Download ECG

ECG
Dr Mahvash Khan
MBBS, MPhil
• The ECG is a record of the overall spread of
electrical activity through the heart.
• The electrical currents generated by cardiac
muscle during depolarization and
repolarization spread into the tissues around
the heart and are conducted through the body
fluids. A small part of this electrical activity
reaches the body surface where it can be
detected using recording electrodes. The
record produced is an electrocardiogram or
ECG.
• An ECG is a recording of that part of the
electrical activity present in body fluids from
the cardiac impulse that reaches the body
surface, not a direct recording of the actual
electrical activity of the heart.
• The ECG is a complex recording representing
the overall spread of activity throughout the
heart during depolarization and repolarization.
It is not a recording of a single action potential
in a single cell at a single point in time.
• The recording represents comparisons in
voltage detected by electrodes at two different
points on the body surface, not the actual
potential. For example, the ECG does not
record a potential at all when the ventricular
muscle is either completely depolarized or
completely repolarized; both electrodes are
viewing the same potential, so no difference in
potential between the two electrodes is
recorded.
• The exact pattern of electrical activity recorded
from the body surface depends on the
orientation of the recording electrodes.
• ECG records routinely consist of 12 conventional
electrode systems, or leads. The 12 different leads
each record electrical activity in the heart from
different locations—six different electrical
arrangements from the limbs and six chest leads
at various sites around the heart.
• To provide a common basis for comparison and
for recognizing deviations from normal, the same
12 leads are routinely used in all ECG recordings
• Different parts of the ECG record can be
correlated to specific cardiac events.
• Firing of the SA node does not generate
enough electrical activity to reach the body
surface so no wave is recorded for SA nodal
depolarization. Therefore the first recorded
wave, the P wave, occurs when the impulse or
wave of depolarization spreads across the atria.
• In a normal ECG, no separate wave for atrial
repolarization is visible. The electrical activity
associated with atrial repolarization normally
occurs simultaneously with ventricular
depolarization and is masked by the QRS
complex.
• The P wave is much smaller than the QRS
complex, because the atria have a much
smaller muscle mass than the ventricles
and consequently generate less electrical
activity.
• At the following three points in time no net
current flow is taking place in the heart
musculature so the ECG remains at
baseline
• During the AV nodal delay. This delay is
represented by the interval of time between the
end of P and the onset of QRS complex. This
segment of the ECG is known as the PR
segment.
Current is flowing through the AV node but
the magnitude is too small for the ECG
electrodes to detect.
• The ECG can be used to diagnose abnormal
heart rates, arrhythmias and damage of
heart muscle.
When the ventricles are completely depolarized and
the cardiac contractile cells are undergoing the
plateau phase of their action potential before they
repolarize, represented by the ST segment.
• When the heart muscle is completely
repolarized and at rest and ventricular filling is
taking place after the T wave and before the
next P wave. This period is called the TP
interval.