Transcript Microshock. - Home - KSU Faculty Member websites

‫فيز ‪ : 101‬محاضرة رقم ‪:7‬‬
‫الكهرباء‬
‫‪Electricity.‬‬
The Universe
is made up of
Energy (ability to do work) & Matter (has physical form).
Electricity is a some of Energy.
Atoms
Hydrogen
Oxygen
Hydrogen
Oxygen
Conductor: Matter which supports electron flow.
Metals – Salty Fluids – Human Body.
Insulator: Matter which does not support electron flow.
In these substances the out electrons are rigidly held in place.
Rubber – Glass – Wood.
Conductor carries the flow
of electrons – current.
Insulator prevents the conductor
coming into contact with other
Conductors (including people).
Some materials are half way between a Conductor and Insulator.
We say they allow current to flow but offer some resistance.
It’s a Conductor but its not a very good one.
It resists the flow of electrons.
Electrons release energy as they push their way through
Such materials – examples the wire in the element of an electric fire.
You can see it glow red hot when you switch on the fire and
Electricity flows through the wire.
Basic Circuit Theory
V
e_ e_ e_ e_
Voltage is equivalent to pressure.
V
e_ e_ e_ e_
Current is equivalent to flow.
e_
V
e_
e_
e_
Chemical Energy
Electrical Energy
Light Energy
Ohms Law.
V = I.R
Voltage = Current x Resistance
Electricity is an efficient way to move energy around.
The ESB turn the kinetic energy of the water moving in
the Shannon into electrical energy – this is what a
power station does.
The electricity is piped (through copper wires) to the hospitals
and home we live in.
We then turn the electricity back into the form of energy
we need either – light, or movement etc.
Here is a simple explanation of what the ESB do.
Shannon River
(source of
Mechanical Energy)
e_
ESB
e_
A+ A+ A+ A+
e_
e_
Hospital
e_
e_
e_
ESB
Hospital
e_
A+ A+ A+ A+
e_
e_
ESB
Hospital
e_
A+ A+ A+ A+
e_
e_
ESB
Hospital
e_
e_
A+ A+ A+
ESB
Hospital
e_
e_
A+ A+
ESB
Hospital
e_
A+
ESB
Hospital
e_
e_
ESB
e_
Hospital
e_
e_
e_
A+ A+ A+ A+
I = V/R
ESB
Hospital
V = 220
I = V/R
ESB
Hospital
V = 220
I = V/R
ESB
Hospital
V = 220
I = V/R
ESB
Hospital
V = 220
G
Hospital
Thermal Energy
Kettle/heaters
Mechanical Energy Food processor
Electrical Energy to……….
Radiation Energy
Light Bulb
Acoustic Energy
Hi-Fi
Chemical Energy
Mobile Phone
Battery Charger
If the insulators break down and you come in contact with the mains conductors
then the current might flow down to ground through you giving you an electric
shock.
In fact the path of the current in through you, through the mass of the earth,
and back to the transformer via capacitance.
Nursing student do not need to be able to explain capacitance, but you
Must be aware that the mains current will try and flow to ground, through you or
your patients.
To protect from electric shock we add in an extra wire called the earth wire.
Now if the insulation breaks down, the current will take the easy path back
through the green wire, rather than going down through you.
Normally current that flows if up to 13 amps. Its is limited by the resistance
of the device you plug in.
In the event of a break down in insulation, there is no resistance to limit the current
and 100’s of amps can flow.
Current limited by resistance
of the load – typically 1 amp.
Fuse/MCB.
A fuse is a conductor which will melt if the current exceed a certain limit.
The 13 amp fuse in most plugs will blow and disconnect the circuit making it safe,
If the current exceeds 13 (usually associated with a failure in insulation).
Current limited by resistance
of the earth wire – typically 50 amp
- Fire Hazard.
Fuse/MCB.
Circuit Isolated.
Fuse/MCB.
Earthing provides a Safe Low impedence path for Fault Current.
Fuse/MCB.
Leakage Current
N.B. There are allways some small (and usually safe) currents flowing the
casing to earth, and also from parts we connect to patients such as ECG Leads.
These are called Leakage Currents and Manufacturers must design equipment
To ensure all leakage currents are within safe limits.
Fuse/MCB.
The most common electrical fault in hospitals because cable are pulled by accident.
Fuse/MCB.
N.B. If this happens you have lost
the safety offered by the earth wire.
Fuse/MCB.
You get the shock
as the current has
no safe path
What is an electric Shock ?
First we will review nerve conduction.
Then we will see that the normal action of nerve cells is
interputed as current is passed through the body,
Finally we will look at the different effects as the amount
of current increases.
How Human Nerve Cells Transmit Signals.
Dendrites
Axon
Synapse
How Human Nerve Cells Transmit Signals.
Dendrites
Synapse
Axon
Na+
The Axon maintains a chemical
balance with more potassium ions
inside the cell and sodiom ions
outside the cell.
K+
Na+
How Human Nerve Cells Transmit Signals.
Dendrites
When signal is transmitted the myelin
sheet changes so that the sodium
and potassium ions change places.
This results in an electrical change in
the cell and this in turn causes the
next section of myelin to change.
Synapse
Axon
Na+
Na+
K+
K+
Na+
How Human Nerve Cells Transmit Signals.
Dendrites
Axon
K+
Synapse
How Human Nerve Cells Transmit Signals.
Dendrites
Synapse
Axon
K+
How Human Nerve Cells Transmit Signals.
Dendrites
Axon
Synapse
K+
External Electrical Stimulation of Human Nerve Cells
Electricity flowing through the human body can cause enough of
a change in the electrical environment around a nerve cell to stimulate it.
Effects of Mains Derived Current
on the Human Body.
Macroshock:
Direct Body Contact
(only some of the current goes through the heart).
Effects of Mains Derived Current
on the Human Body.
As current increases the effects
get more severe.
Tingly feeling
Perception
Stimulates muscles
And you cant let go
Can not let go Current
Interuption of Normal Cardiac Function
Effects of Mains Derived Current
on the Human Body.
Macroshock.
Ventricular Fibrillation:
I > 50
mA
Can’t let go:
I> 5
mA
Tingling Sensation
I > 0.5 mA
A mA is one thousenth of an Amp.
So very small current can cause physiological effects.
Leakage Currents can be dangerous.
Electrical Safety.
1. Properly fitted plugs
2. Plugs should have the correct fuse
3. Routine Safety Testing
4. Do not place fluids on top of electrical devices
5. Do not plug in equipment when you have wet hands
6. Report all faults
7. Report all frayed cables, broken plugs or any plugs that feel warm.
Now going to talk about Microshock.
Read pages 265 to 268 in..
Science in Nursing and Health Care
And an even better explanation on page 334 in….
Science in Nursing by Laurie Cree and Sandra Rischmiller
4th Edition
Published by Mosby.
ISBN 0 7295 3260 7
Effects of Mains Derived Current
on the Human Body.
Microshock: Direct Cardiac Contact all current goes through the heart.
Microshock can occur in Hospitals.
We often make a conductive connection with the heart
- pacing wire
- Central Line: saline fluid is conductive
Even tiny current which we can not feel (below 0.5 mA)
Are enough to cause heart failure (if greater that 0.05 mA).
C.D. Swerdlow, W.H. Olson, M.E. O’Connor, D.M. Gallik, R.A. Malkin & M.Laks.
Cardiovascular collapse caused by
Intracardiac Leakage Current.
Circulation 1999 May 18; 99 (19): 2559 - 64
Intermittant VT
0.02 mA
Continuous VT
0.03 mA
Ventricular Fibrillation
0.05 mA
Effects of Mains Derived Current
on the Human Body.
Macroshock.
Ventricular Fibrillation:
I > 50
mA
Can’t let go:
I> 5
mA
Tingling Sensation
I > 0.5 mA
Microshock.
Ventricular Fibrillation:
I > 0.05 mA
Effects of Mains Derived Current
on the Human Body.
Safe Limits for allowable
Currents that can flow
from Devices.
Macroshock.
Ventricular Fibrillation:
I > 50
mA
Can’t let go:
I> 5
mA
Tingling Sensation
I > 0.5 mA
0.1 mA
Cardiac Contact.
Microshock.
Ventricular Fibrillation:
Body Contact.
I > 0.05 mA
0.01 mA
Effects of Mains Derived Current
on the Human Body.
Macroshock.
Body Contact
Staff or Patients.
Tingling Sensation
I > 0.5 mA
Cardiac Contact.
Microshock.
Ventricular Fibrillation:
0.1 mA
I > 0.05 mA
0.01 mA
For Electrical Parts that Come in Contact with the Body,
the Leakage Current Must be below 0.1 mA.
This symbol indicates that parts are safe to connect to a patients Body.
Patient Body
Connection.
For Electrical Parts that Come in Contact with the Heart,
the Leakage Current Must be below 0.01 mA.
This symbol indicates that parts are safe to connect to a patients Heart.
Patient Direct Cardiac
Connection.
If you are handling patient connections that make direct cardiac contact you need
to take particular care. Even if all devices are safe, the tiny currents of 0.01 mA
can flow from the casing of of one device to another, through you or the patient
and cause a Microshock to the Patient.
The current is so small you wont feel it !
Micro Shock Risk – Reduced by use of Equipotential Bonding
In areas where this happens (ICU’s CCU’s Cardiac Angio Labs etc)
Extra bonding cables are used reduce the risk of Microshock.
These are long green cables attached to the normal mains leads and they
are plugged into special connectors.
Electrical Safety.
1. Properly fitted plugs
2. Plugs should have the correct fuse
3. Routine Safety Testing
4. Do not place fluids on top of electrical devices
5. Do not plug in equipment when you have wet hands
6. Report all faults
7. Report all frayed cables, broken plugs or any plugs that feel warm.
Microshock Reduction.
8. Use the extra bonding systems
9. Never touch pacing wire or CVP line and equipment at same time.