Some materials are not perfect conductors of electricity; that is, they display electrical resistance. In fact, one can think of a conductor as a material having very low (ideally zero) resistance while insulators can be thought of having very high (ideally infinite) resistance to the flow of electrons. The current that passes through an electrical resistor when a voltage difference is applied across it is given by Ohm’s law (see ◘ Box 36.1):

Where:5

Note that Ohm’s law (V = I × R) is analogous to the physiologic equation describing systemic blood pressure:

That is, the difference between the mean blood pressure and the right atrial pressure is equal to the cardiac output (CO) times the systemic vascular resistance (SVR), where SVR is in Woods units. Finally, to convert vascular resistance in Woods units to the more commonly used dyn s cm⁻⁵ units, multiply by 80:

The electrical power (W) consumed by an electrical device is measured in watts:

Where V is the voltage applied to the device and I is the electrical current that passes through the device (see ◘ Box 36.2).

The watt-second (or joule, J) is commonly used to denote electrical energy expended in doing work. The energy produced by a cardiac defibrillator is measured in watt-seconds (or joules), while the kilowatt-hour is frequently used to measure larger quantities of electrical energy. As an example, electrical utility companies charge their customers on the basis of kilowatt-hours of electricity consumed. The formula to use here is:
where W is the power consumed in watts and T is the time in seconds over which the power is consumed (see ◘ Box 36.3). Note that a kilowatt hour (kWh) is equivalent to 3,600,000 watt-seconds or joules.

A capacitor consists of any 2 conductors (such as parallel plates) that are separated by an insulator. A capacitor stores charge (electrons). In a DC circuit the capacitor plates are charged by a voltage source (ie, a battery) and there is only a momentary current flow as the capacitor charges. No further current can then flow unless a resistance is connected between the 2 plates and the capacitor is subsequently discharged. In contrast to DC circuits, a capacitor in an AC circuit permits current flow, depending on the impedance presented by the capacitor at a given frequency of alternating current.

Capacitors are a key component in the design of cardiac defibrillators (◘ Figs. 36.3 and 36.4), where they serve as an energy storage device according to the formula:
where E is the energy in joules (J), C is the capacitance in farads, and V is the voltage in volts. For example, if the capacitance of the capacitor is 1000 μF (microfarad) and the voltage applied to it is 1000 V then the stored energy is 500 J based on the following calculation:

Note that the energy delivered during an initial monophasic defibrillation is usually 200 J, with less energy typically used for biphasic defibrillation.

Stimulation with electricity can cause muscle cells to contract, and can thus be used therapeutically in equipment such as pacemakers or defibrillators or diagnostically when a nerve stimulator is used to assess the degree of neuromuscular blockade. However, contact with a large electrical voltage (such as a power line), whether AC or DC, can lead to injury or even death, often as a result of ventricular fibrillation. It takes approximately 3 times as much DC current as AC current to cause ventricular fibrillation.

◘ Figure 36.5 illustrates a typical AC electrical power arrangement in schematic form. A typical electrical outlet consists of 2 wires (“hot” and “neutral”) in conjunction with a third “ground” connection (◘ Fig. 36.6). The wire designated as “hot” carries the current to the load while the other (“neutral”) wire returns the current to the source. The potential difference between the 2 is typically 110 and 120 volts. To receive an electrical shock, one must be in contact with the electrical circuit at 2 points, and there must be a voltage supply that causes current to flow through an individual (◘ Fig. 36.7).

When an electrical shock occurs, damage can occur in 1 of 2 ways. In the first mechanism, the electrical current can disrupt the normal physiological function of cells. Depending on its magnitude and path, the current can contract muscles, paralyze respiration, or lead to cardiac arrest via ventricular fibrillation. The second mechanism involves the dissipation of electrical energy throughout the body’s tissues: An electrical current passing through any resistance raises the temperature of that substance, sometimes sufficiently to produce a burn. Basically, the electricity cooks the tissue it passes through.² ◘ Table 36.1 summarizes the physiological effects of various currents passing through the body for a 1-second duration.

The severity of an electrical shock is determined by the amount of current (amperes), its path through the body, and the duration of the current flow. For the purposes of this discussion, it is helpful to divide electrical shocks into 2 categories. Macroshock refers to large amounts of current flowing through a person, which can cause harm or death. Microshock refers to very small amounts of current (in the microampere and milliampere range) and applies only to the electrically susceptible patient, such as an individual who has an external conduit that is in direct contact with the heart. This can be a pacing wire or a saline-filled catheter such as a central venous or pulmonary artery catheter. In the case of an electrically susceptible patient, even minute amounts of current (microshock) may cause ventricular fibrillation.

In the electrically susceptible patient, ventricular fibrillation can be produced by a current that is below the threshold of human perception. The exact amount of current necessary to cause ventricular fibrillation in this type of patient is unknown (the experiments would be unethical), but based on animal experiments is believed to be as little as 20 μA.