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WAVE FREQUENCY

The effects of current on people is almost independent of the frequency, up to some 1,000 cycles/s, making no difference whether it is continuous or alternate. Under this value is when thermal phenomena appear, mainly electrolytic and faradic.

 

For frequencies above 350 KHz, the currents do not interfere with the nervous processes and can only produce heat. We can therefore understand how and why the currents chosen for electro surgery are developed in frequencies above 500 KHz (0.5 MHz).

 

At these frequencies, electrical conduction and organic absorption of the waves becomes more complex. As the frequency increases, the energy tends to radiate. Two mechanisms that produce heat appear:

• By the Joule effect, due to electrical resistance of tissue to current conduction.
  
  
• • By absorption of electromagnetic radiation, due to the molecular structures. Rotational movement of the water molecules (which are dipole structures) in alternate electromagnetic fields and the corresponding electromagnetic wave. The water molecules are stimulated by the incidental electromagnetic wave that causes fast rotation.
  
  The water molecules in rotation produce a mechanical friction effect that is transformed into heat. With the fast polarity changes, resistance inherent to movement of ions and the rotation of the water molecules produces heat.
  
  Resistance and heat production depends on the impendence of the treated tissue The depth reached with the change in electromagnetic wave phase corresponds with the applied RF field, to the change of delivered power, the application time and the cooling conditions.

One effect or another will have more relevance as the pulses are increased. In electro surgery, both are important at frequencies up to one MHz. For frequencies between one MHz and three MHz cycles, electromagnetic radiation is dominant.

 

The delivery frequency of Radiofrequency influences, at a higher frequency (less wavelength) in less penetrability. Furthermore, the effect that the passing of the electromagnetic radiation in the form of radiofrequency generates in the surface tissues of the organism is dependent on its frequency.

 

With radio frequencies equal or inferior to 10-15 MHz, the thermal effect originates is due to the Joule effect, the tissues offer resistance to the passing current and the radiofrequency energy is transformed into heat. Over 15 MHz, the energy of radiofrequency produces its effect according to molecular friction.

Radiofrequency delivery can be classified into three groups:

 

• RF over 10 GHz: These emissions are absorbed by the skin surface and little energy reaches the inferior tissue. For these emissions to produce hazardous effects on the health such as cataracts or skin burns, an output over 1,000 W/m2 is required.
  
  
• RF between 1 MHz and 10 GHz: This type of radiofrequency penetrates the exposed tissue and produces their heating due to the absorption of energy. RF penetration depends on the electrical behaviour of the tissue, the tissue size in relation to the output and the shape of the tissue, geometry and orientation with respect to the radiation. The result of tissue heating can reach ascents in temperature of up to 11º C.
  
  
• RF less than 1 MHz: This type of radiofrequency does no induce significant heating but electrical currents and fields in the tissues that are measured in current density in amps/m2. These electrical currents can produce interferences with the physiological currents resulting from the organism's chemical reactions and produce involuntary contractions or arrhythmia, they should be sufficiently powerful for this. In a normal physiological situation, the organism’s chemical reactions are accompanied by the movements of electrical charges and currents.
  
  The density of these currents is 10 mA/m2. For currents induced by radiofrequency applications with frequencies under one MHz to interfere with the normal functioning of the human body, their density must be at least 100 mA7m2, ten times higher. (Gonzalo Piédrola)¹⁶

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