Anatomy & Physiology Anatomy & Physiology for the EP professionalfor the EP professional

Part II

ObjectivesObjectives

• Identify the venous system of the heart

• Identify the electrophysiology properties of the heart

• Describe the flow of conduction through the heart

• List the properties of cardiac cells• Identify the internal structures of the heart• Describe cardiac innervation and the effects on heart

rate. • Identify the components of cardiac output

• Describe preload and afterload

Cardiac Venous SystemCardiac Venous System

• Cardiac veins lie next to the arteries.

• Coronary sinus (CS) is the largest vein

– Traverses posterior in the AV sulcus and lies next to the LCX.

• The CS receives blood from the Great, middle and small cardiac veins, the oblique vein of the LA and posterior vein of the LV.

• Anterior cardiac venules drain directly into the RA.

• Ostium of the CS is located in the RA posterior and slightly inferior to the MV structure.

Coronary Sinus AnatomyCoronary Sinus Anatomy

Coronary Sinus Anatomy: Coronary Sinus Anatomy: AP viewAP view

CS Os

Middle

Posterior

Postero-lateral

Great

Lateral

Antero-lateral

Anterior

Coronary Sinus Anatomy: Coronary Sinus Anatomy: LAO viewLAO view

Anterior Cardiac Vein

Lateral Cardiac Vein

Postero-Lateral Cardiac Vein

Posterior Vein

Coronary Sinus Anatomy: Coronary Sinus Anatomy: RAO viewRAO view

Anterior Cardiac Vein

Middle Cardiac

Vein

Posterior Cardiac Vein

Anatomy of specialized conduction system

Specialized conduction system

• Sinoatrial (SA), or Sinus Node

• Atrioventricular (AV) Node

• His Bundle

• Left Bundle Branch

• Right Bundle Branch

Conduction SystemConduction System

• Electrophysiology properties– SA Node– AV Node– His Bundle– Right and left bundle branches to

include the Purkinje fibers

Normal sinus rhythm

Intracardiac tracings show the normal intervals

between

• initiation of atrial depolarization A

• His bundle activation H

• ventricular depolarization V

• AH + HV = PR interval

Right Atrium conductionRight Atrium conduction

• Right Atrium

– SA node (SAN)

– Internodal pathways

– Interatrial pathways

– Bachmann’s Bundle

– AV node

• Triangle of Koch

Right Atrium – SA node Right Atrium – SA node conductionconduction

• Located in the RA at the junction of the anterior RA and the SVC

• 2 cm long and .05 cm wide• Thought to extend inferiorly in to the Christa trerminalis• SA node has the fastest automaticity generating

impulses at 100-110 bpm– vagal tone suppresses this… 60-100bpm– Conduction speed w/i SAN: 0.05m/sec

• Calcium dependent action potential• Transitional cells at the border of the node.

SA Node CellsSA Node Cells

• P cells (pole cells)– main characteristic: automaticity– the fastest depolarizing conduction system cells– aka. pacemaker cells

• Transitional cells – form a network of fibers– mainly found on the SA node periphery (border) – attached to the P cells on one side and to the

atrial myocardium on the other side– transmits activation from the SA node to the

atrium

SA Node - stained cross-SA Node - stained cross-sectionsection

atrial myocytes

Cross section RA wall

epicardiumendocardium

adipose

SAN artery SA node

3 Internodal Tracks3 Internodal Tracks

• There is evidence for preferential spread of atrial activation between SA & AV nodes by way of intranodal pathways but whether these are distinct “true” tracts or simply areas of preferential conduction is unclear

• Text have described 3 Internodal tracks:– Anterior – Bachman’s Bundle –Travels concurrently from

the SA node to the left atrium and AV node. (white arrows) – Middle -Travels from the SA node posteriorly around the

SVC, down the inter-atrial septum to the AV node, (red arrows)

– Posterior -Travels from the SA node posteriorly through the Crista Terminalis, to the posterior interatrial septum to the AV node. (yellow arrow)

SA Node & Internodal tracksSA Node & Internodal tracks

AV NodeAV Node

• Impulse reaches the AV Node.– AV node is located at the base of the RA at the

apex of the Triangle of Koch.• Triangle of Koch comprised of the Tendon of Todaro,

septal leaflet of the Tricuspid valve and the Os of the CS

– AV node is comprised of specialized conducting tissue allowing the impulse to travel through the fibrous septum.

– The AV node is slower to conduct (calcium dependent).

– Acts as the “gate Keeper”

AV nodeAV node

• Specialized collection of conducting cells provide the only link between the A & V passage of a wave of depolarization

• Oval-shaped, 60 x 30mm in size• Located subendocardially on the interatrial septum

– RA side: within the triangle of Koch.– LA side: next to the base of the mitral valve annulus

• Anteriorly and inferiorly, continuous with the His bundle • Rich blood supply from AV nodal artery branch of RCA in 90%• 2 types of cells

– rod-shaped: contains Na channels– ovoid-shaped: responsible for spontaneous activity

AV nodeAV node

• AVN conduction speed is approximately 0.1msec• AVN conduction speed is inversely proportional to

prematurity of the impulse received • The cells of the AV node are responsible for the

slowing of conduction between atria and ventricles• This functions to permit sequential atrial then

ventricular contraction• AVN node is richly innervated…

– sympathetic fibers: increase the speed of conduction– parasympathetic fibers: have the opposite effect

Cells of the AV JunctionCells of the AV Junction

•AN (atr io-nodal) cells •in the transitional region•activated shortly after the atrial cells •Action potential (AP) visual appearance is between the fast & brief atrial AP and the slower nodal APs

•N (nodal) cells•"most typical" of the nodal cells•have slow rising and longer AP duration•the site of Wenckebach

•NH (nodal-His) cells •typically distal to the site of Wenckebach block •AP’s closer in visual appearance to the fast rising and long AP of the His bundle.

AV nodeAV node

The His BundleThe His Bundle

• The impulse reaches the His bundle.– The His bundle is the most proximal

portion of the His Purkinje System– Rapidly conducting– Branches to form the right bundle branch,

and left anterior and posterior bundle branches, located within the AV septum

– Further branches to Purkinje fibers within the ventricular myocardium

Bundle BranchesBundle Branches

• His purkinje trifurcates

• Right bundle within the membranous septum

• Exits septum via the moderator band

• Moderator band attaches to the RV free wall

Bundle branchesBundle branches

• At trifurcation of the His...

• Left bundle further bifurcates

• Anterior bundle traverses the interventricular septum to the apex

• Posterior bundle traverses posteriorly, in a fan like fashion, innervating the basilar portion of the LV

His Purkinje systemHis Purkinje system

• specialized heart muscle cells for electrical conduction

• Diameter: 3 mm • Length: 12 to 40 mm

– (penetrating 8-10 mm)• begins at the AV node and passes through the right

annulus fibrosis to the membranous part of the interventricular septum

• starts to branch at the lower part of the membranous septum

• named after the Swiss cardiologist Wilhelm His, Jr., who discovered them in 1893

His Purkinje SystemHis Purkinje System

Purkinje f ibersPurkinje f ibers

peripheral branching of the bundle branches made up of individual myofibrils divided by collagen fibers that prevent the lateral

spread of activation form a subendocardial network that spreads among

the ventricular muscle fibers directly innervate the myocardial cells and initiate the

ventricular depolarization cycle Purkinje conduction speed

subendocardial: 2-10m/secventricular muscle proper: 0.3m/sec

Four Propert ies of Cardiac Four Propert ies of Cardiac CellsCells

• Rhythmicity (Automaticity)

• Excitability

• Conductivity

• Contractility

Rhythmicity:Rhythmicity: (Automaticity) (Automaticity)

• Pacemaker cells can discharge an electrical current without an external stimulus.

• Once the cells are stimulated, the flow is passed from one to another in the conduction system until the muscle contracts.

ExcitabilityExcitability

The ability of the cardiac cell to respond to an electrical stimulus with an abrupt change in its electrical potential. (All or none phenomenon)

ConductivityConductivity

The ability of the cardiac cells to rapidly pass on the electrical stimulus to neighboring cells.

ContractilityContractility

The ability of the fibers to shorten when stimulated.

Right Atrium internal - Right Atrium internal - StructuresStructures

• Endocardium divided into • Anterior trebeculated portion

• Posterior smooth portion

• Christa Terminalis • Separation of the smooth & trebeculated atrium

• External landmark – Phrenic Nerve• Internal:

• Christa Terminalis laterally• Eustachian Ridge inferiorly

Right Atrium internal - Right Atrium internal - StructuresStructures

• Tendon of Todaro- – Fibrous strand from the union of the Eustachian and

Thebesian valves which inserts into the AO root passing thru the atrial wall myocardium.

• Triangle of Koch- – Comprised of the membranous septum, the Os of

the CS and the tricuspid annulus– Location of the AV Node

• Eustachian Ridge – – fold of tissue at the anterior of the IVC

• Thebesian Valve – – Fold of tissue that guards the Coronary Sinus

Right Atrium internal Right Atrium internal structuresstructures

Triangle of KochTriangle of Koch

Right Ventricle internal Right Ventricle internal structuresstructures

• Divided into the postero-inferior portion and the antero-superior outflow portion.

• Demarcated by prominent muscle bands• Parietal band, septal band and the moderator band.• Inflow wall is heavily trebeculated• Outflow portion few trebeculae• Sub-Pulmonic area smooth muscled• Papillary muscles anchor to ventricular myocardium• Chordae tendonae anchor papillary muscles to the

valve leaflets.

Right Ventricle internal structures

Left Atrium internal Left Atrium internal StructuresStructures

• Mainly smooth walled structure – Atrial septal surface smooth– Left appendage offers small pectinate muscle

• Posterior wall may be .05 – 4.0 mm thick• On the right – two, sometimes, three

pulmonary veins enter• On the left – two, sometimes one, pulmonary

veins enter.

Structures that are external to Structures that are external to the Left Atriumthe Left Atrium

• External structures to the LA– Right superior PV – Aorta– Right & Left Superior PV – Pulmonary

Artery– Posterior medial to lateral atrial wall –

Esophagus

Left Atrium internal Left Atrium internal StructuresStructures

Left Ventricle internal Left Ventricle internal StructuresStructures

• Thickness 3 times greater than RV

• Majority of LV trebeculated

• Basilar third is smooth walled

• Two papillary muscles anchor the Mitral Valve

• Ventricular septum muscular.

Left ventricle internal Left ventricle internal structuresstructures

Autonomic InnervationAutonomic Innervation

Autonomic nervous system (ANS) influences:Heart rateConductivityContractility

Parasympathetic Parasympathetic StimulationStimulation

• Right and left vagus nerves • Innervate heart at SA and AV

nodes–Primary postganglionic

neurotransmitter is acetylcholineSlows heart rateSlows AVN conduction

Sympathetic StimulationSympathetic Stimulation

Sympathetic fibers arise in accelerator center in medulla oblangata

Primary postganglionic neurotransmitter is norepinephrine

Receptor SitesReceptor Sites• Alpha

– Vascular smooth muscle

• Beta-1– Heart

• Beta-2– Bronchial smooth muscle– Skeletal blood vessels

• Dopaminergic– Coronary arteries, renal, mesenteric, and

visceral blood vessels

Effect of NorepinephrineEffect of Norepinephrine

• Alpha– No cardiac effect– Peripheral vasoconstriction

• Beta-1– Increased heart rate– Increased conductivity– Increased contractility

Chronotropic EffectChronotropic Effect

• Refers to change in rate–Positive chronotropic effect results

in increased heart rate–Negative chronotropic effect results

in decreased heart rate

Inotropic EffectInotropic Effect

• Refers change in myocardial contractility– Positive inotropic effect results in

increased contractility– Negative inotropic effect results in

decreased contractility

BaroreceptorsBaroreceptorsSpecialized nerve tissueFound in internal carotid arteries

and aortic archDetect and respond to changes in

blood pressure

ChemoreceptorsChemoreceptors

– Located in internal carotid arteries and aortic arch

• Detect and respond to changes in:–Oxygen content of the blood –pH –Carbon dioxide tension

Cardiac OutputCardiac Output

• Volume of blood ejected from the heart over 1 minute

• RV and LV output usually equal• CO = HR X SV

SSx of Decreased COSSx of Decreased CO

• Cold, clammy skin

• Color changes in skin/mucous membranes

• Dyspnea• Orthopnea• Crackles (rales)

• Changes in mental status

• Changes in blood pressure

• Arrhythmias• JVD• Fatigue• Restlessness

Stroke VolumeStroke Volume

• Amount of blood ejected during one contraction

• Dependent on:– Preload– Afterload– Myocardial contractility

PreloadPreload

• Force exerted by the walls of the ventricles at the end of diastole

• Influenced by venous return–Hypovolemia decreases preload–Heart failure increases preload

Frank-Starling Law of the Frank-Starling Law of the HeartHeart

• Up to a limit, the more a myocardial muscle is stretched, the greater the force of contraction (and thus stroke volume)

–Influenced by both preload and afterload

Frank-Starling Law of the Frank-Starling Law of the HeartHeart

AfterloadAfterload

• Afterload is the pressure or resistance against which the ventricles must pump to eject blood

– Results in • Increased myocardial workload• Increased myocardial O2 demand

AfterloadAfterload

• Vasoconstriction results in increased afterload and increased myocardial work and O2 demand

• Vasodilation results in decreased afterload decreased myocardial work and O2 demand

Blood PressureBlood Pressure

• Force exerted by circulating blood volume on arterial walls

• Influenced by peripheral vascular resistance (PVR) – Resistance to blood– Determined by vessel diameter and

muscle tone• BP = CO X PVR

Normal Heart PressuresNormal Heart Pressures

Pressure Measurement Normal

RA Mean 2-8 mm Hg

RV Systolic 25-35mm Hg

End Diastolic 2 -8 mm Hg

PA Systolic 25-35mm Hg

End Diastolic 8-12mm Hg

Normal Heart PressuresNormal Heart Pressures

Pressure Measurement Normal

PCW Mean 8-12mm Hg

Aorta Systolic 90-140mm Hg

Diastolic 60-90mm Hg

LV Systolic 90-140mm Hg

End Diastolic 8-12mm Hg

ReferencesReferences

• Netter, F H (1987) The CIBA collection of medical Illustrations. Volume 5 The heart. Colorpress, New York NY

• Mazgalev, T.N., Tchou, P.J. (2000) Atrial AV Nodal Electrophysiology; A view from the millennium. Futura, Armonk, NY

• Podrid, P.J., Kowey, P.R. (2001) Cardiac Arrhythmia, Mechanisms, Diagnosis & Management, 2nd Ed. Lippincott, Williams & Wilkins. Philadelphia PA

• Anderson, R.H. (2000) Electrical Anatomy of the Atrial Chambers. Medtronic USA

• Anderson R.H., Ho, S.W. ( n.d.) The Anatomy of the Atrioventricular Node. Article review by Sechler, D.A., RN