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 Table of Contents  
Year : 2015  |  Volume : 1  |  Issue : 1  |  Page : 16-20

Determination of Electrical Conductivity of Cadaver Skeletal Muscle: A Promising Method for the Estimation of Late Postmortem Interval

Department of Forensic Medicine, Forensic Medicine Institute, Henan University of Science and Technology, Henan, China

Date of Web Publication29-May-2015

Correspondence Address:
Yaonan Mo
Forensic Medicine Institute, Henan University of Science and Technology, No. 31 Anhui Road, Jianxi District, Luoyang, Henan 471003
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Source of Support: Basic & Frontier Study of Technology Project of Henan Province (Grant No. 112300410082), the Doctor Foundation and the Youngs’ Foundation of Henan University of Science and Technology (Grant No. 09001309, 13000914 and 13000696).., Conflict of Interest: None

DOI: 10.4103/2349-5014.155554

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The electrical conductivity (EC) of extracted muscle fluid has been extensively used to evaluate meat freshness and shelf life in the field of food sanitation for decades. The opposite of freshness is the corruption that increases with time. Based on the freshness/corruption principle, we investigated the relationship between long postmortem intervals (PMIs) and EC in cadaver skeletal muscle. EC values of extracted fluid from rat muscles were measured at different PMIs for 10 days. The results indicate that there was a significant correlation between PMI and EC, and the data fit well to the cubic polynomial regression equation y = - 0.01x 3 + 0.264x 2 -13.657x + 1769.148 (R 2 = 0.925). In addition, the EC of different dilutions of these muscle extracts showed strict quadratic correlation (R 2 = 1) with the dilution ratios, suggesting that EC can be measured with very small quantities of muscle sample. Our study suggests that determination of the EC of cadaver skeletal muscle extracts may be a useful method for estimating long PMIs.

Keywords: Electrical conductivity, forensic science, muscle extraction liquid, muscle fluid extract, postmortem interval estimation

How to cite this article:
Xia Z, Zhai X, Liu B, Mo Y. Determination of Electrical Conductivity of Cadaver Skeletal Muscle: A Promising Method for the Estimation of Late Postmortem Interval. J Forensic Sci Med 2015;1:16-20

How to cite this URL:
Xia Z, Zhai X, Liu B, Mo Y. Determination of Electrical Conductivity of Cadaver Skeletal Muscle: A Promising Method for the Estimation of Late Postmortem Interval. J Forensic Sci Med [serial online] 2015 [cited 2023 Feb 7];1:16-20. Available from: https://www.jfsmonline.com/text.asp?2015/1/1/16/155554

  Introduction Top

Estimating the postmortem interval (PMI) is a major challenge for death investigations in forensic science. PMIs as used in forensic practice are usually based on postmortem changes, such as hypostasis, rigor mortis, rectal temperature, or different supravital reactions. [1] Although numerous studies have been conducted to develop more accurate methods, no single method can be used to accurately estimate PMIs, especially after long PMIs and in more extensively decomposed corpses. Due to the inherent limitations of preserving biological materials, there are still no practical and appropriate methods that can be used as prompt and accurate PMI estimators at crime scenes. [2]

Bodies undergo postmortem changes in several stages. The changes begin at the molecular level and sequentially progress to changes in microscopic and macroscopic morphologies. [3],[4] As bacteria grow in muscle tissues, their metabolic products, most of which are ionic, start to accumulate. [5] The decomposition of macromolecules such as proteins and DNA into smaller, charged molecules results in an effective increase in conductivity; thus, the electrical conductivity (EC) of cadaver muscle could provide a novel basis for estimating longer PMIs.

EC analysis is a rapid, relatively accurate, and economical method for estimating meat freshness for different kinds of animals and has been extensively used in the field of food sanitation for decades. [6],[7],[8] The opposite of freshness is the corruption that increases with time. However, few studies using EC have been reported in the field of forensic science. As there are great similarities between PMI estimation and meat shelflife evaluation which are both mainly dependent on degrees of protein decomposition caused by bacteria, we designed the present study in order to establish a method for estimating long PMIs.

  Materials and Methods Top

Forty-eight healthy Sprague-Dawley rats of either sex, weighing 260-380 g, were euthanized by cervical vertebrae dislocation. The cadavers were randomly divided into 16 groups of three rats each and kept at 19 ± 1°C. The muscles of the left lower hind limbs of the rats were removed at PMIs of 0 h, 4 h, 8 h, 12 h, 16 h, 20 h, 24 h, 48 h, 72 h, 96 h, 120 h, 144 h, 168 h, 192 h, 216 h, and 240 h. The muscles removed were immediately sent to the laboratory for EC testing.

The extracted muscle fluid was processed using the method described by Ekanem and Achinewhu, [9] with some modifications. Muscle in 5 g was homogenized with 50 mL of Mili-Q water and stirred while dynamic EC was determined in the liquid. The mixture was filtered until dynamic EC reached the maximum. The EC of extracted muscle fluid under the precisely controlled temperature of 25°C was measured with a conductivity meter (DDS-11A, Qiwei Instrument Co. Hangzhou, China). With PMI as the independent variable and EC at 25°C as the dependent variable, the data were evaluated by regression analysis ( SPSS version 13.0, IBM , Chicago, USA).

Temperature compensation coefficients (TCCs) were calculated for the same fluids at 20°C and 18°C using the formula α = (κt - κ25 )/[κ25 (t - 25)]. [10] TCCs in the groups at 20°C and 18°C were separately averaged and the results are indicated as α20 and α18 . All the EC values at 20°C and 18°C were compensated for using the formula κc = κt [1+ (25 - t)α] [10] ( Microsoft Office Excel 2007, Microsoft Corporation). Paired-sample t-tests were separately performed using EC values at 25°C and the compensated ones at 20°C and 18°C (SPSS version 13.0). The overlapping curves of all the uncompensated and compensated values at different PMIs were drawn using OriginLab OriginPro version 8 software (OriginLab Corporation, Massachusetts, USA).

Fluids extracted at 0 h, 120 h, and 240 h were separately diluted 5, 10, 30, 40, 50, 60, 70, 80, 90, and 100 times with ultrapure water and the EC was determined in each diluent. Regression curves between EC values and dilution ratios were drawn using OriginLab OriginPro version 8. With the dilution ratio as the independent variable and EC as the dependent variable, the data were evaluated by regression analysis (SPSS version 13.0).

  Results Top

EC values at 25°C at different PMIs

EC values at 25°C at different PMIs are shown in [Table 1]. The data were best fit to the cubic polynomial regression equation y = - 0.01x 3 + 0.264x 2 -13.657x + 1769.148 (R 2 = 0.925). The regression curve is shown in [Figure 1].
Figure 1: The regression curve between PMI and EC (25°C).

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Table 1: EC (25°C, 20°C, and 18°C) at Different PMIs

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EC values at 20°C and 18°C at different PMIs and TCCs

EC values at 20°C and 18°C are shown in [Table 2], and the EC values at 25°C are also included for comparison. These data show that EC values decreased as fluid temperature decreased. The averages of TCCs were α20 = 1.9912 percent per degree centigrade and α18 = 1.9791 percent per degree centigrade [Table 2]. For simplicity, the final TCC for all extracted fluids was determined to be α = 2.0 percent per degree centigrade.
Table 2: The TCCs

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Compensated EC values at 20°C and 18°C are shown in [Table 3]. The paired-sample t-test results showed no significant differences between EC at 25°C and the compensated EC at 20°C and 18°C. Overlapping curves show good agreement between the data after compensating for the EC values at 20°C and 18°C [Figure 2] and [Figure 3].
Figure 2: The curves between PMI and EC (25°C, 20°C, and 18°C) with no temperature compensation.

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Figure 3: The curves between PMI and EC (25°C, 20°C, and 18°C) with TCC 2 percent per degree centigrade.

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Table 3: Compensated EC (20°C and 18°C) at Different PMIs

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EC values of different dilution ratios

The experiments using different dilution ratios of the extracted muscle fluids indicated that the EC values decreased with increasing dilution factor [Table 4]. The curves between EC and different dilution ratios indicate that EC increased smoothly as the concentration of diluents increased [Figure 4]. Regression analysis revealed a strict quadratic correlation between relative concentration and EC values. The equations are y = - 208.19x 2 + 1096.694x + 5.402 (R 2 = 1), y = - 401.713x 2 + 1800.309x + 5.304 (R 2 = 1), and y = - 863.727x 2 + 2998.874x + 4.360 (R 2 = 1).
Figure 4: The tendency curves between EC and relative concentrations of multiple diluents.

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Table 4: EC of Different Concentrations of Multiple Diluents

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  Discussion Top

EC is used to measure how well a solution conducts electricity and has been widely employed in many fields such as food science, geology, [11],[12],[13] clinical medicine, [14],[15] and forensic environmental science. [16],[17] To the best of our knowledge, this is the first report showing the application of the EC of extracted muscle fluid to PMI estimation in forensic science.

Electrochemical studies have shown that the conductivity of an electrolyte solution is positively correlated with the temperature, with a significant increase by 1.5-5.0% per degree centigrade {Taghizade Mortezaee, 2014 #2}. To eliminate potential discrepancies caused by measuring EC at different temperatures, conductivity readings are commonly converted to the values at the same reference temperature, typically 25°C.

EC was used in our study because it reflects the total dissolved conductive substances from the cadaver muscle tissue. To simplify the study, EC values were measured with the extracted fluid maintained at 25°C. Our results show that EC values increased more significantly after a PMI of 24 h than within 24 h. For the first 24 h, the EC values showed no obvious change. However, EC started to increase slowly during the next 48 h, and rapidly from 72 h (day 3) to 192 h (day 8), after which it reached the peak value and plateaued. Rectal temperature has been considered a useful parameter for estimating short PMIs. Currently, there is no objective, accurate method for estimating long PMIs. Therefore, estimation of long PMIs in forensic practice is more challenging compared to short PMIs. We believe that measurement of the EC of cadaver skeletal muscle is likely to become a useful method for the estimation of long PMIs.

It is not only time-consuming but also unsatisfactory to control the temperature of the extracted fluids for prompt PMI estimation at the crime scene. Knowing the TCC makes it possible to determine EC at any temperature by merely adjusting the default TCC on conductivity meters; EC values in the present study were determined at 20°C and 18°C to acquire this important coefficient. Interestingly, the calculated coefficient was nearly identical to the default one (2 percent per degree centigrade) on the conductivity meter.

Estimating PMIs in forensic science is as important as evaluating meat freshness in food science. Some improvements in preparing the extracted muscle fluid were made on a method previously reported by Ekanem and Achinewhu. [9] Dynamic monitoring of EC was performed to save time and ensure that the conductive substances in the muscle were fully extracted in the water. In addition, to experimentally mimic situations in which PMIs are estimated using small amounts of cadaver tissue from bodies in more advanced stages of decomposition, we measured the EC values of different concentrations of diluted muscle extract. Our results indicate that EC can be determined in trace amounts of tissue sample and used to estimate PMI, if care is taken to maintain the precision of electronic EC meters.

EC has been mainly applied in the field of food sanitation to estimate the freshness and shelf life of meats such as fish, [6],[18] pork, [19],[20] beef, [21],[22] lamb, [23] and poultry, [24] . However, forensic PMI estimation involves multiple factors. Of these, environmental temperature has been identified as one of the most decisive factors in cadaver decomposition. [25] Further study is needed to measure the EC of human cadaver skeletal muscle to determine the correlation between the PMI and the EC at different temperatures.

  Conclusion Top

In conclusion, the determination of EC of cadaver skeletal muscle may be a useful method for the estimation of long PMIs.

  References Top

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  [Figure 1], [Figure 2], [Figure 3], [Figure 4]

  [Table 1], [Table 2], [Table 3], [Table 4]

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