cardiac electrical conduction system

big picture

SA node, internodal pathways/routes, AV node, HIS bundle (also called common bundle), bundle branches, purkinje system

what is the significance of the HIS bundle being insulated?

allows more controlled systematic contraction from apex to base

what is the location of the SA node?

subepicardially in the posterolateral RA wall, immediately inferiorly and just lateral to SVC opening in the sulcus terminalis/terminal groove

what is important about endocardial ablation of SA node?

requires more energy

deeper description of SA node

small flattened, ellipsoid strip of specialized cells

histologically: minimal contractile filaments

connects directly to atrial muscle fibers via gap junctions

property of automaticity/self-excitation

how can you have no pulse and still electrical activity?

enough action to stimulate activity but not enough to stimulate contraction

internodal pathways » preferential anatomical conduction routes

1. microscopically identifiable

2. not insulated from other cardiomyocytes - implications?

3. “preferential alignment” of the cardiomyocytes permits recognition of “bands” or “bundles” or “routes”

4. shortest electrical route

5. node to node conduction

middle tract (wenckebach pathway)

SA node - runs posteriorly to SVC then descends within atrial septum — likely joins anterior bundle as it enters AV node

posterior tract (thorel’s pathway)

SA node through crista terminalis, eustachian valve past coronary sinus to posterior portion of AV node

anterior tract (bifurcates to 2)

anterior tract - descends along intertribal septum and connects with anterior part of AV node

Bachmann’s bundle - also called interatrial band/bridge

Bachmann’s bundle

most superficial of myocardial layers (subepicardial) - nearly parallel alignment of myocardial strands/cells

what suggests substrate for Afib in Bachmann’s bundle?

change of fiber orientation as it crosses at boundary of bundle

what is another name for AV node?

Tamara’s node

at apex of koch’s triangle

what are the inferior extensions of AVN thought to account for?

slow pathway of nodal reentry pathway

slow pathway

longer conduction time and shorter refractory period

fast pathway

faster conduction time and longer refractory period

AV node is composed of

gap junctions with electrical communication proteins (connexion)

AV node responsible for delay/decremental conduction

1. small diameter nodal myocytes

2. complex arrangement of myocytes

3. extensive connective tissue

4. poor expression of Na+ channels

5. poor connexin coupling

what area is often ablated in AVNRT?

vestibular area/slow pathway

three portions of the AV node/HIS bundle

1. penetrating portion of AV bundle

2. non branching portion

3. branching portion

what does the bundle of his penetrate?

the cardiac skeleton (central fibrous body)

most proximal part of His-purkinje system is…

the His bundle

tell me more about the HIS bundle

insulated by fibrous sheath » receives input from AV node and sometimes transitional fibers » unlike AV node - NOT heavily innervated with minimal blood supply » bifurcates once leaving central fibrous body » HIS potentials used as a landmark for AV node ablation

bundle of Kent

most common atrioventricular bypass - connects atrial myocardium to ventricular myocardium » pre excites ventricles (delta wave) and can lead to WPW

mahaim pathway

atriofascicular bypass » decremental conducting connections between the distal RA (AV node) and RBB

James pathway (atrionodal)

originate in the low atrium and terminate in the low AV node (bundle of His)

fasciculo-ventricular bypass

connect distal conducting system to ventricular myocardium

orthodromic AVRT

AP travels antegrade thorugh AV node and retrograde through accessory pathway (normal QRS)

antidromic AVRT

AP travels retrograde through AV node and integrate through accessory pathway (wide QRS)

why rapid conduction in purkinje fibers?

1. gap junctions

2. fast voltage gated Na+ channels

3. high glycogen content

what drives calcium homeostasis?

SERCA pumps

what inhibits SERCA pumps?

phospholamban

what drives automaticity or spontaneous rhythmicity?

funny channels

recall phases for nodal vs non-nodal cells

nodal » 4, 0, 3

non-nodal » 0, 1, 2, 3, 4

what is override suppression?

suppression of the “lower level pacemakers”

CAB

class 1C - most effective, class 1B - least effective

Nice Boys Know Christ

Na+ channel blockers

Beta blockers

K+ channel blockers

Ca2+ channel blockers

shape of AP reflects

conduction velocity, refractory period, automaticity

shape of AP affected by…

internal and external stimuli, meds

closed channel state

activation gate closed

inactivation gate open

open channel state

activation gate open

inactivation gate open

inactive channel state

activation gate open

inactivation gate closed

what happens if stimulus triggers in relative refractory period?

risk of RonT and PVCs