Electrotherapy is a section of physiotherapy, which is concerned with the treatment of pathological conditions by the passage of electrical current or radiation rays through the body. Electric currents passed through a biological system can produce thermal, physiochemical and physiologic effect. To understand the mechanism of action whereby electrical currents produce these affects, some of the basic terminology and concepts of electricity are needed to be clarified.

ELECTRIC CURRENT

Electric current is the flow of electrons through a conducting medium when a potential is placed across the ends of the conducting pathway. The direction of the current flow is exactly the reverse of the direction of the flow of electrons.

The essential factors for the production of an electric current are the difference of potential (PD), a conducting pathway between the points of potential difference. This potential difference is achieved by the use of a battery or electromagnetic induction with a dynamo.

Current may be

- Alternating current (AC)
- Direct current (DC)

Electromotive Force (EMF) (E)

The force that causes the movement of electrons is called electromotive force (EMF) and it is measured in volt.

Resistance (R)

It is the property inherent in any material, which opposes an electrical current flow. The unit of electrical resistance is Ohm. One Ohm is equivalent to the resistance offered by a column of Mercury of 106.3 cm long and 1sq mm in cross-sectional area at a temperature of 0°C.

The material of the conductor, length, cross-sectional area and temperature, 2all determine the resistance of a pathway.

For a given material the availability of free electrons to conduct a current determines the resistance of the material. The grater the number of free electrons the lower is the resistance.

For example, in rubber the electrons are bound closely to their nuclei and have few free electrons and that is why it is a poor conductor of electricity. So rubber acts as an insulator.

Magnitude of Current (I)

The intensity or magnitude of current (I) is the rate of flow of electrons through the conductor per second. It is measured in Ampere. One Ampere is the rate of flow of 1Coulomb of electrons per second (6.26 × 10

^{18}electrons).RELATIONSHIP AMONG MAGNITUDE OF CURRENT FLOW, VOLTAGE AND RESISTANCE

The relationship of the above three factors is stated in Ohm's law, which states that the magnitude of an electric current varies directly with the EMF and inversely with the resistance.

The formula expressing Ohm's law –

I = E/R

That is, Amp = (Volt/ Ohm),

Where, | I | = | current flow measured in Ampere. |

E | = | emf measured in Volt. | |

R | = | resistance in Ohms. |

RESISTANCE IN SERIES

Components parts of an electrical circuit can be connected either in series or in parallel. Let us discuss the connection of three resistances in series:

- As the current has to pass through each resistance in turn, then the total resistance equals to the sum of the individual resistance (Fig. 1.1).R (Total resistance)=r
_{1}+ r_{2}+ r_{3}=60 + 30 + 10=100 Ohms. - The intensity of current flowing through each component equals toI (Total amount of current)=(V/R)=(200/100)=2 Amp.
- The voltage drop across each resistance equals toV=I × RSo V
_{1}at r_{1}=I × r_{1}=2 × 60=120 Volt.Similarly V_{2}at r_{2}= 60 Volt.and V_{3}at r_{3}=20 Volt.

RESISTANCE IN PARALLEL

In a parallel circuit the current has the liberty to flow in alternate pathway not in a fixed path. Thus the current flow in each of the parallel pathways is inversely proportional to the resistance of the pathway. The voltage drop at across each of the pathways will be the same while the total resistance will be less than any of the individual resistances (Fig. 1.2).

- The total resistance equals to
- The current flow equals to
- The current flow across each path is

ELECTRICAL POWER

Power is the rate of doing work. To calculate this, time has to be considered. It is measured in Watts.

Power (in Watts) = emf (Volt) × current (Coulomb)

FREQUENCY

It is the number of events occurring in unit time, i.e. number of complete waves passing any fixed point in one second and is measured in Hertz.

By low frequency alternating current in electrotherapy we mean current with frequencies between 50 to 100 cycles per second (50-100 Hz). High frequency alternating current has a frequency of 1,000,000 c/sec (1 MHz). Very high frequency alternating currents are in range of 1-50 MHz.

The characteristics of electrotherapeutic currents include their direction, pulse, shape and amplitude. Indirect current (DC) there is a constant flow of electron in one direction that is the polarity of the electrodes are kept constant. A modification in the DC is, pulsed of interrupted DC. In interrupted DC the direction of the current flow is not held constant.5

In an alternating current (AC) the magnitude of flow of electrons constantly change and the direction of flow reverses periodically. Since there is a constant reversal of polarity of electrodes we never speak of positive or negative poles.

Figures 1.3a to c shows examples of alternating current.

Figs 1.3a to c: (a) Alternating current (sine wave), (b) Original faradic current and (c) Square waves

The configuration that includes pulse and shape of both AC and DC can take on many forms. In physiotherapy, we are more concerned about the rate of rise of current. We can have immediate rise or can have slow rise. The rate of rise of current directly affects the current's ability to excite nervous tissues (Figs 1.4a to c).

The Duration of Current Flow

It is the period of time the current flows for each individual wave or pulse. This period can vary from milliseconds for interrupted DC or AC to minutes in uninterrupted DC.

The Amplitude of the Current Flow

It is the magnitude of current. The peak current is the maximum amplitude of the current.

Surging of Current

In a surging current the intensity of each successive pulse gradually increases in such a manner that each impulse reaches to higher intensity than that of the preceding one and after the peak levels it either falls suddenly or gradually (Fig. 1.5).

In physiotherapeutic practice most of the work is concerned with electrical currents. So, it is important to know what type of current may be used or desirable and their harmful effects.

TYPES OF CURRENTS

Following types of currents are used in practice:

- Direct current
- Low frequency, High frequency and Very High frequency Alternating currents.

With DC the direction of flow of current is always the same. This type of current may be allowed to flow continuously or it may be interrupted at regular interval to short pulse of direct current. With alternating current the direction of flow is regularly reversed. The most common type of alternating current is sinusoidal or sine wave current.

CLASSIFICATION OF CURRENTS

Therapeutic current can be classified on the basis of direction, frequency, voltage, amperage and biophysical effects.

*On the basis of direction of flow of current:*- Alternating current—which flow in both direction
- Direct current—which flow in one direction.

*On the basis of frequency:**Low frequency currents*—in the range of 50 to 100 Hz per second. The primary use is stimulation of nerve and muscles. Various types of currents are used in this category like direct current, interrupted direct, high voltage pulse galvanic current, and TENS.*Medium frequency currents*—in the range of 100 to 4000 Hz per second. These are basically used to stimulate deep-seated muscles and nerves. Example, interferential currents.*High frequency currents*—here the frequency is more than 1MHz and are used for deep voluminous heating of tissues. Example, Short Wave Diathermy (SWD), Microwave Diathermy (MWD), and Ultrasound Therapy (UST).

*On the basis of voltage:**Low voltage currents*—where voltage is less than 100 Volt as in low frequency currents.- High
*voltage currents*—where voltage is greater than 100 Volt as in high frequency currents.

*On the basis of amperage:*- Low amperage currents—where amperage is in the range of 1 to 30 mAmp. Example, high TENS.
- High amperage currents—where amperage remains from 500 to 2000 mAmp.