Bioelectricity electron (from atomic model)
Electricity
F = k
Electric Field : concept for the force acting at a distance without direct contact (M. Faraday ) E = F/q : lines of force
Voltage(V), or electromotive force (EMF), is an expression for electric potential or potential difference. [V:volt=joules/coulomb]
Vba = Vb - Va = - Wba / q
Current(I) is a flow of electrical charge carriers.[A:ampere=C/s]
I = dQ/dt
F=E=0
Na+
Cl-
K+
A-
+
-
Membrane Potential(Voltage example)
1. (resistance) : R = rL/A - -
- I = V/R 2. (capacitance) : Q = CVba - (capacitor) :
- I = C dV/dt - : U = Q2/2C
3. (inductance) : L = NF/I - (coil, inductor)
- V = -L dI/dt, - : U = LI2/2
+ + + +
A
B
V
A
B
Two dissimilar metals in contact with a liquid separator
Steady indication of potential
Metallic electricity
Electronics
Muscle convulsion
Animal electricity
Injury/resting/action potential
Galvani-Volta controversy
an action potential is conducted at speeds that range from 1 to 100m (3 to 300 feet) per second, depending on the properties of the fibre and its environment.
Conduction of Action Potential
“Sensor” “Actuator”
Electrode() Electrode()
x
i i m m
i o i i
v i x t i x dx t adx c j
t
i x t di i x t i x dx t dx
x
Current cross surface BB’
Consider the external medium. Current io is injected at the origin of an infinite homogeneous medium of conductivity o.
io
o
r
2
r r
i i q v r Edr dr cf v r
r r r
r x r
A B
[1 ( )cos ]
r x r x
r
P r v
r
Active cell bathing medium body surface (Constant Current Source) (Volume Conductor) (Potential Difference)
Na+ Cl-
ECG lead EEG lead
Acute anterior MI
EMG Waveform
The international 10-20 system seen from (A) left and (B) above the head. A = Ear lobe, C = central, Pg = nasopharyngeal, P = parietal, F = frontal, Fp = frontal polar, O = occipital. (C) Location and nomenclature of the intermediate 10% electrodes, as standardized by the American Electroencephalographic Society. (Redrawn from Sharbrough, 1991.).
EEG – electrode position
(A) Bipolar and (B) unipolar measurements.
Note that the waveform of the EEG depends on the measurement location.
EEG – waveforms
Biopotential Measurement-electrode
Electrode - definition
– Pure electrolytes: charge carriers are ions, no separate flow
of electron.
extracellular liquids contain ions free to migrate.)
• Electrode :
source and sink of electrons
– The electrode is the site of a charge carrier shift, a charge
exchange between electrons and ions.
Electrode - types
• Two Uses
– Delivery of current to living tissue : high iD
• Three Types
tissue interfaces
diathermy)
chemical transducer
EEG Single Disc Electrodes Disposable ECG Electrodes
Electrode Types
charge distribution in a region.
Perfectly polarized(nonreversible) : electrodes in which
no net transfer of charge occurs across the metal-
electrolyte interface
unhindered exchange of charge is possible
Real electrodes lie between these idealized limits
Other meaning of “polarization”: the condition in which
the electrode-electrolyte potential is altered by the
passage of current
Polarized Electrode : no net transfer of charge across the metal/electrolyte interface
-capactive-
-resistive-
Ag
AgCl
e-
Cl-
Metal
Electrolyte
Several F (Due to small spacing of the double layer)
Electrical Stimulation & Shock
physiological response to current
muscle)
Electrical Shock - Definition
e- Charge carrier
Electricity Conduction Mechanism
Type Current range Physiological (60Hz) (mA) effects Threshold 1 – 5 tingling sensation Pain 5 – 8 intense or painful sensation Let-go 8 - 20 threshold of involuntary muscle contraction Paralysis >20 respiratory paralysis and pain Fibrillation 80–1,000 ventricular and heart fibrillation Defibrillation 1,000 - sustained 10,000 myocardial contraction and possible tissue burns
Physiological effects of Electricity
Electrical Shock - Types
Effect of entry point on current distribution
Fibrillation current : lead I > lead II, III For dog: 20A, human : 80~600 A Safety limit : 10 A
Important susceptibility parameters
Ex) 120V, 500W instruments, power-cord resistance 0.25
Then the voltage drop across the neutral to ground is 0.25(500/120) = 1.04V
Distribution of Electric Power
Macroshock Hazards
Microshock Hazards
passed through the human body, no sensation was
perceived as the frequency was increased beyond 2500-
5000Hz.(100W light bulb connected two human
subjects (arm to arm) burn brilliantly with 1A
current)
Perception Threshold
World War I Navy ship spark-gap transmitter at Harvard Univ.
1925 : W.T. Bovie Harvey Cushing for neurosurgery
1950 : widely used with nonflammable anesthetics
Electrosurgical Unit(ESU) - Bovie
voltages : 1,000 - 10,000 volts peak-to-peak
breakdown field intensity in air : 30kV/cm 0.33cm @ 10,000 V
mode select : cutting, coagulation
return electrode : low resistance
Let-go current from arm to arm as a function of
frequency
How doesn’t ESU produce electrical shock in patients? - 6.3A is required @ 500kHz to generate action potential - causing the cells to vaporize rather than depolarize
Electrical Safety - ESU
Energy levels
- Capacitor charging time: 8sec to 360J/4sec to 200J.
Defibrillation pulse
- Synchronized delay: @ 40 ms from R-wave trigger
Defibrillation electrodes
Pediatric adapter (17 cm2).