Introduction

An intact dry 1 cm2 patch of skin has a typical electrical resistance of about 100 kΩ, which is primarily provided by epithelium, the horny outermost layer of skin. Under normal circumstances, the skin provides considerable protection against brief contact with electrical potentials even as high as 120 V. However, if the skin is wet or perspiring, the electrical resistance is greatly reduced and dangerous currents can be conducted. If the skin has a cut, or if Ag(AgCl) electrodes with electrode paste are used, the resistance can decrease to below 1 kΩ. Under these conditions, dangerously high currents can be produced by potentials as low as 12 V. See Table G.1 for the physiological effects of various current levels. With a skin resistance of 100 kΩ, 500 V would be required to produce a current of 5 mA, but with a skin resistance of 1 kΩ, only 5 V will produce the same current. At the other extreme, high-voltage generators with potentials of 100 kV are used in science museums to show safely the effect of static electricity on people whose heads are endowed with large quantities of hair. The important lesson here is that the primary factor in electrical hazards is the current, not the voltage, passing directly through the heart. This is of particular concern to designers of equipment that uses electrodes placed near or on the surface of the heart, such as coronary catheters, or during surgery.