ECG basics

ECG basics


Electrocardiography is a method of visualization
of the electrical activity of the heart. It allows seeing the processes of depolarization
and repolarization in the heart over a certain period of time using electrodes placed on
the patient’s skin. Electrodes are the conductive pads. ECG device registers an electrical potential
difference between the two certain locations. The pair of electrodes may measure the electrical
potential difference between two locations where electrodes are placed. Such pair forms a lead. But, also, a lead can be formed between a
physical electrode and a virtual electrode. It has an average electrical potential between
three limbs, where electrodes are placed. For example, the electrical potential difference
between the right arm and the left leg, where electrodes are placed, is recorded as lead II. When depolarization or repolarization wave
is directed towards the left leg electrode, a positive deflection of line is recorded
on ECG. To better understand this, let’s imagine
only one pair of electrodes: a positive and a negative one. Cells at rest have a negative charge when
the charge on the outside of the cells are positive. During depolarization, cells become charged
positively, and the charge on the outside of the cells is negative. During repolarization cells get negative charge
again. During the conduction of depolarization wave,
there are cells that are already depolarized, and cells that are not yet depolarized. There is a difference of charge across this
set of cells. We can think about this as being a dipole. It may be marked as a vector that is directed
from a negative charge to a positive one. It’s important that electrodes detect the
charge on the outside of the cells. Direction and magnitude of the electrical
forces are defined as a vector. Depolarization or repolarization wave is recorded
by electrodes placed on the skin. But the electrocardiography is based on a
comparison of the “heart vector” and “recording lead vector”. The “heart vector” means direction and
magnitude of the depolarization or repolarization waves, it’s an above-mentioned dipole. The “recording lead vector” concerns a
vector of ECG recording, between two electrodes. If the “heart vector” runs parallel, in
other words, “heart vector” is of the same direction as “recording lead vector”,
we see deflection strongly upward from baseline in this lead, in other words, there is positive
deflection. If cardiac currents are directed obliquely
to the “recording lead vector”, deflection is positive, but its amplitude is less. The amplitude depends on the angle between
the “heart vector” and “recording lead vector”. The “heart vector” is subdivided into
two vectors: the first one that is a projection of the “heart vector” on the line between
two electrodes and the second one, that is perpendicular to this line, and it’s connected
with the end of the “heart vector”. Projective vector is always not bigger than
the initial one. If the direction is perpendicular to the axis
of the lead, there is no deflection. If the direction of the “heart vector”
is opposite to the axis of the lead, downward deflection is registered. If there is no current flow, no deflection
on ECG is seen, we see only an isoline. The high magnitude of “heart vector” causes
a more prominent positive or negative deflection on ECG, except the case, when the “heart
vector” is perpendicular.