The Electrocardiogram
(ECG in English, EKG
in American)
An amplification of
electrical activity in the heart shown on an oscilloscope.
A wave of
depolarisation travelling towards an electrode causes a positive or upward
deflection. A wave travelling away causes a negative deflection. The deflection
seen on the trace is the average signal.
Therefore the
directions of the deflections depend on the position of the electrodes.
Normal rate of paper
feed is 25 mm/sec, this means five large squares per second, so one large
square is 0.2 of a second. There are 5 small squares in a large square.
The machine should be
calibrated to 1 mV = 1 cm ie. two large squares.
The 12 lead ECG
Different leads
"look" from different directions
A lead records the
difference in potential between two electrodes
The
bipolar leads
I, II and III
I = LA - RA
II = LL - RA
III = LL - LA
The right leg is
neutral
The
unipolar leads.
aV, augmented voltage
right, left and foot. Each one is an "exploring electrode"
In Eindhoven`s
triangle the average of I, II and III gives the central terminal R, L and F are
on the limbs
The
6 standard leads
These look from the
vertical plain, ie. up and down, therefore I and aVL look at the left lateral
surface and II, III, aVF and aVR look at the inferior.
aVL and II look from
opposite directions so the trace directions are opposite.
The
6 chest leads
Each looks at the
heart in the horizontal plain
Cardiac
axis
The average direction
of spread of depolarisation through the ventricles as seen from the front. The
axis can be determined from the QRS complexes in I, II and III. The normal is
"11 o clock - 5 o clock"
Right ventricular
hypertrophy will cause right axis deviation, left hypertrophy will cause left
deviation.
In
Children
The normal HR in
neonates is 140 - 160, this falls slowly to about 80 in puberty. At birth the
right and left ventricle have the same thickness, so the ECG indicates, what
for an adult would be a RV hypertrophy.
The
6 chest leads
Each looks at the
heart in the horizontal plain