Electrocardiography (ECG/EKG) - basics

Electrocardiogram (ECG)

  • Purpose: Tool for visualizing the electrical activity of the heart.

  • Alternate Names: Also known as EKG in German and Dutch.

Basics of ECG Tracing

  • Depolarization Wave: Shows positive charge movement during each heartbeat.

  • Electrodes: Configuration impacts tracing; example – Lead II (right arm and left leg) gives positive deflection towards left leg electrode.

Charge Differences in Heart Cells

  • Resting State: Cells are negatively charged relative to their environment.

  • Depolarization: Cells become positively charged during depolarization, creating a charge difference.

  • Dipole Representation: Can represent dipole as an arrow pointing towards the positive charge.

  • Positive Deflection: When a dipole vector points to a positive electrode, results in positive deflection on ECG tracing.

  • Repolarization: A subsequent wave changes the dipole direction, causing negative deflection.

Vector Components in ECG

  • Vector Analysis: The deflection on an ECG is determined by the component of the dipole vector parallel to the electrodes.

    • Perpendicular Component: Does not contribute to deflection.

    • Angle of Depolarization: If the depolarization wave is at an angle, the size of the ECG deflection corresponds to the magnitude of the dipole in the direction of the electrode.

Standard ECG Configuration

  • Electrode Arrangement: 10 electrodes in total:

    • Limb Electrodes: Four (arms and legs)

    • Precordial Electrodes: Six (V1 to V6 around the chest)

  • Right Leg Lead: Typically used as a neutral lead, providing a reference.

Understanding Lead Types

  • Chest Leads (V1-V6): Set up horizontally to detect positive charge waves moving toward them.

  • Augmented Vector Leads:

    • aVR: Right arm (vector up 30 degrees)

    • aVL: Left arm (vector up 30 degrees)

    • aVF: Left foot (downward vector)

  • Bipolar Limb Leads:

    • Lead I: Right arm (negative) to left arm (positive).

    • Lead II: Right arm (negative) to left leg (positive).

    • Lead III: Left arm (negative) to left leg (positive).

Total Leads and Purpose

  • 12-Lead ECG: 6 leads from limbs and 6 from chest leads, allowing for varied views of heart activity.

  • Depolarization Wave (QRS Complex): Registered differently across leads based on their positions and the direction of the electrical activity.

Grouping Leads by Regions

  • Inferior Leads (II, III, aVF): Closest to the inferior wall, supplied by right coronary artery.

  • Lateral Leads (I, aVL, V5, V6): Near lateral wall, supplied by left circumflex artery.

  • Septal Leads (V1, V2): Near the interventricular septum, supplying arteries from the left anterior descending artery.

  • Anterior Leads (V3, V4): Closest to the anterior wall, also receiving from the left anterior descending artery.

Summary

  • Key Takeaway: ECG utilizes 10 electrodes to create 12 leads to monitor heart activity. The tracing reflects depolarization wave movement towards or away from electrodes, which informs on the heart's function and structure.