ELECTROCARDIOGRAPHY
|
ELECTROCARDIOGRAPHY
AIM
To determine the ECG of a given subject
PRINCIPLE
Prior to each contraction an electrical impulse is generated in the S.
A. node and
thus
transmitted
to AV node,
Bundil of
this
Purkinje
fibres,
ventricular
muscle fibre
and lastly
to the
surrounding
tissue. Body
is a volume
conductor
and heart
muscle being
the
electrode
generator
with 2
opposite
poles bathed
in it.
Underthis
condition
electrical
impulses
which is
initiated in
cardiac
muscle will
be
transmitted
throughout
body. If
suitable
electrodes
are placed
on the body,
opposite to
the heart
and
connected to
a very
sensitive
Galvanometer
with a
recording
device. Then
the
electrical
potential
can be
recorded.
The record
is
electrocardiogram
(E. C. G.).
The machine
is
electrocardiograph
by which it
is recorded.
APPARATUS
12 Leads, recording device, ECG graph paper, cardiogelly
PROCEDURE
When electrocardiographic connections are made between two parts of the
body then
the specific
arrangement
of
each pairof connections is designated as lead. 12 leads are used. The
different
leads
connected
are
1. Standard limb lead
2. Chest lead
3. Augmented unipolar limb leads
Standard limb lead Lead I, Lead II and Lead III
Lead I: Combination will be made by connecting the left arm to the +ve
pole of the
galvanometer
and right
arm to
-ve pole of galvanometer. Lead II: The combinations of lead II will be
made by
connecting
right arm to
the -ve pole
and left leg
to +ve pole.
Lead III :
The
combinations
will be made
by
connecting
left arm to
-ve pole and
left leg to
-ve pole.
Einthoven’s triangle is an equilateral triangle drawn arbitorately
around the
area of
heart.
Einthoven’s
law states
that, if ECG
is recorded
through
standard
limb leads
then some
total
voltage of
QRS
(ventricular
complex) in
leads land
Ill
isequivalent
to lead II.
Normal ECG recorded in standard limb lead. ECG recorded simultaneously
in 3 leads.
It shows
mostly
similar in
shape,
contour and
feature.
QPRT waves
are positive
waves in all
the lead.
Interpretation of ECG
It shows the following 5 consecutive waves PQRST. There are two
isoelectric
period -
shortes are
between S
and T. P,
Rand Tare
upward
deflections,
while Q & S
are downward
origin. The
waves are
therefore
alternatively
up and down.
P is of
atrial
origin hence
called
atrial
complex
while QRST
being of
ventricular
complex.
P - Wave
This is first upward deflection. It is small but constant wave having a
rounded or
pointed top.
It is
depolarised
wave of
atria,
caused by
the passage
of atrial
current over
atria. Its
average
duration is
about .15 1
amplitude
.25 my,
prepolarization
wave of
atria is
submerged
within the
ventricular
complex.
The impulse arrives at theAV node at about the summit P. Normal P
indicate. 1.
impulse is
originating
in SA node.
2. spread
over atria
in usual
direction.
3. There is
no
deflection
conduction.
Q-Wave
As soon as impulse arrives at muscular part of septum the latter
contracts
producing
the first
wave. Q - T
interval
- It measure the ventricular total systolic time measured from onset of
Q wave to
each of T
wave. It is
about 0.365.
RS - T interval - sagging indicate hypoxia
T - P interval - It is measured from and of T wave to begning of P wave.
T - P
interval
actually
measure
diastolic
period of
heart.
Unipolar limb leads
Electrodes are placed respectively on the left arm, left leg and right
arm. They
are
connected
together to
form a
central
terminal
which could
passes
through a
suitable
resistence
and kept
almost 0
potential.
This is
indifferent
electrode.
Other
electrode is
placed
different
part of body
is called
exploring
electrodes,
records the
local
unmodified
action
current.
The following unipolar leads are Lead V R, Lead VL, Lead VF, Lead VR.
Right arm is
+ve and left
arm and foot
is -ve.
When lead is vertical,
VL resembles VR. ‘R’ wave is small, followed by a big S wave, T wave is
inverted.
Lead VF - Left foot is +ve, right arm and left arm is -ve. Large R wave
followed by
S wave T
wave is
erect.
Augmented unipolar limb leads The unipolar limb leads amplitude are
small.
Chest lead
There are six chest lead, Vi, V2, V3, V4, V5, V6. In chest leads
electrode is
placed on
the anterior
surface of
the chest
and
connected to
posture pole
of
galvanometers
and other
electrode
the
undifferent
electrode
place
anywhere in
the body is
connected to
-ve pole of
galvanometer.
One such
combination
makes the
pre-cardial
positions.
They are:
V1 - fourth intercostal space at 2.54 cm away from right sternal border.
V2 - fourth
intercostal
space at
2.54 cm away
from left
sternal
border. V3
-At the
midpoint
between V2
and V4.
V4 - Fifth intercostal space at left midclavicular line.
VS -At point where anterior axillary line intersects perpendicularly the
horizontal
line
extended
from V4. V6
-At point
where
midaxillary
line
intersects
perpendicularly
the
horizontal
line
extended
from V4.
PROCEDURE
The electrographic connections are made according to the different leads
and ECG
recorded.
Significance
of various
leads and
their
limitation
1) Standard
limb leads
are most
valuable for
diagnosis of
arrythma and
also
functional
abnormalities
of heart. 2)
Pre -
cardial
chest leads
are
important
fordiagnosis
for a)
localisation
of recent or
old
ventricular
damage b)
bundle
branch block
c) detection
of
ventricular
hypertrophy
3)Augmental
limb leads
are most
valuable for
a) determine
the position
of heart b)
confirming
the
significance
of Q and T
wave in
standard
leads. C)
Confirming
the evidence
of
ventricular
damage or
hypertrophy.
Clinical Importance
It gives a fairly accurate information of the condition of atria and
ventricles.
In cardiac
abnormalities
characteristic
variations
occur in the
electrocardiogram
along with
side by side
clinical
findings may
act as a
dependable
guide to the
diagnosis,
prognosis
and
treatment.
Questions:
1i. Draw and label the normal ECG from lead II?
2. Enumerate the uses of ECG? 3. Drawthe ECG of the following conditions? a. Myocardial Infarction b. Complete heart block.
|
|
|
|
|