What happens to the tissues at this moment? Is there any danger? We need to answer three questions:
- What current is considered adequate?
- What current parameters are safe?
- What is the acceptable current frequency?
Type of electric current. Flow of direct, or unidirectional, current through electrolytes, biological tissues in particular, leads to irreparable chemical changes on electrodes through which the object is connected to the external part of the electric chain, therefore medical diagnostics applies only alternating electric current of rather high frequency (as a rule, above 1 kHz). The acceptable current value is limited by its biological impact, and rises alongside with the increasing frequency.
Impedance-based equipment uses external electrical current, which is not supposed to receive a response from the excitable cells when initiating the action potential. Criteria, which are used during stimulation, are known and they cannot be overpassed when using electrical impedance equipment. Cell response can be received when the electric charge reaches the cellular membrane. Damages can be caused by high current applied for a short period and low current applied during a long period of time.
Current frequency. Tissues may have different response when stimulated by electrical current depending on the type of tissue, which may be manifested as increased sensitivity, pain or contraction. Mechanisms of electrical sensitivity of a biological tissue change depending on the frequency. At low frequencies (below 300 Hz) even low current causes some response. When medium frequencies (10 Hz – 10 kHz) are applied, the prevailing mechanism will be nervous stimulation. Two negative effects should be avoided: stimulation of recurrent nerve, which may lead to cardiac arrest, and fibrillation of atrial myocardium or ventricular myocardium resulting in heavy circulatory injuries. At higher frequencies (10-100 kHz), heating of the tissue will be the prevailing biological mechanism. Taking in account the delay in response to excitation typical for nerve fibers, due to low speed of distribution of the excitation pulse along the nerve, alternating current with frequency of 20 kHz or higher can be used for diagnostic purposes without any risk of heart damage, damage of muscles or nervous system. This knowledge is essential for cardiovascular physiology, cardiology and on the whole for electrical safety within a hospital.
Electric current intensity. The drawing shows the basic effects of electric current on the body. During weak stimulation (0.3-10 mA), only some sensations are felt. In the range from 10 to 100 mA, it is possible to have contractions, feel pain, fatigue. Electric current higher than 15 mA may lead to respiratory arrest, fatigue and intensive pain, depending on the current path. In the range from 50 mA to 6A, fibrillation of muscles takes place. Values that are even higher may cause myocardial contractions or burns.
Drawing 1. Overall effect of electric current of low frequency (60 Hz) using copper electrodes located on raised arms of a man with 70 kg of weight, at the 1 and 3 seconds of exposition ( from Olson, reproduced by permission of Houghton Mifflin Co).
Electric current used for biological electric impedance measurements should always be below the limit on the curve of excitable tissues located along the electric current path. In a human, the most excitable nerve fibers require the length of current impact of 0.2 ms for the minimum excitation, which is equivalent to 5 000 Hz. This is the threshold value of frequency safety. Frequency interval from 20 to 100 Hz should not be used for medical application.
For the purpose of getting the picture of distribution of electrical conductivity, the recommended frequency values should stay within the limits of 20-100 kHz and current intensity from 0.5 to 1 mA. In order to get the picture of distribution of permittivity, it is necessary to use the current with frequency higher than 100 kHz and current intensity 1-5 mA.
When using electrical impedance tomography as means of medical diagnostics, it is necessary to maintain the electric current parameters, which would be safe for the patient, and which would ensure acceptable impedance measurements. Characteristics of electric current used in electrical impedance mammograph are allowed for medical use and are safe for the doctor and patient, and are significantly lower of the threshold values, i.e. they don’t cause any pain or other sensations in the person.