ECG Findings in Medicine

Sinus Rhythm

• Normal Heart Rhythm
  • Regular P waves followed by QRS complex.
  • Represents normal electrical conduction from sinoatrial node through atrioventricular node into ventricles.
  • PR interval remains constant.
  • QRS complex represents ventricular depolarization, typically narrow (80-100 ms).
  • Rate below 60 bpm: Sinus Bradycardia.
  • Rate above 100 bpm: Sinus Tachycardia.

Atrial Fibrillation

• Abnormal Electrical Activity
  • Atrial muscle fibers contract at different times, leading to quivering.
  • No clear P waves; irregular QRS complexes not preceded by P wave.
  • Leads to irregularly irregular heart rhythm (variable R wave intervals).
  • Often due to atrial dilatation and electrical triggers, e.g., from pulmonary veins.
  • Rates > 100 bpm: Atrial fibrillation with rapid ventricular response.
  • Rates < 60 bpm: Atrial fibrillation with slow ventricular response.

Atrial Flutter

• Coordinated Atrial Electrical Activity
  • Due to re-entry circuit, usually around 300 bpm.
  • Narrow complex tachycardia with "sawtooth" pattern on ECG.
  • Described by conduction ratio (non-conducted to conducted beats).
  • Common 2:1 ratio; higher ratios indicate higher AV block.
  • 1:1 conduction possible, associated with instability/ventricular fibrillation.

Premature Contractions

• Common ECG Findings
  • Premature Ventricular Contractions (PVCs):
    • Originate from His-Purkinje region, appear as wide complex beats (>120 ms).
    • Compensatory pause follows.
  • Premature Atrial Contractions (PACs):
    • P wave with different morphology, followed by narrow QRS.
    • May appear as a pause but are regular in timing.

Bundle Branch Blocks

• Left or Right Block Patterns
  • Wide QRS complexes (>120 ms).
  • Left Bundle Branch Block:
    • Deep S Wave in V1, prolonged R wave in V6 ("W" and "M" shapes).
  • Right Bundle Branch Block:
    • RSR' wave in V1, prolonged S Wave in V6 ("M" and "W" shapes).
  • ST segment affected, challenging interpretation of ST changes.

Atrioventricular Blocks

• First Degree AV Block
  • Prolonged PR interval (>200 ms).
• Second Degree AV Block
  • Mobitz I (Wenckebach): Progressive PR interval lengthening until a non-conducted P wave.
  • Mobitz II: Non-conducted P waves without PR interval change.
• Third Degree AV Block (Complete Block)
  • No association between atrial and ventricular contractions.
  • Independent regular P waves and QRS complexes.

Ventricular Tachycardia and Fibrillation

• Ventricular Tachycardia (VT):
  • Broad complex regular tachycardia, monomorphic or polymorphic (e.g., Torsades de Pointes).
  • Diagnostic uncertainty often treated as VT.
  • Can lead to cardiac arrest or degenerate to ventricular fibrillation.

ST Elevation

• Important ECG Feature, Not an Arrhythmia
  • Reflects various conditions, prominently acute myocardial infarction.
  • Requires >1 mm elevation from baseline, except in leads V2 and V3.
  • Different leads represent distinct heart territories (lateral, inferior, anterior, septal).
  • Reciprocal changes (ST depression) indicate opposite territory effects.

Additional Resources

• Video links provided for further understanding of atrial fibrillation and heart blocks.
• Recommended reading: Life in the Fast Lane, ECG Waves.