Download Interaction of electric and magnetic fields with a human body

Interaction of Electric and
Magnetic Fields With a Human
Body
Ján Jakuš
Electricity
- results from elmg. forces between + protons and –
charged electrons
- in a metal wire electricity depends on a movement
of free electrons
- in solutions electricity depends on a flow of
negatively charged -Anions, which are repealed by
a negatively charged –Cathode, and attracted by a
positively charged +Anode.
- However, the direction of electric current was generally aknowledged as from the +Anode to –
Cathode
Electric Units
Electric Energy (power supply-U) is measured in
VOLTS (V)
Electric Current (intensity-I) in AMPERES (A)
Electric Resistance (R)
in OHMS (Ω)
OHM´S Law is a relationship between:
I= U/R, U=I.R,
R=U/I
Metals - the best electric conductors (of 1st order)
Living bodies are 2nd order electric conductors,
contain salt solutions
Types of Electric Currents
Direct Current (DC) – half wave shaped current.
Sources are batteries and rectifiers.
Alternating Current (AC)– sinusoidally shaped,
220V, 50Hz ( in Slovakia) USA: 120V, 60Hz
Human Body and Electricity
Body is a non-uniform space conductor where
an electric current flows according to electric
tissue conductivity.Body tissues have different resistances (impedances), and therefore
different conductances (spreading).
The highest el. resistance (Impedance) have
the fat and bone tissues (R = 40-50 Ω/m)
The lowest el. resistance (Impedance) have a
blood, liquor, lymphatic fluid (R = 0.8 Ω/m)
El. resistance for dry skin is high (R=1000 Ω/m)
El. resist. for moistured skin is low (R=50 Ω/m)
Impedance, Resistance and Reactance
Biological Effects of Electric Currents on
Human Body
Both DC and AC Currents have 3 main effects :
Stimulatory, Electrolytic, Thermal.
I. Stimulatory – is typical for AC current with
f = 50-100 Hz, but also for DC current
Max. harmless AC current is 5 mA, and for DC
current is 25 mA!!
Stimulatory effects of DC current (a) and AC (b)
are used in electrodiagnostic method:
a) Chronaximetry (e.g. chronaxi for sceletal muscle
is 1.0 msec, but for nerve is 0.1 ms. Thus, an
excitability of nerve tissue is 10-times higher
than that one for muscle tissue)
b) Therapy with stimulatory (diadynamic) currents
Stimulatory Currents in Medicine
1.Rectangular DC Currents mostly used for stimulation of muscles and nerves in rehabilitation
2.Faradic (Triangle) Currents used for stimulation of
sceletal muscles ( muscle twitch, summation,
superposition, tetanus)
3. Sinusoidally shaped AC current
4.Sinusoidally modulated AC currents- relieve a pain
Methods of Electric Stimulation with AC
Currents (see also Therapeutic Methods)
1.Electroconvulsive therapy (electroshocks)
2. Defibrilation (Cardioversion)
They are used in Psychiatry and in Emergency Medicine
Point: Condenser current 2-5 kV / 2-5 ms is
used.
Effects: Re-setting of either 1. brain
function
or the
2. heart action
Methods of Electric Stimulation with
DC Current
Pacemaking
It is a useful method of treatment „sick sinus syndrome“
(i.e. the sinoatrial (SA) node is not working properly,
and as a result the bradycardia and collapse may
appear in patient.
Point: A pacemaker device (battery operated generator
of DC current) is sewed under the skin around a clavicula, or abdomen. Under X-ray method the wires with
hook electrodes are introduced through caval veins
into the heart right atrium, or into both the right atrium
and the right ventricle ON DEMAND pacemaker- it
works (stimulates the heart) only when SA node is
silent.
Heart Pacemaker
II. Electrolytic Effect
Is a typical feature for DC current, when
passing the salt solution
E.g.: NaCl when dissolved in water breakes into Na+ + Cl - ions
When DC electric current passes such
solution, then Na+ cations run towards the
– Cathode. Similarly Cl – anions run towards the + ANODE
Methods that use Electrolytic Effect of DC
Current (see also Therapeutic methods)
IONOPHORESIS is an electric method of an introducing
of drugs dissolved in water to the eye, knee, joints,
skin ...
Point: The method based on the fact that ions with the
same charges are repelled. Contrary, the ions with the
opposite charges are attracted.
E.g. Drugs as anesthetics when dissolved in water
create Cations+ . Thus, when Anodal + Current is applied, then drugs are electrically pushed to the body. Or
drugs as antibiotics, aspirine or iodine salts create
Anions-. When Cathodal - Current passes the solution
then dissolved drugs are pushed to the body.
III. Thermal Effect
This effect is typical for AC current, mainly at
frequencies above 100 kHz.
 As a result- a heat is produced in tissues - this
property of High Frequency AC Currents is used
in thermotherapeutical method namedDiathermy.
Types: short wave (band 27 MHz)
ultra short wave (434 MHz)
microwave (2400 MHz)
Magnetic Fields
Permanent (stable) magnets as Earth (it results
of rotation of hot magma within the Earth core)
Non-stable artificial magnetic fields – arise in a
vicinity of wire (within a coil) under electricity
Magnetic fields with low frequency - increase
the excitability of brain, decrease the blood flow
in vessels with a high diameter - aorta (this in
turn, may decrease the blood pressure)
Magnetic fields with high frequency- have
mainly thermal effects (used in Diathermy)
Both types of fields are used in the Nuclear
Magnetic Resonance Scanning (NMRS)
Electromagnetic Field
Patient Protection Against Electric
Shock –Basic Rules
1.
At a time of Electric Recording (e.g. ENG,
ECG..) or at a time of Defibrilation
remember: NEVER ground a patient ! For
diagnostic and treatment goals doctors must
use only devices approved for their use in
Medicine, being equipped with „separatory“
transformes. Hence, in the time of an
accident i.e. „a short circuit“- the transformer
is able to disconnect the primary el. circuit ,
220V, from the secondary (patient) circuit.
The harmful effect of electricity on a
living body depends on Intensity of el.
current, on the impedance (resistivity of a
tissue),and on the insulation of the body
against the ground.
Electric shock is caused only by Electric Curent, which is determined both by a Voltage and body Resistance.(OHM´S LAW)