2.6 Electrocardiogram

An electrocardiogram (ECG or EKG) is a recording of the electrical impulses that travel through the heart's conduction system during the cardiac cycle. The instrument used to record these impulses is called an electrocardiograph. The electrical changes, resulting from the depolarization and repolarization of cardiac muscle fibers, are detected by electrodes placed on the skin.

Phases Of Heartbeat→

ECG Waveforms and Intervals

The standard ECG waveform consists of specific deflections (waves) and time intervals that correspond to electrical events in the heart.

• P Wave: Represents the depolarization of the atria, which is the electrical signal that triggers atrial contraction (systole). During this wave, the ventricles are in diastole (relaxed).

• QRS Complex: Represents the depolarization of the ventricles. It is composed of three parts:

  • Q wave: The initial downward deflection.
  • R wave: The large, sharp upward spike.
  • S wave: The downward deflection following the R wave.

This complex precedes ventricular contraction (systole), during which blood is ejected from the heart.

• T Wave: Represents ventricular repolarization, the electrical recovery of the ventricles. This event occurs just before the ventricles relax (diastole).

Segments and Intervals

These are the time periods between the waves, which are crucial for assessing the heart's function.

• P-R Interval: The time from the beginning of the P wave to the beginning of the QRS complex. It indicates the time taken for the electrical impulse to travel from the SA node (atria) to the ventricles.

• S-T Segment: The period between the end of the S wave and the beginning of the T wave. Represents the time when the ventricles are completely depolarized.

• J (junction) Point: The specific point where the QRS complex ends and the ST segment begins.

Normal ECG Interval Durations

Uses of an Electrocardiogram

An ECG is a painless, non-invasive diagnostic test used to assess the heart's electrical activity and overall function. It can help detect:

• Arrhythmias (irregular heartbeats).
• Conduction defects (problems with the electrical pathways).
• Abnormal size and position of heart chambers.
• Damage to the heart muscle (e.g., from a heart attack).
• Impaired blood flow to the heart muscle (ischemia).
• The effects of cardiac medications.
• The function of artificial pacemakers.

Heart Problems→ Cardiovascular Disorders→