Summary of "EKG Basics | How to Read & Interpret EKGs: Updated Lecture"

This comprehensive lecture covers the fundamental concepts behind EKG (electrocardiogram) interpretation, starting from the physics and physiology of cardiac electrical activity to understanding the 12-lead EKG system and waveform analysis. The instructor systematically explains how electrical signals in the heart generate the characteristic EKG waves and segments, how to interpret deflections based on electrode placement, and the clinical significance of each part of the EKG waveform.

Main Ideas and Concepts

1. Basic Physics and Physiology of EKG Signals

Electrical activity in the heart is generated by depolarization and repolarization of cardiac cells.
Positive ions (sodium, calcium) entering cells cause depolarization, creating a flow of positive charge.
Electrodes detect the direction of this electrical activity relative to their polarity:
- Positive charge moving towards a positive electrode → upward deflection on EKG.
- Positive charge moving away from a positive electrode → downward deflection.
- Negative charge moving towards a negative electrode also produces an upward deflection.
If electrical activity moves perpendicular to the lead axis or there is no net movement, an isoelectric (flat) line appears.

2. Understanding Lead II and the EKG Waveform Components

Lead II is the most commonly used lead for rhythm strips, with the negative electrode on the right arm and positive on the left leg.
The EKG waveform components and their physiological correlates:
- P wave: Atrial depolarization originating from the SA node moving toward the AV node → upward deflection.
- PR segment: Electrical delay at the AV node (about 0.1 seconds), causing an isoelectric line.
- Q wave: Septal depolarization by the left bundle branch moving away from the positive electrode → downward deflection.
- R wave: Ventricular depolarization dominated by the larger left ventricle → upward deflection.
- S wave: Late ventricular depolarization at the bases moving away from the positive electrode → downward deflection.
- ST segment: Entire ventricular myocardium depolarized, no net electrical movement → isoelectric line.
- T wave: Ventricular repolarization, negative charge moving towards the negative electrode → upward deflection.

3. 12-Lead EKG System Overview

Limb Leads (I, II, III): Bipolar leads forming Einthoven’s triangle.
- Lead I: Right arm (-) to left arm (+), views high lateral wall of left ventricle.
- Lead II: Right arm (-) to left leg (+), views inferior wall.
- Lead III: Left arm (-) to left leg (+), also views inferior wall.
Augmented Unipolar Limb Leads (aVR, aVL, aVF):
- aVR: Positive electrode on right arm, views basal septum and right ventricle; waveforms are typically inverted compared to limb leads.
- aVL: Positive electrode on left arm, views high lateral wall (similar to Lead I).
- aVF: Positive electrode on left leg, views inferior wall (similar to Leads II and III).
Precordial (Chest) Leads (V1-V6):
- Unipolar leads placed on the chest at specific anatomical landmarks.
- Provide a horizontal (transverse) plane view of the heart.
- Lead placements:
  - V1: Right 4th intercostal space, sternal border (right ventricle).
  - V2: Left 4th intercostal space, sternal border (basal septum).
  - V3: Between V2 and V4.
  - V4: Left 5th intercostal space, midclavicular line (anterior wall).
  - V5: Left 5th intercostal space, anterior axillary line (lateral wall).
  - V6: Left 5th intercostal space, midaxillary line (lateral wall).
- R wave progression: R wave amplitude increases from V1 to V6.
- S wave progression: S wave amplitude decreases from V1 to V6.
- V1-V3 reflect right ventricle and basal septum; V4 reflects anterior wall; V5-V6 reflect lateral wall.

4. EKG Strip Measurement Basics

Grid size:
- One large box = 5 mm wide = 0.20 seconds in time.
- One small box = 1 mm wide = 0.04 seconds.
- Height corresponds to voltage: 5 mm = 0.5 mV;