How an EKG Readout Is Made
Believe it or not, but
the
human body is full of electrical signals which can be picked up
and
analyzed.
These signals come from nerve impulses, which can cause muscle
contractions.
Voltages range from microvolts (brain waves and some muscle
contractions)
up to around 2 millivolts p-p (peak-to-peak). Certain
conditions
or exercise can increase voltages further. Currents tend
to be no
more than probably a couple microamps or so. Thanks to
God's
wonderful
design of the body, these signals can be picked up and analyzed
non-invasively
through the use of skin electrodes. Because skin
resistance can
be
high (usually in the mega-ohm range), conductive pastes or gels
are
often
used to make it easier to pick up these signals. Luckily,
newer
equipment
eliminates a lot of mess, due to higher input impedances (input
resistance).
It is the varying VOLTAGES that are picked up and
analyzed. These
voltages are amplified greatly (typically by a factor of 1000 X
on EKG
and around 10,000 X for EEG). The output then can go to a
video
monitor,
paper plotter, radio transmitter, and/or to an analog or digital
recording
system.
How to Wire Up Thy Neighbors
Notes
Please note that limb leads ( I, II, III, aVL, aVR, and aVF) do NOT require exact placement. However, better R-waves can be seen when using the limbs as opposed to the chest. However, the chest versions of these leads are more convenient when monitoring for several hours or days at a time. Unless you are doing an EKG for diagnostic reasons, many people would probably oppose the idea of shaving hair off their chests. Also when using limb connections, be sure to place the electrodes over fleshy areas (NOT over bony areas).
Please note that the
green
marks on the drawings represents the ground electrode. The
black
marks represent the positive electrode. The white marks
represents
the negative electrode. If you want to do a quickie and
use just
two electrodes, just short the negative and ground electrodes
together.
However, this technique does make the output MUCH MORE
susceptible to
outside
interference, such as people walking around (not necessarily the
patient).
LEAD 1
LEAD 2
LEAD 3
Chest version of LEAD 1
Chest version of LEAD 2
Chest version of LEAD 3
Modified Chest Lead V1 (MCL1) (or MCL)
This connection is more picky about
the locations of the electrodes.
Modified Chest Lead V4 (MCL4)
This connection is more picky about
the locations of the electrodes.
EKG Amplifier Circuit
Safety Notes on Above Circuit
First of
all,
be sure to check your work when building this circuit. I
will NOT
BE LIABLE for injury or death resulting from the use of this
circuit.
Built it at your own risk. When I built this circuit
using 9-volt
batteries for its power, I had a maximum of 7 microamps
leaking from
any
two electrodes. Leakage currents greater than 20
microamps
renders
such hardware NOT ACCEPTABLE for medical use. Moreover,
the
medical
profession prefers leakage currents to be no more than 10
microamps.
Please be aware that some people can begin to feel a shock
when even
200-300
microamps are passed through the body. However, be sure
to test
all
three electrode connections (positive to negative, negative to
ground,
and positive to ground) after building this circuit.
Please be
aware
that maximum leakage currents will vary due to supply
voltages,
evenness
of individual supply voltages, and tolerances of the
components
themselves.
If you have any doubt of any voltage spikes coming out of a
computer,
oscilloscope,
or A/D (analog-to-digital) converter, please add
opto-coupling.
In
order to make this possible, the output of this circuit will
need to
have
5-9 volts (depending on individual supply voltage)
added, and
then
have a voltage-to-current circuit added. The output of
this extra
circuit is to then go to an opto-coupler chip. Please be
aware
that
if opto-coupling is used, a small amount of extra programming
is
required
due to the loss of linearity. Probably an exponential
function is
in order here.
Here I am--getting wired up!
Here I am--all wired up!
Here is my actual EKG as of September 28, 2001.
Want some wires on YOUR chest?
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