ELECTRICAL SHOCK TRAUMA
ColinMcFaul, PhD, Mei Li, MD,Ze Liang, and Raphael C. Lee, MD, ScD
Chicago Electrical Trauma Research Institute
Shanghai Power Hospital
Peking Union Medical College
Pritzker School of Medicine, The University of Chicago,
Chicago, Illinois 60637, U.S.A.
KEY WORDS: electrical injury, burn, membrane, electroporation, denaturation, tissue
Shortened Title: Electrical Injury
Send Proofs to:Raphael C. Lee, M.D., Sc.D.
Department of Surgery, MC 6035
The University of Chicago
5841 South Maryland Avenue
Chicago, IL 60637
Phone: (773) 702-6302Fax: (773) 702-0661
e-mail:
Contents
Contents
Introduction
Electrical Transport within Tissues
Physicochemistry of Tissue Injury
Low Frequency Electric Shocks
Direct Electric Force Damage.
Thermal “Burn” Injury
Electro-conformational Denaturation of Transmembrane Proteins
Radiofrequency (RF) and Microwave Burns
Lightning Injury
Common Clinical Syndromes Following Electrical Injury
Diagnostic Imaging of Electrical Injury
Summary and Conclusions
Acknowledgements
References
Introduction
The development of electrical power has been one of the most impactful engineering feats of modern humanity. As electric power penetrates further into society, it poses greaterrisk to human society in terms of safety. Manycitizens have experienced electric shock at least once in their lifetime. The pain and fear generated from encounters with electricity usually prevents us from further tampering with such a dangerous force. However, no matter how careful, accidents can and do occur, especially among electrical workers who handle commercial electrical power lines everyday. Recently, use of electrical power in non-lethal weapons carried by law enforcement has made it into the mainstream in some countries. The purpose of this chapter is to provide a basic overview of the harmful effects of electrical force from both the engineering and medical perspectives.
The range of clinical manifestations of electrical shock is not well documented. Many survivors of accidental electrical shock never seek medical attention. The data that exists is from those who do seek attention or from a few case studies of electrical workers. Furthermore, there is considerable variation in how electrical injury and safety is managed across various countries. In industrializing countries safety practices are often not the top priority, resulting in high rates of injury. A study of burn injuries by Nursalet al.during a one-year (2000-2001) period indicated that 21% of burn patientswere victims of electrical injury (1). In highly industrialized nations, electrical shock rates are on the 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 (2). The rates of injury are highest among electrical workers, mostly caused by ‘live’ electrical equipment, such as wiring, light fixtures, and overhead power lines (3). 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 (4). Another study found that the average age of victims was 37.5 years and the average experience was 11.3 years. In yet another study, the incidence in a 9-year period was 8.3%, 96% of victims were male, and the mean age was 32.7 years (5). For survivors, the injury pattern is very complex, with a high disability rate due to accompanying neurologic damages and/or loss of limbs.
Away from the workplace, most electrical injuries are due to either indoor household low-voltage (<1000V) electrical contact or outdoor lightning strikes (6).An etiology study conducted in India reveals that 60% ofhigh-voltage electric burnswere due to exposed electric wires on farm or agricultural land, while low-voltage electric shocks happenedmainly at home (47%)and at professional utility places (29%)(3).Domestic household 60 Hz electrical shocks are common and usually result 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 by small children (6). Compared to a high-voltage shock that is usually mediated by an arc, 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 (7). For60Hz electrical current, the “no-let-go” threshold for axial current passage through the forearm is 16mA for males and 11mA for females (7,8).
There are roughly 30 human deaths annually in the United States due to lightning strikes and approximately ten times that number of injuries (9,10). The range of lightning injury extent is quite broad, depending upon the magnitude of exposure and the condition of the victim. Usually lightning hits result in surface burns, complex neurological damage similar to blunt head trauma, peripheral neurologic injury, and cardiac damage(11). 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 can cause molecular heating. Electrical trauma may produce such complex patterns of injury because of variations in the source of the electricity, differences in tissue-current interactions by electrical frequency, the variation in current density along its pathway through the body, and variations in body size, body position and use of protective gear. No two cases are the same.
Electrical Transport within Tissues
The fundamental bioengineering perspective is that the human body is considered to be a compartmentalized (or lumped element) conducting dielectric. It consists of about 60% water by weight: 33% intracellular and 27% extracellular (12). Body fluid in both the intracellular and extracellular compartments is highly electrolytic, and these two compartments are separated by a relatively impermeable, highly resistive plasma membrane. Current within the body is carried by mobile ions in the body fluid. The concentration of mobile ions results in a conductivity of approximately 1.4S m-1 in physiological saline. While electrons are the charge carriers in metallic conductors or electrical arcs, in the human body the charge carriersare ions. The conversion from one to the other occurs at the skin surface through electrochemical reactions(13).
At low frequencies (i.e., below radio frequencies), the electric current passing across the body distributes such that the electric field strength is nearly uniform throughout any plane perpendicular to the current path (14–16). As a consequence, the current density distribution depends on the relative electrical conductivity of various tissues and the frequency of the current. Experimental data support this basic concept. Sances et al. measured the current distribution in the hind limb of anesthetized hogs (16). They found that major arteries and nerves experienced the largest current density because of their higher conductivity. It was also observed that skeletal muscles carried the majority of the current due to their predominant volumetric proportions.
At a macroscopic scale, upper limbs are mostly involved in electric shocks, especially the right upper limb, as would be expected from dominant hand interactions with electrical sources(17). Cela et al. modeled this using a multiresolution admittance method. In this method, a human arm was initially split into uniformly-sized voxels. Within regions of uniform composition, voxels were combined, greatly reducing the number of voxels in the model. Fine resolution was maintained at material boundaries(18). The voxels were then connected as a network of conductors, allowing the calculation of current density at each voxel. This model was applied to predict the damage in skeletal muscle caused by cell lysis, and to simulate the injury pattern.The simulation found that current density increases towards the distal part of the arm as the cross sectional area decreases. The wrist has particularly high current density and damage due to the constriction and the high fraction of less conductive bone.The overall impedance of this arm model is 599 Ω, which is consistent with the estimated resistance of the human body(18).Computational models of human high-voltage electrical shock suggest that the induced tissue electric field strength in the extremities is high enough to electroporate skeletal muscle and peripheral nerve cell membranes (19–22) and to possibly cause electroconformational denaturation of membrane proteins.
At a more microscopic scale, low frequency current distribution within tissue is determined by the density, shape and size of cells. The cell membrane acts as an insulating ion transport barrier that mostly shields the cytoplasmic fluid from low frequency electrical current. In addition, the presence of cells diminishes the area available for ionic current and, in effect, makes tissues less conductive. As cell size increases, the membrane has less impact on a cell’s electrical properties, because the volume fraction of the cell occupied by the membrane is inversely proportional to the total cell radius (23). Similarly, the conductivity of skeletal muscle parallel to the long axis of the muscle cells is greater than that perpendicular to this axis. Solid volume fraction of the extracellular matrix can also be important in certain tissues and anatomic locations. For example, the resistivity of cortical bone and epidermis is higher than other tissues because their free water content is lower, as evidenced in the recent work by Kalkanet al.(24).
At higher frequencies, in RF and microwave ranges, the current distribution is dependent on different parameters. The cell membrane is no longer an effective barrier to current passage, and capacitive coupling of power across the membrane readily permits current passage into the cytoplasm. Frequency-dependent factors like energy absorption and skin-depth effects govern the field distribution in tissues. At the highest frequency ranges, including light and shorter wavelengths, other effects such as scattering and quantum absorption effects become important in governing field distribution in tissues. Table 1 provides a categorization of frequency regimes, with the corresponding wavelength spectrum, their common applications, and their effects on tissues as a result of electrical injury. Mechanisms of biological effects are different in each regime. A discussion of injury mechanisms must also be separated according to the frequency regime.
Table 1Frequency - wavelength regimes with general applications and harmful effects
Field Frequency(cycles/sec) / Energy Coupling Mechanism / Tissue Damage
D.C. to 103 / Ionic Currents
Forces on Cell Structures / Joule Heating
Membrane Poration
103 to 107 / Ionic Currents
Field Energy Absorption by cells / Joule Heating
Cell spinning
107 to 109 / Field Energy Absorption by proteins / Macromolecular Heating
109 to 1011 / Field energy absorption by water / Microwave Heating of water
1011 to 1015 / Field Energy Absorption by atomic bonds / Photo-optical protein damage
Physicochemistry of Tissue Injury
Low Frequency Electric Shocks
The biophysical mechanism of injuries caused by contacting electrical power sources remain controversial(5). It has been shown that the pathophysiology of tissue electrical injury is complex, involving thermal, electroporation, and electrochemical interactions (24–27), and blunt mechanical trauma secondary to thermoacoustic blast from high-energy arc (28). The various modes of trauma lead to complex patterns of injury which remain incompletely described.
Different conductivities in different tissuescauses tissue damages to varyfrom swelling to full-thickness burns in electric shock. According to Ohm’s law, tissue damage in electric shock shows typical patterns in which tissues with higher conductivity presenting more severe clinical outcomes. Nerves, blood, mucous membranes, and muscles or moist hands possess the lowest resistance in the human body. Thus theappearance may not reflect the situations of patients properly; internal injuries can be severe, even when skin burns seem moderate. It is worth mentioning one unique complication of electric shock: rhythmic disturbances of the heart. Cardiac muscles are specialized smooth muscles that contract constantly. This synchronization of muscle movement is governed by the sinus node, a group of cells within the heart that spontaneously produce electrical impulses. A current of more than 50–100 mA can disturb these impulses and develop ventricular fibrillation.
The understanding of electric injuries has deepened from simple burns by Joule heating to complicated models of cell damages. These new concepts are helping physicians with better management of electrical injury patients(29).In the most general terms, tissue damage exists when proteins and other biomolecules, cellular organelle membranes or water content is altered. Among all the components of the cells and tissues which can be damaged by an electrical shock, it is the thin cell membrane which has the greatest vulnerability. Thus, the cell membrane appears to be most important determinate of tissue injury accumulation.
The most important function of the cell membrane is to provide a diffusion barrier against free ion diffusion (30). Because most metabolic energy of mammalian cells is used in maintaining transmembrane ionic concentration differences(31), the importance of the structural integrity of the lipid bilayer is apparent. 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,leading to metabolic energy exhaustion. Cell necrosis results if the membrane is not sealed. Thus, in discussing tissue injury resulting from electrical shock, the principal focus is directed at kinetics of cell membrane injury and the reversibility of that process. A simulation study of membranes by Tarek (32) explains the electroporation phenomena in bilayers.
Direct Electric Force Damage.
A cell within an applied DC or low-frequency electric field will experience electric forces which will act most forcefully across and along the surface of the cell membrane. The forces acting across the membrane can alter membrane protein conformation and disrupt the structural integrity of the lipid bilayer. The magnitude of the forces acting across the membrane is related to the induced transmembrane potential Vm. Vm depends on a variety of factors, such as the intra- and extracellular medium conductivity, cell shape and size, the external electric field strength E as well as how the electric field vector orients with respect to the point of interest on the cell membrane(22–24).
Given that most cells are either somewhat spheroidal or cylindrical in shape, the relationship between the externally applied electric field and the induced transmembrane potential can be simplified to either of two simple forms. Under physiologic tissue conditions, the peak magnitude of induced transmembrane potential Vm(VPm) at the electrode-facing poles of spherical cells can be expressed as:
V pm=1.5Rcell cos (· (1+(f / fs)2)-1/2·Epeak, (1)
WhereRcellis the radius of the cell, Epeakis the peak field strength in the tissue surrounding the cell, is the angle off axis from the field direction, fs is the sub--dispersion frequency limit below which the cell charging time is short compared to rate of field change, and f is the field frequency(23). For cylindrical shaped cells, such as skeletal muscle and nerve cells, aligned in the direction of the field (herein assigned the z coordinate), the induced transmembrane potential takes a different form. Under these circumstances an electrical space constant parameter becomes useful in describing the electrical properties of the cell. The induced transmembrane potential can be expressed as a function of z:
Vpm(z)Am sinh (z / m) (1+(f / fs)2)-1/2 Epeak (2)
Where m is the electrical space constant of the cell, A is a variable that depends on cell length, the position z = 0 corresponds to the mid-point of the cell(34). The bottom ofFigure 1 illustrates schematically the spatial variation of Vpm(z) on the cell size for both of these cases. Of course, physically larger cells like skeletal muscle and peripheral nerve oriented in the direction of the electrical field would experience an induced transmembrane potential of greater magnitude than smaller cells.
Under normal physiological conditions, the cell’s outer plasma membrane is an electrical transport barrier, restrictingcurrent passage through the cell. That leads to an induced transmembrane potential (23).Thus larger cells are more vulnerable to membrane disruption by electrical shock current.
Equations (1) and (2) are valid as long as the electrical properties of the cell membrane remain constant. The natural transmembrane potential of mammalian cells has a magnitude of less than 100 mV(36).When an extracellular imposed electric field results in an induced transmembrane potential difference magnitude of greater than 200-300 mV across a mammalian cell membrane, molecular alterations occur thatcan lead to membrane disruption with subsequent loss of membrane transport barrier function(23). Gaylor et al. also showed that crowding of cells increases the induced transmembrane potential by preventing the electric field lines from diverting around a given cell. One effect of this is that cells in the interior of a muscle are more susceptible to direct damage from an applied electric field than are cells that are adjacent to more conductive tissue.
Figure 1.Top: Dependence of the induced transmembrane potential difference on cell size and position when cells are exposed to the same electric field. Electrical current lines are the same as electric field lines (shown as stream lines with arrows). Constant voltage lines are shown in darker color without arrows. Bottom: the transmembrane potential difference as a function of position along the direction of the applied electric field. The longer cell has a much larger transmembrane potential at its ends than the shorter cell.
The principal mechanisms of damage are electroporation of the lipid bilayer and electro-conformational denaturation of the membrane proteins. Electro-conformational damage to membrane proteins has been well documented for voltage-gated membrane protein channels(37,38). The processes occur quickly, on the order of milliseconds, after strong fields are applied.
Electroporation is the biophysical process of cell membrane disruptionresulting from an electrical field in the plasma membrane of such magnitude that re-ordering of lipids in the lipid bilayer takes place(33,37,38). Investigation of electroporation of many cells within a tissue was initially driven by the need for a better understanding of the pathophysiology of electrical injury (39–41). In the early 90’s, it was studied in connection with cardiac defibrillation shocks (42,43). More recently, tissue electroporation has begun to be envisioned as a potential therapeutic tool in the medical field. It has found useful applicationsat both the single and multicellular level in 1) enhanced cancer tumor chemotherapy (electrochemotherapy, (44,45); 2) localized gene therapy ((46,47); 3) transdermal drug delivery and body fluid sampling (48–50);4) introduction of foreign DNA into cells; 5) introduction of enzymes, antibodies, viruses, and other agents or particles for intracellular assays; 6) cell fusion; 7) to insert or embed macromolecules into the cell membrane; 8) to sample microenvironments across membranes(51); 9) gene delivery in human embryonic stem cells (52),gene transfer in whole embryos(53) and gene repair in mammalian cells(54). Recent reviews and books published have extensively treated this subject (33–35,50,55–58).