Physiological Effects of 60-Hz Electric Shocks

April 17, 2015
The muscles in your body are controlled by electrical signals, and the heart, being a muscle, is also dependent on electrical signals. The heart functions by a carefully controlled flow of electricity through the heart muscle. Cardiac muscle cells contract when an electrical signal triggers them. The cells within the heart atria and within the heart ventricles are electrically connected. When one area of the heart muscle is stimulated, this electrical signal is sent to neighboring cells, which also contract, and then send an electrical signal to their neighboring cells. 

In a properly functioning heart, the electrical signal to contract starts at the sinoatrial (SA) node in the upper part of the right atrium. The electrical signal is sent down the atria, which contract, and is then delayed momentarily at the atrioventricullar (AV) node before passing into the ventricles, which contract to eject blood into the pulmonary artery and aorta (Figure 1). The timing and direction of the electrical signal flow throughout the heart muscle are critical for the heart to function properly. If the electrical triggering of a portion of the heart is timed improperly, the heart muscle may not contract in a controlled manner, which could cause the heart to quiver as electrical signals randomly propagate throughout the muscle. This condition, called ventricular fibrillation, is almost always fatal unless corrected rapidly by re-polarizing the entire heart muscle with a defibrillator.1  


Figure 1. The human heart with key aspects of the electrical pathway illustrated.


Electrical shock occurs when a person’s body completes the current path in an energized electrical circuit. The effects of an electrical shock can vary from none, to a slight tingle, all the way to immediate cardiac arrest. The potential severity of the shock depends on several factors, such as the electrical resistance of the body and the voltage across the two points contacted, which determine the amount of current that flows through the body. The path that the current takes through the body is also important in determining the potential severity of the electrical shock. Current flow across a finger will have different physiological ramifications than current flow across the heart. Electrocution, which is defined as death by electricity, is usually caused by current flow disturbing heart function.

For a given pathway in the body, the magnitude of the electrical current, measured in units called amperes (A), is the most important variable in determining the severity of an electrical shock. The magnitude of electrical current is determined by the voltage across the contacted points in the circuit and the resistance of the pathway through which the current flows. Many literature resources detail the effects of electrical shock, and at what magnitude these effects occur, which vary somewhat from study to study (e.g., Reference 2). Table 1 illustrates the magnitude and effect of shocks at a 60-Hertz (Hz) alternating current (AC), which is the most common frequency for household power outlets in the United States, determined by the National Institute for Occupational Health and Safety in their publication, “Worker Death by Electrocution” (Publication No. 98-131) for the chest pathway.3  

Table 1. Estimated effects of 60-Hz AC currents.

 

A voltage that produces only a tingling sensation under one circumstance can produce a lethal shock under other conditions. Many people are familiar with Ohm’s Law: V=I∙R. When a body completes an ideal circuit by connecting two points at different voltages V, the resistance R of the body between these two points will determine the magnitude of the current flow I. The resistance depends on the amount and type of body tissue in the current path. 

Within the human body, electrical resistance varies widely among bone, fluids, and various tissues. Most households throughout the world have AC electrical lines powered at a potential between 100 and 240 volts RMS at a 50- to 60-Hz frequency. The minimum resistance, given in IEC Standard 60479-1, for hand-to-hand current flow, for the most conductive 5% of the population, is about 900 ohms at a 120-V potential, which would allow ~130 mA of current to flow. Actual currents are not usually this high, because the skin increases the overall resistance of the current path. The IEC standard lists the same resistance values for 50-Hz and 60-Hz potentials. Tests conducted on human volunteers and cadavers found that resistance depends on the applied voltage (e.g., at 25 V, resistance was generally above 1,750 ohms; at 225 V, it was generally above 775 ohms). External conditions can affect these values, such as the presence of perspiration and wet environments. Voltages over 600 V can rupture human skin, reducing the hand-tohand resistance of the human body to its internal resistance of about ~575 ohms, which allows more current to flow and thus causes greater damage to internal organs. Entrance and exit wounds generally are present when this occurs.4   

Several studies have found that the threshold at which a person perceives electrical current flow is ~0.5 to 1 mA, where a shock might be perceived but no uncontrolled startle reaction results. Shocks just above the perceptible level can be dangerous, not because they cause physiological harm, but because a person may be startled and his or her reaction may be hazardous (e.g., it may lead to a fall or contact with dangerous equipment). 

Higher current levels will cause involuntary muscle contractions. Depending on the contact points of a body that completes the electrical circuit, the current flow may cause a person to involuntarily grasp a conductor or get locked into a circuit. The person is involuntarily held or frozen to the energized conductor and cannot “let go” unless the power is turned off or he or she is physically removed from contact with the circuit. If the contact is not somehow broken, the person’s skin resistance may decrease due to perspiration, tearing of skin, or a tighter grasp, thereby increasing the current flow and possibly causing death. 

Electrical shocks with a current path through the respiratory centers (RC) can result in respiratory arrest. The main RC is located in the medulla oblongata, which is the lowermost part of the brain stem and controls respiratory movements. Injury to this center may lead to central respiratory failure, which would necessitate mechanical ventilation. Electrical shocks with current flow through the respiratory centers, such as possibly from the head to a limb or between two arms, could lead to respiratory arrest, usually with a grave prognosis. 

As mentioned, the typical mode of death in electrocutions is related to interference with the electrical signals that control heart function. A ~50 mA shock at 60 Hz, with a current path through the chest that lasts longer than 2 seconds, can produce ventricular fibrillation. This threshold increases to ~500 mA for shocks that last less than 0.2 seconds. Cardiopulmonary resuscitation (CPR) is generally administered promptly until more advanced resuscitation means become available. 

Burns are a common shock-related injury; they can occur when an electric current flows through tissue or bone, generating heat that causes tissue damage. The body cannot dissipate the heat generated by current flowing through the resistance of the tissue, and therefore, burns occur. 

This article briefly touched on some of the physiological effects produced by electrical shocks. Several parameters, such as the voltage, frequency, waveform shape, current duration, contact surface area, contact pressure, skin condition, and moisture level all are important in evaluating electrical injury cases. All these parameters could not be detailed here, but several resources are available for further study, including those listed below. 

The original version of this article appeared in the Georgia Defense Lawyer, Winter 2013, published by the Georgia Defense Lawyers Association (GDLA). It is reprinted with permission.

_____________

References

  1. AC Guyton, “Textbook of Medical Physiology-8th Edition,” Philadelphia: Saunders, 1991 (ISBN 0-7216-3087-1.)
  2. EK Greenwald (ed.), “Electrical Hazards and Accidents, Their Causes and Prevention,” New York: John Wiley & Sons, 1991 (ISBN 0-442-23799-5).
  3. “Worker Deaths by Electrocution,” National Institute for Occupational Safety and Health Publication #98-131, Cincinnati, OH: NIOSH, 1998.
  4. “Effect of Current on Human Beings and Livestock,” International Electrotechnical Commission (IEC) Standard 60479.

AUTHORS