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Electrocution

Pathology and Clinical Mechanisms of Electrocution Injury
Dr. Raphael C. Lee

The medical community is becoming increasing aware of the wide range of manifestations of electrocution injury. Almost everyone has once in his or her lifetime been shocked by electricity. The fear reflex generated by the bad experience of pain usually prevents us from further tampering with electricity. However, no matter how careful, accidents do and will happen, especially among electrical workers who have to handle commercial electrical power lines everyday.

Today, rates of electrocution injury among industrial workers range widely from one country to another. Within industrializing countries safety practices are often not the top priority, resulting in high rates of injury. In mature industrialized nations, electrocution shock rates continue to decline. In the United States, electrocution remains the fifth leading cause of fatal occupational injury with an estimated economic impact of more than 1 billion dollars annually (1). Although accurate statistics are difficult to find, it appears that the rates of injury may be highest among electrical workers. A study in Virginia suggested that public utilities have the highest rate of fatal electrical injuries among all industrial sectors. More than 90% of these injuries occur in men, mostly between the ages of 20 and 34, with 4 to 8 years of experience on the job (2). Another source (3) suggests that the average age of victims was 37.5 years and the average years of experience amounted to 11.3 years. For survivors, the injury pattern is very complex, with a high disability rate due to accompanying neurologic damages.

Away from the workplace, most injuries are due to either in-door household low-voltage (<1000 V) electrical contact or to out-door lightning strikes (4). Low-voltage (120 V or 220 V) household power-frequency electrocution shocks are common and usually results in minor peripheral neurological symptoms or occasionally skin surface burns. However, more complex injuries may result depending on the current path, particularly following oral contact with household appliance cord disclosures or outlets in small children (5). Compared to a high-voltage shock that usually generates an arc and resulting explosive thermoacoustic blast, low-voltage shocks are more likely to produce a prolonged, “no-let-go” contact with the power source. This “no-let-go” phenomenon is caused by an involuntary, current-induced, muscle spasm (6). For 60 Hz electrical current the “no-let-go” threshold for axial current passage through the forearm is 16 mA for males and 11 mA for females (7, 8).

Injury often follows contact with higher frequency electrical power as well. There are approximately 200 deaths annually in the United States due to lightning injury. Yet, many more people survive it. The range of lightning injury extent is quite broad, depending upon the magnitude of exposure and the condition of the victim. According to many reports, if lightning strikes in the vicinity of several individuals, usually only one endures serious or fatal injury. Radiofrequency and microwave injuries are less common. Nonetheless, they represent an important medical problem to understand. At higher frequencies, when the wavelength is short enough to couple at the atomic level, the fields can be ionizing as well as cause molecular heating.

In short, electrical trauma may produce a very complex pattern of injury because of the multiple modes of frequency-dependent tissue-current interactions, the variation in current density along the path through the body, as well as variations in body size, body position and use of protective gear. No two cases are the same. Today these variations create challenges in clinical diagnosis that delays effective medical management. Further advancement in medical care of electrocution shock victims will follow the development of more accurate bioengineering models of injury. The objective here is to review the basic considerations for addition bioengineering research.

The current distribution at higher frequencies, in RF and microwave ranges is dependent on different parameters. The cell membrane is no longer an effective barrier to current passage, capacitive coupling of power across the membrane readily permits current passage into the cytoplasm. Factors affecting the field distribution in tissues are frequency-dependent energy absorption and skin-depth effects. At the highest frequency ranges, including light and shorter wavelengths, other effects such as scattering and quantum absorption effects become important in governing tissue distribution. Table 1 provides a biological effect catagorization of the electrocution injury frequency regimes. Mechanisms of biological effects are different in each regime. A discussion of injury mechanisms must also be separated according to the frequency regime.

Until recently, low frequency electrocution injury was considered to be only a thermal burn injury, produced by Joule Heating (6). Over the past ten years, it has been shown that the pathophysiology of tissue electrocution injury is more complex, involving thermal, electroporation, and electrochemical interactions (15, 16, 17), and blunt mechanical trauma secondary to thermoacoustic blast from high-energy arc (18) (Table 2). While these forces can alter all tissue components, it is the thin plasma membrane of cells which has the greatest vulnerability. Thus, the cell’s plasma membrane appears to be the most important structure in determining the rate of tissue injury accumulation.

The most important function of the cell membrane is to provide a diffusion barrier against free ion diffusion. The energy required for moving a solvated ion across a planar, pure phospholipid bilayer in an aqueous, physiological environment approaches ã68 kT (19), indicating the steep energy hurdle. Because most metabolic energy of mammalian cells is ultimately invested in maintaining the ionic difference across the cell membrane (20), the importance of the structural integrity of the lipid bilayer is apparent. If the membrane is permeabilized, the work required maintaining transmembrane concentration differences increases proportionately. The conductance of electropermeabilized membranes may increase by several orders of magnitude. ATP production and in turn, ATP-fueled protein ionic pumps, cannot keep pace which lead to metabolic energy exhaustion. If the membrane is not sealed, biochemical arrest and the permeabilized cell will become necrotic. Thus, in discussing tissue injury resulting from electrocution shock, the principal focus is directed at kinetics of cell membrane injury and the reversibility of that process.

Electroporation. Electroporation is the term ascribed to the biophysical process of electric field driven re-organization of lipids in the lipid bilayer by supraphysiologic electric fields (21, 25, 26). Current electroporation theory indicates that highly electrically polar water molecules are pulled by Kelvin polarization stress into transient defects in the lipid packing order within bilayer leading to quasi-stable or stable pore formation. Although most commonly used to introduce foreign DNA into cells, electroporation of isolated cells has also been used to (1) introduce enzymes, antibodies, viruses, and other agents or particles for intracellular assays; (2) precipitate cell fusion; and (3) insert or embed macromolecules into the cell membrane. Recent reviews and books published have extensively treated this subject (21, 22, 23, 27, 28, 29, 30, 31). In this review we will briefly this phenomenon as it relates to the understanding of electrocution injury.

Tissue Electroporation. Investigation of tissue electroporation had been initially driven by the need for a better understanding of the pathophysiology of electrocution injury (47, 48). In the early 90’s, it was studied in connection with cardiac defibrillation shocks (49, 50). More recently, tissue electroporation has begun to be envisioned as a potential therapeutic tool in the medical field. It has found use in (1) enhanced cancer tumor chemotherapy (electrochemotherapy, 51, 52), (2) localized gene therapy (53, 54), (3) transdermal drug delivery and body fluid sampling (55, 56, 57). Computational models of human high-voltage electrocution shock suggest that the induced tissue electric field strength in the extremities is high enough to electroporate skeletal muscle and peripheral nerve cell membranes (15, 58, 59, 60) and to possibly cause electroconformational denaturation of membrane proteins.

Bhatt and coworkers (15) measured electroporation damage accumulation using isolated, cooled in vitro rat biceps femoris muscles. After the initial impedance measurement, a electric field pulse was delivered to the muscle using current pulses that setup tissue field pulse amplitudes ranging between 30 - 120 V cm-1, which was thought to be typical forearm field strengths in high-voltage electrocution shock. The duration of the DC pulses ranged from 0.5 - 10 ms. These short pulses reduce Joule heating to insignificant levels. Field pulses were separated by 10 seconds to allow thermal relaxation. The change in the normalized low frequency electrical impedance in the muscle tissue following the application of short-duration DC current pulses indicated skeletal muscle membrane damage. A decrease in muscle impedance magnitude occurs following DC electric field pulses that exceed 60 V cm-1 magnitude and 1 ms duration. As seen in Figure 3 the field strength, pulse duration, and number of pulses were factors that determine the extent of electroporation damage.

Based on these results, Block et al (16) electrically shocked fully anesthetized female Sprague-Dawley rats through cuff-type electrodes wrapped around the base of the tail and one ankle (Fig. 4) using a current-regulated DC power supply. The objective was to determine whether electroporation of skeletal muscle tissue in-situ could lead to substantial necrosis. The study involved histopathological analysis and diagnostic imaging of an anesthetized animal hind limb. A series of 4 millisecond DC-current pulses, each separated by 10 seconds to allow complete thermal relaxation back to baseline temperature before the next field pulse, was applied. The electric field strength produced in the thigh muscle was estimated to range from 37 V cm-1 to 150 V cm-1, corresponding to applied currents ranging from 0.5 – 2 A. These tissue fields were suggested to be on the same level as that experienced by many victims of high-voltage electrocution shock. Muscle biopsies were obtained from the injured as well as the collateral control legs six hours post shock and subjected to histopathological analysis. Sections of electrically shocked muscle revealed extensive vacuolization and hypercontraction-induced degeneration band patterns which were not found in non-shocked contralateral controls (Fig. 5). The fraction of hypercontracted muscle cells increased with the number of applied pulses. These results are consistent with the investigators hypothesis that non-thermal electrical effects alone can induce cellular necrosis. The pathologic appearance of the shocked muscle was similar to that seen in the disease malignant hyperthermia indicating that electroporation may lead to Ca2+-influx into the sarcoplasm. Most recently, a similar muscle injury pattern has been described in a human electrocution injury victim published by deBono in a clinical case report (61). These results suggested that direct electrocution injury of skeletal muscle in-situ can lead to the commonly seen pattern of injury in electrocution shock victims even in the absence of pathologically significant Joule heating.

Electrophysiological Responses to Electroporation. Compound muscle action potential amplitude (CMAP) records reflect the vectorial sum of fields produced by individual action potential (AP) conducting muscle cells. Only cells with intact membranes and with active ATP production are capable of generating action AP’s. Thus, changes in CMAP amplitude can be used to quantify extent of tissue injury caused by events which damage cell membranes. In a recent report, CMAP recordings were used to estimate electroporation injury accumulation in the anesthetized rat hindlimb. CMAP’s were produced by magnetic stimulation of the distal spinal cord. CMAP’s were recorded via skin surface electrodes. Using this entirely non-invasive protocol, CMAP’s changes in response to a series of applied 150 V cm-1 field pulses were recorded as a function of the number of field pulses applied. The data is shown in Figure 6. A saline injection was given intravenously 30 min. after the electric field application to simulate fluid resuscitation and as a sham-treatment for therapeutic investigations. The CMAP amplitudes decreased drastically after the electrocution shocks were applied, then they recover gradually with time. The larger the number of shocks, the larger the initial drop in CMAP, and the slower the recovery.

Using the in vivo rat hind limb electrocution injury model described by Block, Matthews et al (63) monitored the uptake of Tc99m-PYP in the electrically shocked tissue as a function of the magnitude of the DC current. Either 0.5, 1.0 or 1.85 A of direct current was applied to the rat’s hind limb. Intravenous saline infusion was used as sham-treatment. For each animal, a series of Tc99m-PYP incorporation images (at 2 min. intervals) over the period of four hours was recorded. Their results supported earlier reports indicating that Tc99m-PYP does accumulate in electroporated tissue. The plots of Tc99m-PYP incorporation in Figure 7 suggests that the level of the tracer accumulation is positively correlated to the tissue field pulses applied. This indicates that quantitative imaging of Tc99m-PYP uptake may be developed further as an indicator of the extent of electroporation or other membrane injury.

These experimental studies have shown that electroporation can lead to skeletal muscle tissue necrosis in-vivo. For several reasons, electroporation damage accumulation dynamics at the tissue level is different than for the case of isolated cells including the reduction of membrane lipid mobility caused by adhesion to large molecular weight biopolymers in the extracellular matrix of tissues. In addition, the distribution of electric fields, and in turn the induced transmembrane potential in tissue, is influenced by the packing density of the cells. Collectively, these results suggests that electroporation is likely to be an important mechanism of injury in electrocution shock victims.

If the field strength becomes sufficiently intense, those field-induced changes can cause irreversible damage to a membrane protein. In particular ion channels and pumps with their selective, voltage gated charge transport mechanisms (e.g., Ca2+ specific channel) are highly sensitive to differences in Vm. Chen and coworkers investigated the effects of large magnitude Vm pulses on voltage-gated Na+ and K+ channel behavior in frog skeletal muscle membrane using a modified double vaseline-gap voltage clamp. They found in both channel types, but more drastically in K+ channels, reductions of channel conductance and ionic selectivity by the imposed Vm (76). In their most recent work, Chen et al. were able to demonstrate that these changes are not caused by the field-induced huge channel currents (Joule heating damage) but rather the magnitude and polarity of the imposed Vm (77). The consequences of this effect may underlie the transient nerve and muscle paralysis in electrocution injury victims.

Exposure to ambient microwave fields are known to cause burn trauma. Microwave burns have different clinical manifestations than low frequency electrocution shocks (79, 80, 81, 82). At low frequency the epidermis is a highly resistive barrier, whereas in the microwave regime, electrical power readily passes the epidermis in the form of “capacitive” coupling with very little energy dissipation. Consequently, the epidermis may not be burned unless it is very moist. The microwave field penetration into tissue has a characteristic depth in the range of 1 cm, resulting in direct heating of sub-epidermal tissue water. The rate of tissue heating is dependent not only on the amplitude of tissue electric field, but also on the density of dipoles. For example, microwave heating is much slower in fatty tissues (83).

The LT curves are helpful to estimate the likelihood of significant thermal damage following a certain electrocution shock exposure. Unfortunately, the contact time of an actual electrocution shock injury is almost never known. It seems from the work of Jones and others (91), that for accidents involving high-power electrical sources, contact times are very likely to be on the scale of fractions of a second. This is because the acoustic blast resulting from arcing is likely to push the victim away. Whereas longer contacts are more likely with lower voltages. Thus, the usefulness of the LT analysis rests in its suggestion that the duration of contact is the most significant parameter in determining the extent of thermal injury to subcutaneous tissues in high-voltage, high-current electrocution shock.

IMAGING ELECTROPORATION DAMAGE PATTERNS. In most electrocution injury cases, the treating physician does not know the voltage, current and the contact time experienced in a particular high-voltage accident. Since the muscle damage sustained lies invisible underneath the skin, non-invasive imaging methods such as magnetic resonance imaging (MRI) have rbeen explored as important tools in early diagnosis. MRI allows the detection of the edema (T2-weighted imaging sequences) and evaluation of permeability changes (contrast enhanced T1-weighted sequences) in electrically induced muscle injury with high spatial resolution.

Using the previously described rat hind limb electrocution injury model, Jang et al acquired multi-slice T2-weighted and contrast agent (Gd-DTPA) enhanced T1-weighted images to obtain information about edema localization and contrast agent distribution volume, respectively (92). Images of relative contrast agent distribution volume in the electrically shocked hind limb were obtained by subtracting the pre-injection T1-weighted image sets from the respective post-bolus injection T1-weighted image sets (93). MRI scans of non-shocked rat hind limbs served as a control for image interpretation. Significant differences in both the T2 - and contrast enhanced T1 -weighted images were observed between control (non-shocked) and electrically shocked animals (Fig. 11). The investigators found a strong regional correlation between edematous tissue (T2) and areas of increased contrast agent distribution volume (T1) confirming the cell membrane permeabilizing (electroporating) nature of the muscle injury imaged.

Because muscle bundles are encapsulated in layers of perimysium and epimysium, severe edema will increase the local interstitial hydrostatic pressure so much that it obstructs the local blood flow (compartment syndrome). The prolonged ischemia alone will also cause muscle necrosis. This effect is especially pronounced in deep muscles injured by electric shock(5). As discussed in Lee, 1997 (5), if edema is present in a muscle group, damage should be expected. The MRI images of electrically shocked hind limbs shown in Figure 11 demonstrate also that electrocution injury is not uniformly distributed in the bulk muscle tissue: some muscle flaps are spared and others endure more severe damage. The inhomogeneous injury pattern of muscle electrocution injury is probably a combination of the following factors: [1] Non-uniform distribution of electrical current intensity in different muscle flaps; [2] Non-uniform electrical resistance distribution of muscle itself and the break-in barrier resistance of muscle sheaths; [3] Differences in relative orientation of different muscle cell bundles and muscle flaps with respect to the electrical current entrance and exit points;. Although at this time the cause for the inhomogeneous injury pattern can only be speculated on, the early differentiation of injured vs non-injured muscle flaps via MRI will greatly help treating physicians on surgery planning and management.

COMBINED THERMAL AND ELECTROCUTION INJURY MECHANISMS. The preceding discussion indicates that there are thermal and direct electrical mechanisms of tissue injury in victims of electrocution shock. These mechanisms produce different cellular injuries, suggesting that therapeutic strategies will be different. Thus it is important to discuss this important matter further.

Given the importance of electrical power to human culture, the problem of electrocution injury is one that will continue to exist for the foreseeable future. Electrocution injury has been poorly understood and perhaps less than optimally managed in the past. Improvement requires a better understanding of injury mechanisms, anatomical patterns of injury and therapy. A prompt, accurate clinical diagnosis of electrocution injury is one of the most difficult tasks in the medical field (5) because it usually calls upon an understanding of the interaction between electric current and human tissue. Specifically speaking, the difficulty involves the following:

1. The exact tissue damage mechanism and damage level depend on a host of parameters: the characteristics of the power source (DC or AC current, voltage, frequency, etc.), path and duration of closed circuit, area and impedance of contact spot, etc. Correspondingly, there is a whole spectrum of damage characteristics depending on the values of these parameters. The physician needs to do a 4-dimensional (spatial plus temporal) detective work in order to arrive at a correct diagnosis.

2. Electrical damage to the tissues is not easily detectable by visual inspection or physical examination. And often times its sequelae will not manifest themselves after a certain period of time: electrically injured tissue may initially appear viable, only to become visibly necrotic at a later point (in a number of days) (94, 106, 107).

The molecular structure of biological systems can be severely altered by the effects of high-energy commercial frequency electrical power. The mechanisms of damage include cell membrane electroporation, Joule heating, electroconformational protein denaturation, and others.

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