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An experimental evaluation of electrical skin conductivity changes in postmortem interval and its assessment for time of death estimation
Computers in Biology and Medicine ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Computers in Biology and Medicine journal homepage: www.elsevier.com/locate/cbm
An experimental evaluation of electrical skin conductivity changes in postmortem interval and its assessment for time of death estimation İsmail Cantürk a,n, Fethullah Karabiber b, Safa Çelik c, M. Feyzi Şahin c, Fatih Yağmur d, Sadık Kara e a
Electronics and Communications Engineering Department, Yıldız Technical University, İstanbul, Turkey Computer Engineering Department, Yıldız Technical University, İstanbul, Turkey c Council of Forensic Medicine, İstanbul, Turkey d Faculty of Medicine, İstanbul Medeniyet University, İstanbul, Turkey e Institute of Biomedical Engineering, Fatih University, İstanbul, Turkey b
art ic l e i nf o
a b s t r a c t
Article history: Received 12 August 2015 Received in revised form 12 December 2015 Accepted 15 December 2015
In forensic medicine, estimation of the time of death (ToD) is one of the most important and challenging medico-legal problems. Despite the partial accomplishments in ToD estimations to date, the error margin of ToD estimation is still too large. In this study, electrical conductivity changes were experimentally investigated in the postmortem interval in human cases. Electrical conductivity measurements give some promising clues about the postmortem interval. A living human has a natural electrical conductivity; in the postmortem interval, intracellular fluids gradually leak out of cells. These leaked fluids combine with extra-cellular fluids in tissues and since both fluids are electrolytic, intracellular fluids help increase conductivity. Thus, the level of electrical conductivity is expected to increase with increased time after death. In this study, electrical conductivity tests were applied for six hours. The electrical conductivity of the cases exponentially increased during the tested time period, indicating a positive relationship between electrical conductivity and the postmortem interval. & 2015 Published by Elsevier Ltd.
Keywords: Time of death estimation Forensic medicine Postmortem interval Electrical conductivity Body mass index
1. Introduction Time of death (ToD) estimation is one of the most critical concepts in forensic medicine, and ToD studies are ongoing worldwide. ToD estimation is used to predict the elapsed time after death using different physical and chemical techniques. One of the legal criteria for ToD determination is the availability of witnesses who witnessed the death. If there are no witnesses, or witness declarations must be verified, scientific methods are employed to reveal the ToD. ToD is crucial from various perspectives, especially in the criminal justice system. Precise ToD determination helps identify perpetrators and decreases the number of suspects. In some cases, ToD determination may be essential to verify witnesses' declarations. ToD can clarify whether suspects were at the site of the murder or not. Additionally, death order, even if different by only a few seconds, is important for determining inheritance in cases
n Correspondence to: Elektronik ve Hab. Müh. Böl. Yıldız Teknik Üniversitesi Davutpaşa Mah., Davutpaşa Cad. 34220 Esenler İstanbul/TURKEY. Tel.: þ90 (212) 383 59 25. E-mail address: [email protected] (İ. Cantürk).
where the corpses of two or more related persons are found at the same time [1]. In the literature, various physical and chemical techniques have been proposed to estimate ToD within some error margins. Some of the physical techniques are algor mortis [1–12], rigor mortis [13–17], supravital reactions [18–20], livor mortis [21–23], and postmortem decomposition [24–27]. For some chemical methods [28–32], body fluids (e.g., blood, vitreous humour) are taken and changes in the electrolytes are evaluated with respect to time [33]. Scientists have fit curves to the data obtained from these physical and chemical methods to try to predict ToD [34]. Since ToD estimation is challenging and obtaining real data is difficult, ToD estimation has not been adequately studied in the literature. The results of previous studies based on conventional methods are not satisfactory [1], and there is a need to develop new techniques to more accurately estimate ToD. The physical methods are not easy to evaluate quantitatively; natural disintegration of the body can be easily altered by environmental factors, like humidity and temperature. Additionally, since chemical methods are based on taking body fluids multiple times, natural postmortem progress is likely to be disrupted. All these methods need significant amounts of time and experienced researchers.
http://dx.doi.org/10.1016/j.compbiomed.2015.12.010 0010-4825/& 2015 Published by Elsevier Ltd.
Please cite this article as: İ. Cantürk, et al., An experimental evaluation of electrical skin conductivity changes in postmortem interval and its assessment for time of death estimation, Comput. Biol. Med. (2015), http://dx.doi.org/10.1016/j.compbiomed.2015.12.010i
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In this study, changes in electrical conductivity were experimentally investigated in the postmortem interval in human cases. Electrical conductivity measurements give some promising clues about the postmortem interval. An alive human has natural electrical conductivity; however, in the postmortem interval, intracellular fluids gradually leak out of cells. These leaked fluids combine with extracellular fluids in tissues, and since both of these fluids are electrolytic, this mixing of intracellular and extracellular fluids helps to increase conductivity and thus the level of electrical conductivity is expected to increase after death. In this study, a clinical electrical conductivity test was applied to corpses that were within the early postmortem period (24 h after death) at the Istanbul Council of Forensic Medicine (ICFM). The rest of the paper is organized as follows. The experimental procedure and information about the cases are discussed in the Materials and Methods section. The experimental results are detailed in Section 3 and discussed in Section 4. The paper is concluded in Section 5.
2. Materials and methods 2.1. Materials Cases transferred to ICFM were kept in a cold room (5 °C). Corpses received during working hours were taken to autopsy. The ones received after working hours were kept in the cold room until the next day. The cases that came after working hours comprised our experimental group because only the corpses received after working hours stay overnight at the institution. Electrical conductivity was measured in 32 cases. In order to have the same environmental conditions, we selected only the corpses that passed away between the times of 14:00–15:00 in July and August in Istanbul, when the weather conditions and average temperatures are very similar; cases that passed away in other months were eliminated from the research group. Additionally, we only examined cases that were under the same weather conditions (27 °C temperature, sunny days). Therefore, the study group consisted of cases that died under identical environmental and temperature conditions. Additionally, corpses with edema were excluded. The measurements were started at 16:30 and finished at 22:30 for all cases. Having the same environmental factors among the cases is important because environmental factors may affect electrical conductivity. According to the case selection criteria, 11 cases that satisfied the conditions were included in the research group; information about these 11 cases is given in Table 1. The stimulating voltage level and temperature of the room were assumed to be fixed during the experiment. Finally, eiet and fluid intake prior to death may have a small influence on conductivity; since this situation of cases prior to death was unknown, it was assumed to be the same in all cases. 2.2. Methods A computer-aided automation system (see Fig. 1) was designed to measure electrical conductivity changes. The Biopac MP150 data acquisition unit was utilized to record the signals. Electrical stimulation signals were generated by an isolated linear stimulator. The system was automated to produce rectangular pulse signals every 15 min for six hours. The stimulating signals had a magnitude of 10 V and duration of 1 ms because short pulses are generally preferred to long pulses in clinical tests [35]. The polarity of the stimulator and recording electrodes were arranged as shown in Fig. 1. Positive electrodes were placed closer to each other than the negative electrodes were. In this system,
Table 1 Information about the cases.
Case Case Case Case Case Case Case Case Case Case Case
1 2 3 4 5 6 7 8 9 10 11
Gender
Age
Height (cm)
Weight (kg)
Cause of death
Male Male Male Male Male Male Male Female Male Male Female
22 50 42 29 30 27 55 38 25 44 47
180 176 165 166 174 172 170 166 175 175 168
70 60 80 55 80 58 65 59 66 56 57
Drugs Fall from height Remaining under rubble Drugs Car accident Drugs Hanging Stab wounds Drugs Alcohol Car accident
stimulations were delivered above the elbow. Response signals were recorded 25 cm away from the stimulating electrodes. To determine the distance, the mid-points of the recording and stimulating electrodes were taken as reference points. Needle electrodes (0.3 mm in diameter) were used for stimulation after being inserted 1.5 cm through the skin, and surface electrodes were utilized for recording. Surface electrodes were pre-gelled single Ag/AgCl electrode conductors, 11 mm in diameter, with a 95 mm2 conductive contact area. Since stimulation was delivered by needle electrodes under the skin, this experiment can also be called a tissue conductivity measurement. After the response signals were perceived by the recording electrodes, they went into a signal amplification unit. Since these signals had low magnitudes, they were amplified in this unit. Response signals were automatically recorded and monitored on a computer. Fig. 2 demonstrates the utilized rectangular pulse signal generated by the stimulator and the response signal, which was recorded by the recording unit. The stimulating signal had one phase of a rectangular wave. The response signal was biphasic, i.e., it had parts on both the positive and negative cycles.
3. Results The results of 11 cases, depicted in Fig. 3, demonstrate that as the time in the postmortem interval increases, tissue conductivity continues to increase. As mentioned in Section 2, the response signal has portions in both the positive and negative cycles. As the postmortem interval increases, the amplitude of the response signal continues to go up in both cycles, and thus we calculated the integrals of the cycles for each case and for each measurement. In Fig. 3, the X-axis is elapsed time (h) after we started the measurements. The Y-axis shows the integrals of the signals (mV s). As shown in Fig. 3, there is a rapid change in electrical tissue conductivity for the initial parts of the graphics. Afterward, this rapid change slows down for the latter parts of the time axis. The graphics do not start from a zero point because a living human has natural electrical conductivity. After death, the level of electrical conductivity starts to go up gradually. During the experimental time, the conductivity levels of the cases increased. Since we took periodic measurements at discrete times, Fig. 3 denotes the measurement values that were taken at those times; increments in the graphics for all cases were continual but their change percentages were different. Electrical conductivity tests were applied for six hours; the increases in conductivity for the first three hours were greater than last three hours. Although the average percentage change in conductivity was 15.2% at the end of one hour with respect to the initial conductivity levels, it reduced to almost 4% after five hours. Fig. 3 also depicts 11 cases with fitted curves. Measurements were conducted every 15 min across six hours; therefore, the number of measured points for each case was 25. Polynomial
Please cite this article as: İ. Cantürk, et al., An experimental evaluation of electrical skin conductivity changes in postmortem interval and its assessment for time of death estimation, Comput. Biol. Med. (2015), http://dx.doi.org/10.1016/j.compbiomed.2015.12.010i
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Fig. 1. The system for recording electrical conductivity signals.
Fig. 2. Stimulating rectangular pulse signal and biphasic response signal.
curve fitting was applied to connect the discrete data points; the order of fitting was selected based on the curve’s ability to cover the data points of the cases. For the curve to follow the trends of the data points, fifth-order polynomial curve fitting was suitable for all the cases because fifth-order polynomial curves followed the trends of the data points and represented the data points better than other orders; smaller orders led to more linear curves, which did not cover the data points, and higher orders produced curves that did not follow the trends of the data points. Fig. 4 gives insight about the correlations between baseline conductivity level, body mass index (BMI-the body mass divided by the square of the body height, expressed in units of kg/m2), and age. The baseline conductivity level defines the initial conductivity level differences of the cases when the measurements were started. BMI was significantly negatively correlated with the baseline conductivity level (r ¼ 0.76); larger BMI values led to smaller baseline conductivity levels. As shown in Fig. 4(b), the correlation between age and baseline conductivity level was very low (r ¼ 0.07) and therefore age may not be related to the baseline conductivity level. 4. Discussion Electrolyte levels in tissues change in the postmortem interval. Cellular membrane integrity is impaired after death; due to natural
disintegration processes, the cellular membrane stops working and cell permeability is disrupted. Therefore, intracellular fluids start to leak out of cells and combine with extracellular fluids in tissues; these leaking electrolytic fluids increase conductivity by increasing the number of ions in the tissues over time. Ions increase electrical conductivity by lowering electrical impedance, and lower electrical impedance means that more electrons can be carried at a time. This decrease in impedance level leads to higher currents and higher conductivity. The impedance of a system can be described as its ability to oppose electrical current; if the impedance is high, a small current flows into a system, and otherwise a high-amplitude current is generated. Here, since fluids leaking out of cells lower the impedance of the cases, electrical conductivity should increase as the postmortem interval increases [28–32,36], which is supported by our findings here. Electrical conductivity measurements seem to be a potential method to estimate ToD. Statistical analysis of 11 cases proved that differences in baseline conductivities were negatively related to BMI (r¼ 0.76) but not age (r¼0.07). It can be said that the BMI of a human is proportional to the amount of adipose tissue within the body and thus that high BMI values indicate significant amounts of adipose tissue. High BMI values lead to low baseline conductivities because adipose tissue has low conductance. Our results prove that a higher amount of adipose tissue leads to lower baseline conductivity.
Please cite this article as: İ. Cantürk, et al., An experimental evaluation of electrical skin conductivity changes in postmortem interval and its assessment for time of death estimation, Comput. Biol. Med. (2015), http://dx.doi.org/10.1016/j.compbiomed.2015.12.010i
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Fig. 3. Electrical conductivity changes of the cases and 5th order polynomial curve fitting.
Fig. 4. (a) Correlation between BMI and baseline conductivity levels, (b) correlation between age and baseline conductivity levels.
Different layers of the upper extremity have different properties. The conductivity of the adipose tissue is lower than the other tissues in the upper extremity. In this study, needle electrodes were used for stimulation and inserted 1.5 cm through skin, thus delivering stimulation below the adipose tissue. Although adipose tissue has low conductivity, it also decomposes in the disintegration
process. Since surface electrodes were utilized for recording, we recorded the total tissue response. Electrical stimulations were given above the elbow and the response signals were recorded 25 cm away from the stimulating electrodes. The recording point was close to the wrist. Longer and shorter distances than 25 cm may alter conductivity; longer
Please cite this article as: İ. Cantürk, et al., An experimental evaluation of electrical skin conductivity changes in postmortem interval and its assessment for time of death estimation, Comput. Biol. Med. (2015), http://dx.doi.org/10.1016/j.compbiomed.2015.12.010i
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distances might result increase conductivity and shorter distances might decrease conductivity. There were some limitations to our study. We were not able to start measurements just after death; the transfer of a corpse to the institution takes almost two hours. Therefore, experiments started, approximately, in the first two hours after death. In addition to that, the number of cases was limited due to not having enough cases that satisfied our criteria.
5. Conclusion A clinical test was designed to explore the relationship between electrical conductivity and postmortem interval for human cases. The obtained results indicate that there is a positive relationship between electrical conductivity and postmortem interval, i.e., conductivity goes up as the postmortem interval increases. In this study, we chose cases that had identical environmental conditions and lacked edema. In addition, BMI affected baseline conductivity but age did not. A novel contribution of this paper is the investigation of electrical tissue conductivity changes in the postmortem interval in human cases, which has not been previously studied. The experiments in this paper showed a positive correlation between conductivity and increased postmortem intervals. Further studies are needed to verify that electrical conductivity is a valid tool for ToD estimation and to study the effects of different environmental factors on electrical conductivity in postmortem human cases.
Conflict of interest None Declared.
Acknowledgments This study was approved by İstanbul Medeniyet University, Clinical Researches Ethic Commission under decision no: 2014/ 0055 and supported from Fatih University under Project no: P58011401-B. This study was conducted at Istanbul Council of Forensic Medicine under decision no B.03.1.ATK.0.01.00.08/96. Additionally, we applied for a patent for this method (2015/03140).
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Please cite this article as: İ. Cantürk, et al., An experimental evaluation of electrical skin conductivity changes in postmortem interval and its assessment for time of death estimation, Comput. Biol. Med. (2015), http://dx.doi.org/10.1016/j.compbiomed.2015.12.010i
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Contents lists available at ScienceDirect
Computers in Biology and Medicine journal homepage: www.elsevier.com/locate/cbm
An experimental evaluation of electrical skin conductivity changes in postmortem interval and its assessment for time of death estimation İsmail Cantürk a,n, Fethullah Karabiber b, Safa Çelik c, M. Feyzi Şahin c, Fatih Yağmur d, Sadık Kara e a
Electronics and Communications Engineering Department, Yıldız Technical University, İstanbul, Turkey Computer Engineering Department, Yıldız Technical University, İstanbul, Turkey c Council of Forensic Medicine, İstanbul, Turkey d Faculty of Medicine, İstanbul Medeniyet University, İstanbul, Turkey e Institute of Biomedical Engineering, Fatih University, İstanbul, Turkey b
art ic l e i nf o
a b s t r a c t
Article history: Received 12 August 2015 Received in revised form 12 December 2015 Accepted 15 December 2015
In forensic medicine, estimation of the time of death (ToD) is one of the most important and challenging medico-legal problems. Despite the partial accomplishments in ToD estimations to date, the error margin of ToD estimation is still too large. In this study, electrical conductivity changes were experimentally investigated in the postmortem interval in human cases. Electrical conductivity measurements give some promising clues about the postmortem interval. A living human has a natural electrical conductivity; in the postmortem interval, intracellular fluids gradually leak out of cells. These leaked fluids combine with extra-cellular fluids in tissues and since both fluids are electrolytic, intracellular fluids help increase conductivity. Thus, the level of electrical conductivity is expected to increase with increased time after death. In this study, electrical conductivity tests were applied for six hours. The electrical conductivity of the cases exponentially increased during the tested time period, indicating a positive relationship between electrical conductivity and the postmortem interval. & 2015 Published by Elsevier Ltd.
Keywords: Time of death estimation Forensic medicine Postmortem interval Electrical conductivity Body mass index
1. Introduction Time of death (ToD) estimation is one of the most critical concepts in forensic medicine, and ToD studies are ongoing worldwide. ToD estimation is used to predict the elapsed time after death using different physical and chemical techniques. One of the legal criteria for ToD determination is the availability of witnesses who witnessed the death. If there are no witnesses, or witness declarations must be verified, scientific methods are employed to reveal the ToD. ToD is crucial from various perspectives, especially in the criminal justice system. Precise ToD determination helps identify perpetrators and decreases the number of suspects. In some cases, ToD determination may be essential to verify witnesses' declarations. ToD can clarify whether suspects were at the site of the murder or not. Additionally, death order, even if different by only a few seconds, is important for determining inheritance in cases
n Correspondence to: Elektronik ve Hab. Müh. Böl. Yıldız Teknik Üniversitesi Davutpaşa Mah., Davutpaşa Cad. 34220 Esenler İstanbul/TURKEY. Tel.: þ90 (212) 383 59 25. E-mail address: [email protected] (İ. Cantürk).
where the corpses of two or more related persons are found at the same time [1]. In the literature, various physical and chemical techniques have been proposed to estimate ToD within some error margins. Some of the physical techniques are algor mortis [1–12], rigor mortis [13–17], supravital reactions [18–20], livor mortis [21–23], and postmortem decomposition [24–27]. For some chemical methods [28–32], body fluids (e.g., blood, vitreous humour) are taken and changes in the electrolytes are evaluated with respect to time [33]. Scientists have fit curves to the data obtained from these physical and chemical methods to try to predict ToD [34]. Since ToD estimation is challenging and obtaining real data is difficult, ToD estimation has not been adequately studied in the literature. The results of previous studies based on conventional methods are not satisfactory [1], and there is a need to develop new techniques to more accurately estimate ToD. The physical methods are not easy to evaluate quantitatively; natural disintegration of the body can be easily altered by environmental factors, like humidity and temperature. Additionally, since chemical methods are based on taking body fluids multiple times, natural postmortem progress is likely to be disrupted. All these methods need significant amounts of time and experienced researchers.
http://dx.doi.org/10.1016/j.compbiomed.2015.12.010 0010-4825/& 2015 Published by Elsevier Ltd.
Please cite this article as: İ. Cantürk, et al., An experimental evaluation of electrical skin conductivity changes in postmortem interval and its assessment for time of death estimation, Comput. Biol. Med. (2015), http://dx.doi.org/10.1016/j.compbiomed.2015.12.010i
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In this study, changes in electrical conductivity were experimentally investigated in the postmortem interval in human cases. Electrical conductivity measurements give some promising clues about the postmortem interval. An alive human has natural electrical conductivity; however, in the postmortem interval, intracellular fluids gradually leak out of cells. These leaked fluids combine with extracellular fluids in tissues, and since both of these fluids are electrolytic, this mixing of intracellular and extracellular fluids helps to increase conductivity and thus the level of electrical conductivity is expected to increase after death. In this study, a clinical electrical conductivity test was applied to corpses that were within the early postmortem period (24 h after death) at the Istanbul Council of Forensic Medicine (ICFM). The rest of the paper is organized as follows. The experimental procedure and information about the cases are discussed in the Materials and Methods section. The experimental results are detailed in Section 3 and discussed in Section 4. The paper is concluded in Section 5.
2. Materials and methods 2.1. Materials Cases transferred to ICFM were kept in a cold room (5 °C). Corpses received during working hours were taken to autopsy. The ones received after working hours were kept in the cold room until the next day. The cases that came after working hours comprised our experimental group because only the corpses received after working hours stay overnight at the institution. Electrical conductivity was measured in 32 cases. In order to have the same environmental conditions, we selected only the corpses that passed away between the times of 14:00–15:00 in July and August in Istanbul, when the weather conditions and average temperatures are very similar; cases that passed away in other months were eliminated from the research group. Additionally, we only examined cases that were under the same weather conditions (27 °C temperature, sunny days). Therefore, the study group consisted of cases that died under identical environmental and temperature conditions. Additionally, corpses with edema were excluded. The measurements were started at 16:30 and finished at 22:30 for all cases. Having the same environmental factors among the cases is important because environmental factors may affect electrical conductivity. According to the case selection criteria, 11 cases that satisfied the conditions were included in the research group; information about these 11 cases is given in Table 1. The stimulating voltage level and temperature of the room were assumed to be fixed during the experiment. Finally, eiet and fluid intake prior to death may have a small influence on conductivity; since this situation of cases prior to death was unknown, it was assumed to be the same in all cases. 2.2. Methods A computer-aided automation system (see Fig. 1) was designed to measure electrical conductivity changes. The Biopac MP150 data acquisition unit was utilized to record the signals. Electrical stimulation signals were generated by an isolated linear stimulator. The system was automated to produce rectangular pulse signals every 15 min for six hours. The stimulating signals had a magnitude of 10 V and duration of 1 ms because short pulses are generally preferred to long pulses in clinical tests [35]. The polarity of the stimulator and recording electrodes were arranged as shown in Fig. 1. Positive electrodes were placed closer to each other than the negative electrodes were. In this system,
Table 1 Information about the cases.
Case Case Case Case Case Case Case Case Case Case Case
1 2 3 4 5 6 7 8 9 10 11
Gender
Age
Height (cm)
Weight (kg)
Cause of death
Male Male Male Male Male Male Male Female Male Male Female
22 50 42 29 30 27 55 38 25 44 47
180 176 165 166 174 172 170 166 175 175 168
70 60 80 55 80 58 65 59 66 56 57
Drugs Fall from height Remaining under rubble Drugs Car accident Drugs Hanging Stab wounds Drugs Alcohol Car accident
stimulations were delivered above the elbow. Response signals were recorded 25 cm away from the stimulating electrodes. To determine the distance, the mid-points of the recording and stimulating electrodes were taken as reference points. Needle electrodes (0.3 mm in diameter) were used for stimulation after being inserted 1.5 cm through the skin, and surface electrodes were utilized for recording. Surface electrodes were pre-gelled single Ag/AgCl electrode conductors, 11 mm in diameter, with a 95 mm2 conductive contact area. Since stimulation was delivered by needle electrodes under the skin, this experiment can also be called a tissue conductivity measurement. After the response signals were perceived by the recording electrodes, they went into a signal amplification unit. Since these signals had low magnitudes, they were amplified in this unit. Response signals were automatically recorded and monitored on a computer. Fig. 2 demonstrates the utilized rectangular pulse signal generated by the stimulator and the response signal, which was recorded by the recording unit. The stimulating signal had one phase of a rectangular wave. The response signal was biphasic, i.e., it had parts on both the positive and negative cycles.
3. Results The results of 11 cases, depicted in Fig. 3, demonstrate that as the time in the postmortem interval increases, tissue conductivity continues to increase. As mentioned in Section 2, the response signal has portions in both the positive and negative cycles. As the postmortem interval increases, the amplitude of the response signal continues to go up in both cycles, and thus we calculated the integrals of the cycles for each case and for each measurement. In Fig. 3, the X-axis is elapsed time (h) after we started the measurements. The Y-axis shows the integrals of the signals (mV s). As shown in Fig. 3, there is a rapid change in electrical tissue conductivity for the initial parts of the graphics. Afterward, this rapid change slows down for the latter parts of the time axis. The graphics do not start from a zero point because a living human has natural electrical conductivity. After death, the level of electrical conductivity starts to go up gradually. During the experimental time, the conductivity levels of the cases increased. Since we took periodic measurements at discrete times, Fig. 3 denotes the measurement values that were taken at those times; increments in the graphics for all cases were continual but their change percentages were different. Electrical conductivity tests were applied for six hours; the increases in conductivity for the first three hours were greater than last three hours. Although the average percentage change in conductivity was 15.2% at the end of one hour with respect to the initial conductivity levels, it reduced to almost 4% after five hours. Fig. 3 also depicts 11 cases with fitted curves. Measurements were conducted every 15 min across six hours; therefore, the number of measured points for each case was 25. Polynomial
Please cite this article as: İ. Cantürk, et al., An experimental evaluation of electrical skin conductivity changes in postmortem interval and its assessment for time of death estimation, Comput. Biol. Med. (2015), http://dx.doi.org/10.1016/j.compbiomed.2015.12.010i
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Fig. 1. The system for recording electrical conductivity signals.
Fig. 2. Stimulating rectangular pulse signal and biphasic response signal.
curve fitting was applied to connect the discrete data points; the order of fitting was selected based on the curve’s ability to cover the data points of the cases. For the curve to follow the trends of the data points, fifth-order polynomial curve fitting was suitable for all the cases because fifth-order polynomial curves followed the trends of the data points and represented the data points better than other orders; smaller orders led to more linear curves, which did not cover the data points, and higher orders produced curves that did not follow the trends of the data points. Fig. 4 gives insight about the correlations between baseline conductivity level, body mass index (BMI-the body mass divided by the square of the body height, expressed in units of kg/m2), and age. The baseline conductivity level defines the initial conductivity level differences of the cases when the measurements were started. BMI was significantly negatively correlated with the baseline conductivity level (r ¼ 0.76); larger BMI values led to smaller baseline conductivity levels. As shown in Fig. 4(b), the correlation between age and baseline conductivity level was very low (r ¼ 0.07) and therefore age may not be related to the baseline conductivity level. 4. Discussion Electrolyte levels in tissues change in the postmortem interval. Cellular membrane integrity is impaired after death; due to natural
disintegration processes, the cellular membrane stops working and cell permeability is disrupted. Therefore, intracellular fluids start to leak out of cells and combine with extracellular fluids in tissues; these leaking electrolytic fluids increase conductivity by increasing the number of ions in the tissues over time. Ions increase electrical conductivity by lowering electrical impedance, and lower electrical impedance means that more electrons can be carried at a time. This decrease in impedance level leads to higher currents and higher conductivity. The impedance of a system can be described as its ability to oppose electrical current; if the impedance is high, a small current flows into a system, and otherwise a high-amplitude current is generated. Here, since fluids leaking out of cells lower the impedance of the cases, electrical conductivity should increase as the postmortem interval increases [28–32,36], which is supported by our findings here. Electrical conductivity measurements seem to be a potential method to estimate ToD. Statistical analysis of 11 cases proved that differences in baseline conductivities were negatively related to BMI (r¼ 0.76) but not age (r¼0.07). It can be said that the BMI of a human is proportional to the amount of adipose tissue within the body and thus that high BMI values indicate significant amounts of adipose tissue. High BMI values lead to low baseline conductivities because adipose tissue has low conductance. Our results prove that a higher amount of adipose tissue leads to lower baseline conductivity.
Please cite this article as: İ. Cantürk, et al., An experimental evaluation of electrical skin conductivity changes in postmortem interval and its assessment for time of death estimation, Comput. Biol. Med. (2015), http://dx.doi.org/10.1016/j.compbiomed.2015.12.010i
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Fig. 3. Electrical conductivity changes of the cases and 5th order polynomial curve fitting.
Fig. 4. (a) Correlation between BMI and baseline conductivity levels, (b) correlation between age and baseline conductivity levels.
Different layers of the upper extremity have different properties. The conductivity of the adipose tissue is lower than the other tissues in the upper extremity. In this study, needle electrodes were used for stimulation and inserted 1.5 cm through skin, thus delivering stimulation below the adipose tissue. Although adipose tissue has low conductivity, it also decomposes in the disintegration
process. Since surface electrodes were utilized for recording, we recorded the total tissue response. Electrical stimulations were given above the elbow and the response signals were recorded 25 cm away from the stimulating electrodes. The recording point was close to the wrist. Longer and shorter distances than 25 cm may alter conductivity; longer
Please cite this article as: İ. Cantürk, et al., An experimental evaluation of electrical skin conductivity changes in postmortem interval and its assessment for time of death estimation, Comput. Biol. Med. (2015), http://dx.doi.org/10.1016/j.compbiomed.2015.12.010i
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distances might result increase conductivity and shorter distances might decrease conductivity. There were some limitations to our study. We were not able to start measurements just after death; the transfer of a corpse to the institution takes almost two hours. Therefore, experiments started, approximately, in the first two hours after death. In addition to that, the number of cases was limited due to not having enough cases that satisfied our criteria.
5. Conclusion A clinical test was designed to explore the relationship between electrical conductivity and postmortem interval for human cases. The obtained results indicate that there is a positive relationship between electrical conductivity and postmortem interval, i.e., conductivity goes up as the postmortem interval increases. In this study, we chose cases that had identical environmental conditions and lacked edema. In addition, BMI affected baseline conductivity but age did not. A novel contribution of this paper is the investigation of electrical tissue conductivity changes in the postmortem interval in human cases, which has not been previously studied. The experiments in this paper showed a positive correlation between conductivity and increased postmortem intervals. Further studies are needed to verify that electrical conductivity is a valid tool for ToD estimation and to study the effects of different environmental factors on electrical conductivity in postmortem human cases.
Conflict of interest None Declared.
Acknowledgments This study was approved by İstanbul Medeniyet University, Clinical Researches Ethic Commission under decision no: 2014/ 0055 and supported from Fatih University under Project no: P58011401-B. This study was conducted at Istanbul Council of Forensic Medicine under decision no B.03.1.ATK.0.01.00.08/96. Additionally, we applied for a patent for this method (2015/03140).
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Please cite this article as: İ. Cantürk, et al., An experimental evaluation of electrical skin conductivity changes in postmortem interval and its assessment for time of death estimation, Comput. Biol. Med. (2015), http://dx.doi.org/10.1016/j.compbiomed.2015.12.010i
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