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Journal of Rehabilitation Researchand Development Vol . 25 No . 3Pages 57-62

A Technical Note

Biochemical changes in sweat following prolonged ischemia

MARTIN FERGUSON-PELL, PhD, and SATSUE HAGISAWAHelen Hayes Hospital, Orthopedic Engineering and Research Center, West Haverstraw, New York, andKumamoto University, Japan

The authors wish to state that considerable furtherinvestigation is needed to determine whether biochemicalanalysis of sweat will yield a practical technique for earlydetection of pressure damage to tissues . This technicalnote is intended to report a first step in this initiative.Our work illustrates a concept for possible noninvasivedetermination of biochemical changes related to tissuemetabolism rather than suggesting an immediately appli-cable clinical test.

Abstract—Much emphasis has been placed on the mea-surement of physical parameters at the body supportinterface in order to detect and moderate conditionswhich could result in pressure damage to soft tissues.Major difficulties are encountered both in the design ofinstrumentation and interpretation of the data collected.Metabolic processes in sweat glands that control sweatsecretion have been shown to be sensitive to appliedpressure, producing sweating rate suppression andchanges in sweat NaCl concentration . In this study, wehave demonstrated the feasibility of measuring lactateconcentration in sweat collected locally using an electro-chemical stimulation technique (iontophoresis of pilo-carpine nitrate) . Elevated levels of sweat lactate concen-tration during local tissue indentation were detected in agroup of able-bodied subjects . Upon removal of theindentor, however, levels of sweat lactate returned tonormal.

Key words : sweat metabolism, electro-chemical stimula-tion, ischemia, pressure damage, soft tissue.

Address all correspondence to : Martin Ferguson-Pell, PhD, HelenHayes Hospital, Route 9W, West Haverstraw, NY 10993 .

INTRODUCTION

Pressure sores represent a severe problem forspinal cord injured individuals and often requireprolonged hospitalization for treatment or surgicalrepair (7) . If a reliable and practical clinical methodfor early detection of pressure sores could bedeveloped, it is probable that in many cases, givensuitable medical intervention, the process of tissuebreakdown could be reversed or at least limited in itsseverity . Various techniques for early detection ofpressure sores have been investigated to identifychanges in mechanical, physical, or physiologicalproperties of compromised tissues . None to datehave yielded a reliable and clinically practical meansto detect early tissue damage.

Current clinical practice for early detection ofpressure sores relies upon frequent inspection ofskin color . Persistent red areas are often classified"Stage I" pressure sores (1) and appropriate clinicalmeasures taken . Several difficulties and limitationsare, however, experienced with this highly subjectivetechnique . First, it is important for the clinician orpatient to differentiate between persistent rednessand the normal, healthy, short-term redness ofreactive hyperemia . Second, in the more advancedstages of pressure sore formation, the area becomesischemic and cyanotic, with often only a margin ofredness. Third, all skin color changes associatedwith early onset of pressure sores are difficult todetect in nonwhite patients.

In recent years, methods for evaluating skin statusor the effectiveness of different body support

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Journal of Rehabilitation Research and Development Vol . 25 No. 3 Summer 1988

Figure 1.Sweat induction in progress using iontophoresis of pilocarpine.

systems (cushions, mattresses, etc .) have revolvedaround the measurement of pressure and otherphysical parameters at the skin/support surfaceinterface (2,3) . Although these techniques haveproved successful in reducing pressure sore incidenceamong spinal cord injured persons at a number ofspecific centers, they are not in widespread use norhave they been developed to address the needs of thebroader population at risk, especially the elderly.

An alternative approach employs the tissues them-selves as a direct source of information reflectingtheir viability. This approach is attractive, because itoffers an opportunity to bypass the need to establishprescription criteria for physical parameters, if verysensitive techniques for detecting the early stages oftissue distress can be developed.

Techniques have been developed to monitor bloodflow in tissues using noninvasive techniques (ther-mography, laser Doppler, 133Xe clearance), but allshare a similar limitation in their clinical applicabil-

ity . Their response to reactive hyperemia and in-flammation associated with early onset of tissuetrauma is similar and requires the clinician to waitfor reactive hyperemia to resolve before an abnor-mal tissue response can be detected . In this respect,they offer only a marginal advantage (namely theirobjectivity) over skin color observation . Techniquescapable of measuring tissue oxygenation (TCPO 2)on the other hand are promising as they providedirect information about the metabolic status of thetissues.

During the early stages of pressure sore forma-tion, significant changes in tissue biochemistry arethought to occur . Even during an ischemic periodthat results in no adverse tissue response, metabo-lites are thought to accumulate, pH decreases, andP02 decreases with a corresponding increase inPCO 2 (6) . Despite the potential that tissue biochemis-try offers for monitoring tissue viability, this ap-proach has been neglected primarily because it

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FERGUSON-FELL and HAGISAWA : Biochemical Changes in Sweat

implies the need for invasive sampling of local areasat risk . In addition to the difficulties inherent insampling significant quantities of extra-vascularfluids, the risk that injection sites would producefoci for infection has deterred clinicians fromdeveloping biochemical tests.

Sweat glands are richly endowed with a capillaryblood supply that carries biochemicals whose con-centrations may serve as indicators for pressure soreformation . In principle, it would therefore seemfeasible to use sweat as a vehicle for monitoringbiochemical changes in the tissues using simple,established, noninvasive techniques.

Although a number of biochemicals come to mindas candidates for investigation, in this preliminarystudy we have demonstrated the concept for changesin lactic acid and Na + concentration based onprevious research studies involving whole limb re-sponse to ischemia (4) during thermally-inducedsweating .

METHODS

Profuse localized sweating can be induced inhumans and a few animals (including the horse, andthe footpads of cats, rats, etc .), by introducingpilocarpine or acetylcholine intradermally . Analysisof sweat for sodium or chloride ions is a well-established technique for screening children forcystic fibrosis (CF), as the NaCl concentration in thesweat of CF children is significantly higher thannormal . Routine screening by introduction of pilo-carpine nitrate using iontophoresis is readily accom-plished with inexpensive commercial equipment . Inthis study, a sweat stimulator (Westcor Inc ., UT)employing pilocarpine-impregnated agar gel elec-trodes was used (Figure 1), and the sweat sampleswere collected (Figure 2) using a Macroduct dispos-able capillary tube system (Westcor Inc ., UT) . Sweatsamples were obtained from the forearm of maleand female able-bodied volunteers in the age range

Figure 2.Macroduct sweat collector .

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Journal of Rehabilitation Research and Development Vol . 25 No . 3 Summer 1988

Table 1.Comparison of lactate and sodium concentration insweat collected using pilocarpine iontophoresis andthermal induction . One subject repeated on 3 occasionsduring summer months .

Table 2(a) .

Sweat Volume

Collected

Collectedduring ischemia

after ischemia

Concentration ag/ml

Session #

Pilocarpine

Thermal

Lac . Na y Lac . Na +

1 0 .57 1 .37 0 .54 1 .41

2 0 .67 1 .33 0 .70 1 .32

3 0 .63 1 .06 0 .63 1 .08

1 male subject 36 yrs, summer.

of 20 to 40 years . A typical sweat yield for thesesubjects following 5 minutes of stimulation with acurrent density of 25nA/mm2 over an area of 500m 2was 30µl, when collected for 30 minutes.

Lactate levels in 5µ1 sweat samples were deter-mined using a routine lactate dehydrogenase assayavailable in kit form (Sigma Chemicals Procedure#826-UV) . Na + concentration was also measuredroutinely using an atomic absorption spectropho-tometer (Perkin-Elmer Model 306) . Sweat volumewas estimated by weighing the Macroduct beforeand after collection.

A preliminary pilot study was conducted todetermine whether Na' and lactate levels wereinfluenced by the pilocarpine method of sweatinduction . One subject was subjected to the follow-ing regimen on 3 separate occasions during thesummer months . Pilocarpine-induced sweat wascollected from one arm using the Macroduct systemfollowing careful cleaning with alcohol wipes anddistilled water . The area was carefully dried . Follow-ing routine sweat induction and sampling, thesubject was fitted with a Macroduct sweat collectoron the other (non-stimulated) arm, and was asked tojog in an environmental chamber at 35-40C and70-90 percent relative humidity (RH) for 30 min-utes . During this time, an adequate (20-30µl) sweatsample was collected for Na' and lactate analysis.

Two primary studies were undertaken to deter-mine the effects of ischemia on Na + and lactatelevels . In the first study, one forearm of eachsubject was subjected to continuous pressure of150mmHg for 30 minutes using a 25mm diameterpneumatically-controlled indentor . The applied

Ischemic

Control

Ischemic

Control

Mean

18 .0

36 .9

33 .2

39 .5

SD

15 .6

21 .7

15 .1

27 .6

p

<0 .05

NS

n

9

9

force was monitored, using a force transducer andrecorded on a chart recorder . Room temperaturewas maintained at 30C and 70 percent RH. Uponrelease, sweating was stimulated for 5 minutes andthen collected for a subsequent 30 minutes . In thesecond study, the indentor was adapted to accom-modate the sweat collection system (Figure 3).

Figure 3.Sweat collection during indentation.

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FERGUSON-PELL and HAGISAWA : Biochemical Changes in Sweat

Table 2(b) .

Table 2(c).

Sodium Concentration

Collected

Collectedduring ischemia

after ischemia

Ischemic

Control

Ischemic

Control

Mean

1 .36

1 .05

1 .19

1 .24

SD

0 .44

0 .31

0 .30

0 .31

p

NS

NS

n

6

8

Lactate Concentration

Collected

Collectedduring ischemia

after ischemia

Ischemic

Control

Ischemic

Control

Mean

0 .83

0 .60

0 .66

0 .68

SD

0 .13

0 .10

0 .10

0 .10

< 0 .001

NS

n

9

9

Sweating was induced before and collected duringindentation . In both studies, the subject's other armwas used as a control.

RESULTS

Table 1 compares the sweat volume, lactate andNa + concentrations for thermally-induced and pilo-carpine sweating. Although the number of testsperformed was too small for rigorous statisticalanalysis, the results indicate that for this subject, nolarge differences in Na + or lactate concentrationwere introduced by stimulating sweating using theiontophoresis technique.

Tables 2(a), 2(b), and 2(c) compare the sweatvolume, sodium concentration, and lactate concen-tration for samples obtained during and afterischemia against their respective controls . Theseresults were obtained for a total of 9 able-bodiedsubjects (4 male, 5 female) . Sweat volume (Table 2a)was significantly depressed (p<0 .05) for sweatcollected during ischemia, but did not significantlydiffer from the control when collected during thereactive hyperemia phase after ischemia . This effectis well-documented for whole limb ischemia, and isknown as the hemihidrotic effect (5).

Sodium concentration did not change significantlyfor the 2 test protocols . Statistically significantdifferences could not be detected between theischemic and control sites or between the 2 proto-cols, although there is some evidence for elevatedNa + levels in sweat from the ischemic site, whencollected during ischemia . Previous whole limbstudies (4) have identified significant differences inNa' concentrations comparing ischemic versus con-

trot sites . Large differences in Na t concentrationsbetween subjects contribute to the large standarddeviations obtained in this part of the study andthereby tend to mask differences seen for eachindividual.

Lactate concentration (Table 2c) was observed toincrease significantly (p< 0 .001) for the ischemicversus control site during ischemia, but did notdiffer significantly for the sweat samples collectedduring reactive hyperemia . This result comparesfavorably with those obtained in previous studies,and confirms that sweat biochemistry is very sensi-tive to tissue status . It should be noted, however,that the elevated levels of lactate in sweat are notthought to result from lactic acid transfer fromplasma or extracellular fluid to the sweat . The sweatgland itself appears to function as a metabolic unitthat reflects its oxygenation status through reducedoutput rate and increased lactate production . Previ-ous studies (4) suggest sweat lactate concentration isrelatively independent of blood lactate levels (ele-vated, for example, by muscle fatigue during exer-cise).

DISCUSSION

The results obtained in this study confirm thatpilocarpine stimulation of sweating can be usedeffectively to monitor local tissue biochemistry.Tests with a single subject suggest that the ionto-phoresis technique does not modify the levels oflactate and Na + when compared to normal profusesweating induced by high environmental tempera-ture coupled with exercise.

A previous study (4) for whole limb ischemia has

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Journal of Rehabilitation Research and Development Vol . 25 No. 3 Summer 1988

suggested that lactate levels during ischemia areattributable to changes in sweat gland metabolism,and that they are generally independent of capillaryblood lactate levels . In this study, we have con-firmed that similar results can be obtained by localsweat monitoring for locally applied ischemia . Ourresults demonstrate the sweat gland's capacity toreturn to normal lactate production as soon ascapillary blood oxygenation permits normal (aer-obic) metabolism. In this respect, these resultssuggest that, in principle, the sweat gland could beadopted as a physiological ischemia monitor if sweatcan be collected during the ischemic event.

Further research is needed to determine theclinical applicability of sweat chemistry for earlydetection of pressure sores . Additional parallel testsare required to demonstrate that the stimulationtechnique does not produce artifactual changes insweat composition . A collection system needs to bedeveloped that permits monitoring of sweat compo-sition at the loaded body support interface . Ideally,such a system could include an indicator for the

concentration or presence of biochemicals indicativeof tissue stress . However, basic research is requiredto identify pertinent biochemical factors that arereleased specifically in association with the earlyonset of pressure sores.

This study illustrates an approach to the evalua-tion of tissue responses during and following in-duced localized ischemia . If this approach proves tobe viable following more detailed investigation,monitoring sweat biochemistry may help to elimi-nate the technical and interpretation problems asso-ciated with interface pressure monitoring.

ACKNOWLEDGMENTS

The authors wish to express their appreciation forsupport provided by the New York State Department ofHealth, Veterans Administration Grant #B212-2RA, andKumamoto University, Japan . Dr. R . Vosburgh's assis-tance with biochemical techniques is also much appreci-ated.

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