ECG Electrocardiogram
The electrocardiogram (ECG) is a record of the electric potential
changes that occur in the heart during the cardiac cycle. It is recorded from
the surface of the body. The instrument used to record the ECG is called
Electrocardiogram. The waves of the ECG are due to depolarization and not
due to contraction of the heart. This wave of depolarization occurs first
before the contraction of the cardiac muscle begins.
The electrical activity of the heart was first recorded by Waller in
1887 with a capillary electrometer. But the work of Einthoven who recorded
the ECG with a strong galvanometer only lead to the development of modern
electrocardiography. Einthovan was awarded Nobel Prize in 1924.
A normal ECG is composed of five waves designated from left to
right with the letters P,Q,R,S and T. P,R and T are normally upward or
positive waves while Q and S are downward or negative waves.
ECG-’PQRST’ wave
When the cardiac impulse (originating in sinus node which is the
primary pace maker) passes through the heart, electrical currents spread in
the tissues surrounding the heart. A small amount of this current spreads to
the surface of the body. If electrodes are placed on the skin on opposite
sides of the heart, electric potentials generated by these currents can be
recorded. This recording is known as electrocardiogram (ECGorEKG).
P.Wave: It occurs in the auricles. It is an atrial wave. It is due to the
spread of depolarisation in the atria (auricles). Its duration is 0.1 second and
it occurs just before the atrial systole. Its amplitude is about 0.1 to 0.3 mv.The
cardiac impulse reaches the sinu-auricular node at about the summit of the
Pwave. The P wave is a guide to the activity of atria.
Q,R, and S Waves
After the completion of P wave, the isoelectric interval occurs.
Following this, QR and S waves begin. Q wave is a small negative
downward deflection. It is mostly indistinct. It represents atrial septal
depolarization. R wave is a prominent positive wave and S wave is a small
negative wave. R and S are due to depolarization of the ventricular muscle.
The duration of the QRS complex is about 0.08 second and usually does not
exceed 0.1second. The average amplitude to R wave is about 1 mv. Lot of
diagnostic information can be gained from alteration in the QRS complex.
T wave
Following S wave there is an isoelectric interval. T wave begins after
that. It is due to ventricular repolarization. It is a broad wave. Its average
duration is about 0.27 second and amplitude 0.15 to 0.5 mv.
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