LECTURES:
1. Electrical Conduction System of the Heart
2. The Heart & Its Electrical Conduction System
3. Complete ECG Course
4. The EKG Guy Videos

Overview

• Made up of specialized cells w/ pacemaking function + others that allow for impulse transmission
• Main function: create electrical impulse & transmit it in organized manner to rest of myocardium (electrical chemical process => electrical energy => electrodes capture => EKG)
• Pathway: SA node => internodal pathways + Bachmann bundle (RA before LA) => AV node => His bundle => R- & L-bundle branches => Purkinje fibers => cardiomyocytes
• R-bundle branch activates RV
• L-bundle branch activates LV; gives off L-posterior fascicle (short & broad, dual blood supply, resistant to ischemic damage) & L-anterior fascicle
• Pacemaker (PM) = dictates rate at which heart cycles through pumping action
• Every cell along normal conduction system has automatic pacemaking capability & potential to become PM
• If PM fails => backup (“safety net”), but at slower intrinsic rate than preceding cells
• General rule: fastest PM controls heart
• Normal intrinsic rates (adult ranges): rates slow from top to bottom along conduction system
• SA node (60-100 BPM) =>
• Atrial cells (55-60 BPM) =>
• AV node (40-60 BPM) =>
• His bundle (40-45 BPM) =>
• Bundle branches (40-45 BPM) =>
• Purkinje cells (20-40 BPM) =>
• Ventricular myocytes

Electrical Conduction System
Sinoatrial (SA, sinus) node:

• Normal physiologic PM of heart
• Made up of pacemaker (P) cells
• Intrinsic rate: 60-100 BPM
• Location: crista terminalis junction in upper wall of RA & SVC opening
• Blood supply: RCA > LCA > RCA + LCA
• Receives input from diff systems (nervous, circulatory, endocrine) & responds to body’s needs

Internodal pathways:

• Made up of transitional (T) cells
• Intrinsic rate: 55-60 BPM
• Anterior, middle, & posterior pathways
• Transmit impulse from SA node to AV node
• Bachmann bundle: transmits impulse through inter-atrial septum to LA
• Location: RA walls + inter-atrial septum

AV node:

• Intrinsic rate: 40-60 BPM
• Location: RA wall, near coronary sinus opening & septal leaflet of tricuspid valve
• Blood supply: RCA
• Slows conduction from atria to ventricles to allow for atrial contraction & filling of ventricles
• AV nodal conduction makes up majority of PR interval

His bundle (bundle of His):

• Intrinsic rate: 40-45 BPM
• Location: RA wall & interventricular septum
• Only route of communication b/t atria & ventricles

L-bundle branch:

• Intrinsic rate: 40-45 BPM
• Location: interventricular septum
• Fibers innervate LV & L-face of interventricular septum; ends at start of L-anterior fascicle (LAF) & L-posterior fascicle (LPF)
• LAF (or L-anterior superior fascicle, LASF):
  • Location: LV
  • LAF => Purkinje cells => anterior & superior LV (hence, LASF)
  • Single-stranded fascicle => greater risk of block than LPF
• LPF:
• Fan-like structured fascicle => lesser risk of block than LAF
• Location: LV
• LPF => Purkinje cells => posterior & inferior LV

R-bundle branch:

• Intrinsic rate: 40-45 BPM
• Location: RV
• Fibers innervate RV & R-face of interventricular septum
• R-bundle branch => Purkinje cells => RV

Purkinje system:

• Intrinsic rate: 20-40 BPM
• Location: below endocardium of ventricles
• Innervate ventricular myocytes & initiate ventricular depolarization cycle

Abbreviations: L, left; R, right; RA, right atrium/atrial; LA, right atrium/atrial; LV, left ventricle; RV, right ventricle; b/t, between; RCA, right coronary artery; LCA, left coronary artery