Defibrillators, CPR, and Cardiac Conduction

Cardiac Arrest in Popular Culture

• Common TV Trope: Patient flatlines, doctors use defibrillator paddles.
• Misconceptions:
  • CPR (Cardiopulmonary Resuscitation)
  • Defibrillator usage
  • Heart's electrical activity

Understanding Cardiac Arrest

• CPR: Helps prolong heart function but often needs defibrillators to save lives.
• Defibrillators: High-voltage shock stops the heart, not restart it.

Heart Cell Characteristics

• Skeletal Muscles vs Cardiac Muscles:
  • Skeletal muscle: Long, multinucleate; can work independently.
  • Cardiac muscle: Squat, branched, interconnected; must work in unison.
  • Cardiac muscle loaded with mitochondria (25-35% of each cell).
• Connective Tissue: Endomysium between cells, full of capillaries for oxygen.

Electrical Activity in Heart Cells

• Similarities with Skeletal Muscle:
  • Actin-myosin sliding filaments for contraction.
  • Uses action potentials.
• Pacemaker Cells: Unique to Heart
  • Generate their own depolarization without external triggers.
  • Have leaky sodium and potassium channels.
  • Located in the Sinoatrial (SA) Node.

Cardiac Conduction System

• Pacemaker Cells: Trigger action potentials.
• Pathway:
  1. SA Node (Natural pacemaker in right atria)
  2. Atrial cells via gap junctions
  3. Atrioventricular (AV) Node (signal delays)
  4. Atrioventricular bundle (aka Bundle of His)
  5. Purkinje fibers (ventricular contraction)
• Timing: Entire process takes about 220ms.

Heartbeat Mechanism

• Contraction Sequence: Atria first (due to AV node delay), then ventricles.
• Ventricular Contraction: From bottom up like squeezing a tube of toothpaste.

Defibrillation Mechanism

• Fibrillation: Out-of-sync contraction of heart cells.
• Defibrillator: Sends massive electric shock; resets heart rhythm.

CPR vs Defibrillation

• CPR: Keeps blood circulating; does not reset heart's rhythm.
• Defibrillation: Resets heart rhythm by depolarizing all cells simultaneously.

Conclusion

• Pacemaker cells and cardiac conduction system are crucial for maintaining heart rhythm.
• Defibrillators reset the heart’s rhythm, contrary to TV depictions.
• CPR maintains circulation during cardiac emergencies until defibrillation can occur.