The relationship between skin maturation and electrical skin impedance

Journal of Dermatological Science, 2 (1991) 336-340 0 1991 Elsevier Science Publishers B.V. All rights reserved 0923-1811/91/$03.50

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DESC 00102

The relationship between skin maturation and electrical skin impedance *Michelle M. Emery, ‘Adelaide A. Hebert, 2A. Aguirre Vila-Coro and 2Thomas C. Prager Departments of ‘Dermatology and 20phthalmology. Universityof Texas Medical School at Houston, Texas, U.S.A. (Received

Key words: Impedance;

Skin maturation;

1 June 1990; accepted 29 March 1991)

Electrophysiological

testing; Premature

neonates;

Full-term neonates

Abstract

When performing electrophysiological testing, high electrical impedance values are sometimes found in neonates. Since excessive impedance can invalidate test results, a study was conducted to delineate the relationship between skin maturation and electrical skin impedance. This study investigated the skin impedance in 72 infants ranging from 196 to 640 days of age from conception. Regression analyses demonstrated a significant relationship between impedance and age, with the highest impedance centered around full-term gestation with values falling precipitously at time points on either side. Clinically, impedance values fall to normal levels at approximately four months following full-term gestation, Skin impedance values are low in premature infants, but rapidly increase as the age approaches that of full-term neonates. Low impedance values in premature infants are attributed to greater skin hydration which results from immature skin conditions such as 1) thinner epidermal layers particularly at the transitional and corniced layers; 2) more blood flow to the skin; and 3) higher percentage of water composition. These factors facilitate the diffusion of water vapor through the skin. As the physical barrier to skin water loss matures with gestational age, the skin impedance reaches a maximum value at full term neonatal age. After this peak, a statistically significant inverse relationship exists between electrical skin impedance and age in the first year of life. This drop in skin impedance is attributed to an increase in skin hydration as a result of the greater functional maturity of eccrine sweat glands.

Introduction Electrophysiological testing in premature and full-term neonates has facilitated further understanding of the skin’s changing electrical properties during maturation. Electrical impedance, which is the resistance to an applied alternating electrical current, is the parameter used in determining the condition of the skin-electrode inter-

Correspondence to: Thomas C. Prager, Hermann Eye Center, 6411 Fannin Street, Houston, TX 77030, U.S.A.

face prior to tests such as the electroencephalogram (EEG), electrocardiogram (ECG), pattern shift visual evoked potential (PSVEP) and electromyogram (EMG). Since excessive electrical skin impedance, greater than 5 kQ, can invalidate electrophysiological test results, this study was conducted to delineate the temporal relationship between skin maturation and electrical skin impedance. It is important to establish the time period when clinical electrophysiology testing may result in muddled evoked potentials. We previously reported that high electrical skin impedance values

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found at full-term fell to normal levels by approximately four months post full-term delivery. [l] Regression analyses demonstrated a significant inverse relationship between skin impedance and age during the first year of life [ 11. This decline in skin impedance during the first few postnatal months was attributed to an increase in skin hydration as a result of the greater functional maturity of eccrine sweat glands [ 11. To further determine the temporal aspect of this transient elevation in electrical skin impedance [ 11, the current study investigated electrical skin changes in premature infants, full-term neonates and infants up to one year old. Thus, establishing a relationship between electrical skin property and age could serve as a quantitative indicator of skin development. A previous study by other researchers used electrical skin resistance as a method to evaluate skin maturity in premature infants [ 21. The major methodological flaw of this study was the measuring of skin resistance with direct electrical current. Skin resistance measurements that use direct electrical current may lead to the formation of polarization currents within the body, thereby masking the true electrical resistance of the tissues being measured [ 31. This difficulty can be overcome by using low voltage alternating current and measuring the opposition to current flow or impedance. Our study attempts to appropriately establish the relationship between skin maturation and conductivity of the skin using impedance, a more accurate reflection of electrical state than resistance.

impedance values at the forehead and occiput were recorded and all infants were evaluated at six hours or longer following birth. All premature and full-term neonates were born in Hermann Hospital, Houston, TX, U.S.A. The older infants were selected from pediatric patients from San Jose Clinic, Houston, on the basis of their age (less than one year). The investigation was conducted with parental consent, and the study’s protocol was approved by the Committee for the Protection of Human Subjects (University of Texas Medical School). Materials

A battery operated electrode impedance meter (Grass EZM 5 model, Quincy, MA) was used to test the impedance through surface electrodes. This meter is used in diagnostic procedures such as PSVEP, EEG, EMG and ECG. The digital readout can display an impedance range from 0.1 kS1to 200 kO with an accuracy of f 5 % [4]. The frequency of the meter is 30 Hz which approximates EEG and evoked potential (EP) frequency and simulates electrode impedance values under actual recording conditions [4]. Up to ten electrodes can be connected to a panel activated by an electrode selector switch. The meter records the skin impedance through the selected electrode, and the rest of the electrodes, connected in a parallel circuit, serve as a reference [4]. Proper calibration of the meter was checked prior to each measurement. An adjustment potentiometer allowed appropriate calibration changes [ 41. Procedure

Materials and Method Subjects

Subjects for skin impedance measurements consisted of 48 full-term children and 24 premature infants without dermatological or central nervous system disease. Gestational age was determined by the Dubowitz criteria [ 191 and history, where appropriate. Ages at time of impedance measurement ranged from 196 to 640 days from conception. Gestational age, and

Cephalic test sites consisted of active measuring electrodes on the occiput (OZ), and the forehead (International lo-20 System [ 51 for PSVEP testing). Data were accepted when the following criteria were met: 1) the environmental temperature did not exceed 34 ‘C; 2) axillary or rectal temperature was less than 38 degrees centigrade; 3) infants exhibiting crying and/or excessive

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motor activity were excluded from the study; These conditions were established to offset the effect of increased environmental temperature, core body temperature, emotional arousal and motor activity which produce generalized sweating resulting in artifactual reduction in skin impedance [6,7]. Infants exhibiting perspiration were eliminated from the study since their inclusion could have resulted in artificially low and thus inaccurate data. Slight motor activity (crying) may add more variability to the data resulting in a less powerful statistical outcome. The test sites were prepared with gentle skin abrasion using Omni Skin Prepping Paste (D.O. Weaver and Co., Auora, CO), which was free of chloride and acetone, to remove the superficial stratum comeum, surface lipids, glycosaminoglycans and loose debris. All infants had identical skin preparation. Thus the relative reduction of the superficial stratum comeum was expected to be the same at all gestational ages. The skin was abraded by using a horizontal rubbing motion 25 times while attempting to maintain a consistent surface pressure. After abrasion, a small area of pink tissue was exposed. A standard silver EP electrode (area = 1 cm”) was filled with Calcium Chloride-Free EC 2 Electrode Cream (Grass Instrument Co., Quincy, MA). The electrode was then pressed firmly onto the prepared test site and held in place with paper tape. Once the skin-electrode interface was obtained and instrument calibration established, a stabilized impedance measurement through each electrode was taken as datum. Results Skin impedance was recorded from 72 infants ranging in age from 196 days to 640 days from conception with a mean value of 350 days (116 SD). The cephalic electrode recording sites were the occiput (OZ), and the forehead. The average impedance value was 4.9 ka (3.7 SD) across all time points. Fig. 1 depicts a scattergram of occiput impedance data and two linear re-

Correlation Betweeen Age and Electrical Impedance - Occiput

2oT

0 200

I

250

300

350 400 450 AGE from Conception

500 (days)

550

600

650

Fig. 1. Scattergram of electrical impedance recorded from the occiput as a function of age from conception. A linear regression line curve has been determined for conception to full-term and from full-term to 650 days post-conception.

gression lines. A line of best fit was determined from conception to full-term (r = 0.78, F = 45.74, P c 0.0001) and from full-term to 650 days from conception (r = -0.37, F = 7.07, P = 0.01). Note that the skin impedance values were highest at full-term gestation (approximately 275 days postconception) and were lower at ages on either side of full-term gestation. The fourth order correlation between age and impedance at the occiput was 0.46 with a significant goodness of fit (F = 5.52, P = 0.02). Similar fourth order regression values were found for the forehead recording site (r = 0.47, goodness of lit F = 7.09, P = 0.01).

Discussion In this study we found the highest impedance to be centered around full-term gestation with impedance values reduced at time points on either side of full-term gestation. The distribution presents with a positive slope as full-term gestation is approached and a negative slope following full-term gestation. Skin impedance, the electrical opposition to an alternating current, appears to be a function of changes in skin hydration which fluctuate with age. A transient increase in skin impedance which we observed to occur around full-term gestation

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is attributed to the functional immaturity of sweat glands [ 11. Sweat glands are structures which alter electrical impedance by contributing to skin hydration [8,9]. These skin structures are fully developed anatomically after the 28th week of gestation [lo]. However, only a small fraction of these sweat glands are functionally mature with secretory activity at full-term (38th-40th week) [lo]. Clinically, impedance values fall to normal adult levels at approximately four months following full-term gestation, secondary to the greater functional maturity of eccrine sweat glands [l]. In view of the relative lack of sweating capacity in premature infants the fmding of low impedance values, especially in infants 132 weeks of gestation, must be attributed to sources other than eccrine sweat activity [ 111. Low impedance values in premature infants is due to greater skin hydration which results from immature skin factors such as: 1) thinner epidermal layers particularly at the transitional and comified layers [ 121; 2) more blood flow to the skin [ 13,141; and 3) higher percentage of water composition [ 141. These factors facilitate the diffusion of water vapor through the skin. This diffusion of water to the epidermal surface constitutes an Insensible Water Loss (IWL), which also serves as a major route of heat transfer from the skin [ 151. To test the above assumptions, Wilson and Maibach have used a Mecca electrolytic water analyzer to measure the Transepidermal Water Loss (TEWL) of local skin sites [ 161. Results from the Wilson and Maibach study demonstrated a relationship between increasing TEWL with decreasing gestational age [ 161. Thus, because of the greater percentage of water composition and IWL from the skin of premature infants, skin impedance should decrease with the higher levels of hydration. However, since the physical barrier to the diffusion of water vapor becomes more resistant to IWL with advancing gestational age, it was found that premature skin impedance rapidly increased as age approached that of fullterm neonates. It is well established that epidermal stratum

comeum greatly contributes to the impedance of skin [ 3,171. Since histologically apparent stratum comeum develops in fetal skin by 25 weeks of gestation [ 181,the role of the stratum comeum on impedance in premature and full term infants must be considered. It has been found that impedance loci formed by the upper keratin layers of the stratum comeum have the highest impedance value [ 171. As keratin layers are removed by cellulose tape stripping or skin abrasion the impedance values decrease [ 171. Since similar skin preparation techniques were used in all patients we feel that the alteration of the stratum comeum was relatively the same for all subjects. By minimizing the contribution of the insulating properties of the superficial layer of the stratum comeum we noted a significantly lower skin impedance in premature infants and in infants greater than four months as compared to infants at full term gestational age. In summary, linear and polynomial regression analyses demonstrated a statistically significant inverse relationship between skin impedance and age from the full-term period to the first year of life. On the other hand, the correlation between premature skin impedance and age is positive as full-term gestation is approached. These results demonstrate a significant relationship between impedance and age, with peak impedance centered around full-term gestation. This distribution of skin impedance over time allows a better understanding of the electrical properties of premature and neonatal skin. These findings may be of clinical use to the electrophysiologist when determining the condition of the skin-electrode interface prior to EEG, PSVEP, ECG or EMG testing in premature and full-term neonates.

Acknowledgement

We wish to thank The Vale-Asche Foundation for supporting this research.

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