Overview
This lecture introduces the cardiac conduction system, explaining how electrical impulses travel through the heart to create rhythmic contractions and set the foundation for ECG interpretation.
Heart Chambers and Valves
- The heart has four main chambers: right atrium, right ventricle, left atrium, and left ventricle.
- Key valves include tricuspid, mitral, pulmonic, and aortic, which help direct blood flow.
Key Terminology: Electrical Events
- Depolarization refers to muscle cell contraction triggered by changes in ion charge.
- Repolarization refers to muscle cell relaxation as ions return to their original positions.
- In the ventricles, contraction is called systole and relaxation is called diastole.
The Cardiac Conduction System
- The sinoatrial (SA) node, located in the right atrium, is the heart's pacemaker and initiates electrical impulses.
- The SA node fires at a rate of 60–100 beats per minute and can be influenced by the nervous system.
- Depolarization from the SA node spreads through the atria, causing them to contract and move blood to the ventricles.
- Bachmann’s bundle quickly carries the signal from the right atrium to the left atrium for synchronized contraction.
Atrioventricular (AV) Node and Delay
- The atrioventricular (AV) node, located in the septal wall, receives impulses via internodal pathways.
- The AV node acts as a gatekeeper, delaying the impulse by about 120 ms (0.12 sec) to allow complete atrial contraction.
- The AV node can generate impulses at 40–60 beats per minute if needed.
Ventricular Conduction Pathways
- After the AV node, impulses travel through the bundle of His in the ventricular septum.
- The bundle of His splits into right and left bundle branches, with the left branch dividing into anterior and posterior fascicles.
- Purkinje fibers distribute impulses throughout the ventricles, ensuring coordinated contraction.
- Purkinje fibers have intrinsic rates of 20–40 beats per minute if higher pathways fail.
Key Terms & Definitions
- Depolarization — the process causing muscle cell contraction.
- Repolarization — the process causing muscle cell relaxation.
- Sinoatrial (SA) Node — heart’s pacemaker; initiates electrical impulses.
- Atrioventricular (AV) Node — delays impulse to allow full atrial contraction.
- Automaticity — ability of certain cells to initiate impulses without external signal.
- Bundle of His — high-speed pathway conducting impulses from AV node to ventricles.
- Bundle Branches — pathways splitting from bundle of His to both ventricles.
- Purkinje Fibers — network distributing impulses for ventricular contraction.
- Systole — ventricular contraction phase.
- Diastole — ventricular relaxation phase.
Action Items / Next Steps
- Review this overview of the conduction system for foundational understanding.
- Prepare for Part 2, which will cover ECG rhythm interpretation.
- Optional: Review hemodynamic and arterial blood gas lessons as suggested.