ECG: Detailed Notes

What is an ECG?

  • Definition: Electrocardiogram (ECG) is a graphical representation of the electrical activity of the heart during a cardiac cycle.
  • Purpose: Used to monitor the health of the heart.
  • Standard ECG: Obtained by connecting the patient to a machine with three electrical leads:
    • One to each wrist.
    • One to the left ankle.
  • Multiple Leads: For a detailed evaluation of the heart's function, multiple leads are attached to the chest region.

Waves of a Standard ECG

  • P-wave:
    • Represents the electrical excitation (depolarization) of the atria.
    • Leads to the contraction of both atria.
  • QRS complex:
    • Represents the depolarization of the ventricles.
    • Initiates the ventricular contraction (systole).
    • The Q wave is a small downward deflection.
    • The R wave is the first upward deflection.
    • The S wave is the subsequent downward deflection.
  • T-wave:
    • Represents the return of the ventricles from the excited to the normal state (repolarization).
    • The end of the T-wave marks the end of systole.

Interpretation of an ECG

  • Counting Heartbeats: By counting the number of QRS complexes that occur in a given time period, one can determine the heart beat rate of an individual.
  • Clinical Significance: Any deviation in the shape, duration, or frequency of these waves indicates a possible abnormality or disease of the heart.

ECG Segments and Intervals

  • PR Interval: From the beginning of the P wave to the beginning of the QRS complex. Represents the time taken for electrical impulse to travel from the atria to the ventricles.
  • ST Segment: From the end of the S wave to the beginning of the T wave. Represents the period when the ventricles are depolarized and contracting.
  • QT Interval: From the beginning of the QRS complex to the end of the T wave. Represents the total time for ventricular depolarization and repolarization.

Common ECG Abnormalities

  • Arrhythmias: Irregular heart rhythms.
    • Tachycardia: Abnormally fast heart rate (>100 bpm).
    • Bradycardia: Abnormally slow heart rate (<60 bpm).
  • Myocardial Infarction (Heart Attack): Often indicated by ST segment elevation or depression.
  • Ischemia: Reduced blood flow to the heart muscle.
  • Enlarged Heart Chambers: Can be indicated by changes in wave amplitude or duration.