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