the measuredpool of 190 subjects with a clear CIVD response (Fig. 2).These 11 subjects had a RIF of 5.2 Ϯ 1.6, as compared

TABLE I. SCORING SYSTEM OF YOSHIMURA AND IIDA (18)TO DETERMINE THE RESISTANCE INDEX OF FROSTBITE (RIF).

Number of points 1 2 3

Tmin Ͻ 1.5°C 1.6–4.0°C Ͼ 4.1°CTonset Ͼ 12 min 8–11 min Ͻ 7 minTmean Ͻ 4.0°C 4.1–7.0°C Ͼ 7.1°C

Tmin stands for the minimal finger skin temperature, Tmean equals theaverage finger skin temperature, and Tonset corresponds to the time atwhich cold-induced vasodilation (CIVD) starts. All points are addedso that the RIF score ranges from 3 (weak reaction to cold) to 9 (strongreaction to cold).

TABLE II. SEVERITY AND LOCATION OF THE 57 COLDINJURIES IN 54 MARINES.

Hands Feet Head TOTAL

1st degree 7 30 2 392nd degree 7 11 0 18TOTAL 14 41 2 57

Fig. 1. Pain scores (mean Ϯ SEM) on the Borg scale vs. time forimmersion of a finger in ice water; n ϭ 205 for times 5–25 and n ϭ 191for time ϭ 30 min.

PREDICTION OF COLD INJURY—DAANEN & VAN DER STRUIJS




with 7.0 Ϯ 1.6 for the remaining 179 subjects (Fig. 3).This was significantly different (t-test, t ϭ 3.6, df ϭ 188,pϽ 0.001). Three subjects had second-degree injuries of the feet and three others had second-degree injuries of the hands. Three subjects had first-degree injuries of thehands, and two had first-degree injuries of the feet. Onesubject had first-degree injuries of both the hands andthe feet. The injured subjects did not differ from theuninjured group in body mass index (23.1 vs. 23.2 kg ⅐

mϪ2, t ϭ 0.1, df ϭ 188, p ϭ 0.91) or smoking behavior(40% vs. 44%, t ϭ 0.3, df ϭ 187, p ϭ 0.80). Of the 11subjects in the injury group, 10 were Caucasians.

DISCUSSION

The mean RIF-value of the Japanese soldiers was5.7 Ϯ 1.7 (18) compared with the value of 6.9 Ϯ 1.7 of the mixed Dutch population in the current experiment.Contrary to our observations, Hirai et al. (8) observedthat the Japanese had a faster and more pronouncedCIVD-reaction than Caucasians. The difference disap-peared when subjects were living in the same area,showing not only genetic effects but also the effect of the living environment. The comparison between theold Japanese data and the current experiment suggeststhat the Dutch marines have relatively good protectionagainst local cold injuries. This may be due to excellentphysical condition, health, and food status or differ-ences in history of cold exposure or cold history (4).

In our screening group 11% of the subjects werenon-Caucasians. Generally, Afro-Caribbeans have aweaker CIVD-response as compared with Caucasians(9). The RIF values of the Caucasians in our study werehigher than the other ethnicities (7.0 vs. 6.1). Four out of nine subjects with a RIF value of 3 were non-Caucasian:an African, an Indonesian, and two Moroccans. Theseethnic differences cannot be fully subscribed to ambientfactors; they partly remain when people who are bornin hot climates move to colder areas.

The general experience in the Royal NetherlandsNavy is that smokers have a higher risk for local coldinjuries. The results, however, show that smokers havea better CIVD-reaction than non-smokers. It is impor-tant to realize that smoking was not allowed during

training in the cold. Smoking is known to lead to in-stantaneous vasoconstriction, and we speculate that thevasoconstriction reaction due to cold may be blunted insmokers.

The pain ratings during the cold-water immersiontest correlated well with the RIF score. This was sur-prising because the pain was scored in a group andsocial control was expected to dominate. Marines donot easily call events painful. Pain is known to dependon the phase of the CIVD response (11). The vasocon-striction phase is most painful, and during vasodilationpain decreases. Most subjects are expected to be in thevasoconstriction phase 5 min after immersion, and,therefore, their pain score is high. During immersion,core temperature generally decreases slightly due toheat loss to the water and the CIVD magnitude subse-quently decreases. Regression analysis showed thatsubjects experiencing strong pain after 10 min of im-mersion had a worse CIVD reaction and thus may havea higher risk for local cold injuries. Thus, pain can beseen as an effective warning signal.

It is extremely unlikely that the cold-water immer-sion test itself might cause any damage to the immersedfinger. The finger may feel tender after immersion forseveral hours, but 3 h of immersion are needed to resultin overt non-freezing cold injuries (12). Campbell and

Jewitt (2) reported a case of cold-induced angioedemaafter 4.5 min of hand immersion in 0°C water, butpropanolol may have triggered this exaggerated re-sponse to the cold stress. This case is very rare since thecold pressor test is considered to be quite safe. It has

been administered to more than 1700 children ages 4 to18 yr without known adverse physical or psychologicaleffects (16).

Core temperature is well known to affect CIVD.

When core temperature drops, CIVD magnitude alsodecreases. In the experiment, no relation was observed between sublingual temperature and RIF. This observa-tion may be explained by the facts that oral temperatureis just a global estimator of core temperature and that

Fig. 3. Resistance Index of Frostbite (RIF) values for 11 subjects whosuffered cold injuries compared with the other 179 measured subjects.The percentages denote the percentage of cold injuries that occurred foreach RIF value. A RIF of 3 to 4 is classified as a weak reaction, 5 to 7 asaverage, and 8 to 9 as strong (18).

Fig. 2. Schematic overview of the number of involved subjects. Mostsubjects in the screening test participated in the Norway training. How-ever, the exact number could not be determined.





core temperature threshold for hand blood flow variesconsiderably between subjects. Generally, only compar-isons within subjects show a relation between core tem-perature and CIVD parameters.

Scientific evidence for the validity of RIF scores as anindicator for cold injury risk is scarce. Iida (10) andco-workers immersed a fingertip in Ϫ10°C cooling liq-uid for 10 min. Freezing occurred only in absence of vasodilation. The best validation is to determine the RIFscores of a large group of people and to check whetherpeople with cold injuries had different RIF scores thanthose without. This study was a first attempt. Only 11subjects were both in the experiment and suffered coldinjuries. Even though this group is small, the hypothe-sis that cold injuries are related to CIVD response couldnot be rejected. The reference group consisted of theother 179 subjects in the prospective screening test. It isestimated that almost every subject in the screening testwent to the cold training in Norway. However, it can-not be excluded that some of them dropped out duringthe training and did not go.

In the discussions during the experiments we noticedthat subjects with low RIF values were generally awareof their increased susceptibility to local cold injuries. Itis likely that these subjects behave more carefully incold areas than subjects with a good CIVD response.Still, we observed in our study that the group with coldinjuries had a worse CIVD reaction (lower RIF values)than the reference group.

In extreme cold and windy climates, freezing coldinjuries occur extremely fast, even before CIVD canoccur (17). In this case RIF values cannot be consideredas a good indicator of cold injury risk. During theoperation in Norway, the winds were not extreme, sothe occurrence of freezing injuries before CIVD onset

was not very likely.In order to be able to use the finger immersion test asan indicator for the risk of frostbite, the test has to bereproducible. Yoshimura and Iida (18) investigated thereproducibility of the RIF score by measuring the CIVDreaction of five subjects every 2 to 4 d. The maximalvariation was Ϯ 1 point (SD 0.26). This can be consid-ered good reproducibility. Daanen (3) investigated theCIVD-reproducibility in eight subjects and observed anSD in Tonset of about 1 min and an SD in Tmin of 0.7°C.O’Brien recently observed that the within-subject coef-ficient of variation is only 19% for onset time, 21% forTmin and 17% for Tmax (13). She observed that the nail

bed showed better reproducibility than the finger pad,

and thus may be a better place to measure. However,even for the fingertip, the variation within subjects wasalways smaller than the variation between subjects.

The results indicate that subjects with RIF-scores of 3or 4 in the screening test have a higher risk of sustainingcold injuries (Fig. 3). If we assume that 1000 subjectswere to be tested prior to deploying and sustained anexpected 50 cases of cold injuries, 95 subjects would beexpected to have RIF values of 3–4 and thus be stoppedfrom going. In those, the screening would be expectedto prevent 21 cases of cold injuries. However, 29 coldinjury cases would still be expected in the remaining

905 subjects (false negatives). This means that 42% of the cold injuries would be prevented by the screeningtest. On the other hand, screening would have pre-vented 74 subjects from going who would not havesustained cold injuries (false positives).

In conclusion, we demonstrated that the RIF, as cal-culated from the finger skin temperature, was higher inCaucasians than in the other ethnicities. Unexpectedly,smokers had a better RIF score than non-smokers. Sub-

jects experiencing pain after 10 min of immersion hadworse RIF scores and are thus thought to have higherrisks for cold injuries. Of our subjects, 11 acquired coldinjuries during an operation in the cold. These subjectshad significantly worse RIF scores than the other testedsubjects. This study, therefore, shows that the RIF, de-termined in a simple lab test, may be related to the riskfor cold injuries during operations in the field.

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resistance index of frostbite as a predictor of cold injury in arctic operations

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