Heart's Electrical Conduction System

• The information provided helps identify the conduction pathway, focusing on refractoriness and different periods related to cardiac cell excitability and electrical activity.

Refractoriness

• This is the recovery cells need before responding to a stimulus after being discharged.
• This period is longer than the contraction itself.

Absolute Refractory Period (ARP)

• Also known as the effective refractory period.
• Cells cannot respond to further stimulation during this time.
• Myocardial mechanical cells are unable to contract and the electrical conduction system cannot conduct an electrical impulse.
• Tetanic (sustained) contractions are impossible in cardiac muscle.
• Encompasses Phases 0-3 of the cardiac action potential.
• The onset spans from the QRS complex to the peak of the T wave.

Relative Refractory Period

• This is known as the vulnerable period.
• Some cardiac cells have nearly reached their threshold potential.
• These cells can respond (depolarize) to a stronger than normal stimulus.
• It occurs during the downslope of the T wave.

Supernormal Period

• This occurs after the relative refractory period.
• Stimuli that are weaker than normal can cause cardiac cells to depolarize.
• Dysrhythmias or abnormal heart rhythms can develop.
• It concludes at the end of the T wave.

Cardiac Conduction System

• The sinoatrial (SA) node firing initiates an electrical impulse to normalize heart rate.
• In adults, the SA node measures 10-20mm long and 2-3 mm wide.
• Slightly less than half the cells are pacemaker cells and its intrinsic rate is 60-100 bpm
• The SA node acts as the primary pacemaker because of its fastest firing rate.
• Other pacemakers take over if the SA node fails to fire, fires too slowly, or fails to activate the surrounding atrial myocardium.
• The SA node is located in the (where) upper posterior right atrium meeting of the superior vena cava and the right atrium.
• It lies less than 1 mm from the epicardial surface and receives rich para/sympathetic nerve fiber supply.
• HR decrease is enacted with Vagus nerve stimulation/Rest/sleep
• HR increase is enacted with Sympathetic activity/Exercise/Stress
• Blood supply comes from the SA node artery, originating from the R coronary artery in 60% of individuals.
• Fibers from the SA node connect directly with atrial fibers.
• Impulse pathway proceeds, beginning with the SA node R atrium interatrial septum L atrium.
• This contraction of both the right (R) and left (L) atria occurs almost simultaneously.
• Fibrous skeleton that separates atria from ventricles does stops electrical stimulus to pass to ventricles.
• Before atrial depolarization completes, impulse travels through 3 internodal pathways (anterior, middle, posterior).
• These pathways consist of myocardial cells and specialized conduction pathways.
• The anterior internodal pathway (Bachmann's bundle) conducts impulses to the left atrium.
• The middle internodal pathway is Wenckebach's bundle.
• The posterior internodal is Thorel's pathway.
• Internodal pathways merge with cells of the AV node.

Atrioventricular (AV) Junction

• Consists of the AV node and the non-branching part of the Bundle of His.
• It includes specialized conduction tissue, which provides electrical connections between the atria and the ventricles.

Accessory Pathways

• This refers an abnormal that the AV junction bypasses. Extra working myocardial tissue forms a connection between atria and ventricles outside the normal conduction system.

Atrioventricular (AV) Node

• A group of cells located in the floor of the right atrium, near the tricuspid valve, and the opening of the coronary sinus.
• Depolarization and repolarization occur slower here, vulnerable to AV blocks.
• In adults, the AV node is 22 mm long, 10 mm wide, and 3 mm thick.
• Blood is supplied by the right coronary artery in 85-90% of people, other times the left circumflex provides.
• It is supplied by both sympathetic and parasympathetic fibers
• Atrial impulse AV node delays impulse to ventricles because fibers in AV junction have fewer junctions.
• This is for atria and ventricles to contract at different times.
• Allows time to empty into before the contraction.
• Increases in ventricles, raising

AV Node Functional Regions

• The AV node divides into 3 functional regions based on action potentials and electrical/chemical stimulation response:
  • Atrionodal (AN) region, aka Upper regional/Transitional zone
  • Nodal (N) region is located at the Midportion of the AV node
  • Nodal-His (NH) is the lower junctional region the AV node gradually merges with the Bundle of His
• Primary delay occurs in the AN and N areas.

Bundle of His

• The bundle of His is located in the upper portion of the interventricular septum.
• BoH pacemaker cells spontaneously discharge at an intrinsic rate of 40-60 bpm.
• The bundle conducts electrical impulses to the right (R) and left (L) bundle branches.
• Consists normally the only electrical connection between the atria and ventricles.
• It receives dual blood supply from branches of the left anterior and posterior descending coronary arteries, making it less vulnerable to ischemia.

His-Purkinje System

• This is the His-Purkinje network, and consists of the Bundle of branches + Purkinje fibers.

Right & Left Bundle Branches

• Right bundle Innervates the R ventricle.
• Left bundle Spreads impulse to interventricular septum and L ventricle (the L ventricle is thicker than the ventricle).
• Divides into 3 Called fascicles.

Fascicles

• Contain 3 small bundles of fibers.
  • Anterior fascicle spreads impulse to portions of the left ventricle.
  • Posterior fascicle relays impulse to portions of left ventricles.
  • Septal fascicle relays impulse to the septum.

Purkinje Fibers

• Bundle branches divide into smaller branches Purkinje fibers appear. Impulses spread from the septum papillary muscles down to the heart apex creating a web that goes halfway through the ventricular mass.

Pacemaker Sites

• The pacemaker cells fire at a rate of 20-40 bpm.
• SA node is the primary pacemaker/impulse to atria.
• AV junction impulse delays relay to the Bundle of His to contract/receives impulse from SA node/so atrium contracts before ventricles.
• Bundle of His in superior portion impulse from AV for right and left bundle branches.
• Bundle Branches are located in the septum, receives and relays to Purkinje.
• Purkinje fibers in myocardium receive and relay to ventricular myocardium. Electrical impulses spread rapidly right or left right or left ventricle. Endocardium myocardium surface. The walls are stimulated to contract causing ventricles blood into and blood.

Normal Heart rhythm Measurements

• PR interval normal is 0.12-0.20 sec
• Usually QRS complex is usually <0.12 sec
• Interval is normal with heart rate + sex usually about 0.40.

Electrode Placement

• Bipolar are known as standard limb leads with positive and negative.
• Records difference in electrical potential.
• Lead I: Difference between left arm (+) and right arm (-)
• Lead II: Difference between left leg (+) and arm (+)
• Lead III: Difference between left leg (+) and left arm (-)
• Limb leads are numerals precordial are numerals.

Precordial Leads Placement

• Six unipolar from the plan.
• V1 to the of the
• V2 to the of the.
• V3 is between V2 and V4.
• V4 is in mid-
• V5 line.
• V6 that the that.

Common Hospital Leads

• Lead II uses an electrode left and grounded on the shoulder.
• MCL1 uses a electrode right ground electrode right shoulder.

Electrocardiographic

• Positive QRS travelling travelling electrode
• Negative QRS from travelling electrode
• Isoelectric QRS from electrode the
• Flat "written" when impulse.