An electrocardiogram (ECG) records the electrical activity of the heart. A normal ECG waveform contains a P wave, QRS complex, and T wave. The P wave represents atrial depolarization, the QRS complex represents ventricular depolarization, and the T wave represents ventricular repolarization. An ECG also contains isoelectric lines, waves, segments, intervals, and a voltage and time calibration to analyze the tracing.

1. Electrocardiogram (ECG)

2. Normal ECG
• Definition: it is the recording of electrical activity generated by the
cells of the heart that reaches the body surface. OR
• It is the graphical representation of electrical activity in the heart.
• The normal electrocardiogram is composed of a P wave, a QRS
complex, and a T wave. The QRS complex is often, but not always,
three separate waves: the Q wave, the R wave, and the S wave. The
P wave is caused by electrical potentials generated when the atria
depolarize before atrial contraction begins. The QRS complex is
caused by potentials generated when the ventricles depolarize
before contraction, that is, as the depolarization wave spreads
through the ventricles. Therefore, both the P wave and the
components of the QRS complex are depolarization waves. The T
wave is caused by potentials generated as the ventricles recover
from the state of depolarization. Thus, the electrocardiogram is
composed of both depolarization and repolarization waves.

3. Characteristics of Normal ECG
• Baseline or Isoelectric line: The flat or straight line present in the
graph paper of ECG.
• Wave: Any deviation from baseline is called wave. Wave always
starts from baseline and ends on baseline.
• Waveform: Movement away from the baseline, up or down.
• Segment: A line between waveforms or a line (baseline) joining the
two waves (successive waves).
• There are three segments in one ECG:-
• 1) PQ Segment
• 2) ST Segemnt
• 3) TP Segment
• Complex: Two or more waves having no segment between them is
called complex. Eg QRS complex in which there are three waves
having no segment.

4. • Interval: A waveform plus a segment is called interval.
• PR Interval: The time duration between beginning of P
wave and beginning of QRS complex. Normal duration
is 0.12 sec to 0.20 sec.
• QRS Interval: The time duration in which ventricles
depolarise, it is also called ventricular activation (VAT).
Its normal duration is less than 0.11 sec.
• QT Interval: The time duration between the beginning
of Q wave and end of T wave. Its normal duration is
0.35 sec.
• RR Interval: It is the time duration between two
successive R waves. Its normal duration is 0.8 sec.

5. Normal ECG

6. Voltage and Time Calibration of ECG
• Horizantally:
• One small box (1mm)= 0.04 sec
• One large box (5mm)= 0.20 sec
• Five large boxes (25mm)= 1sec
• Vertically:
• One large box (5mm)= 0.5mv
• Two large boxes (10mm)= 1mv