Basic Electrophysiology

2 types of receptors are located in the Aortic Arch and Carotid sinus:

• Baroreceptors

• Chemoreceptors

Baroreceptors

Detect change in BP

Chemoreceptors

Detect change in O₂ Sats

What do baroreceptors and chemoreceptors do?

Detect changes and cause reflex response in sympathetic nervous system

Heart rate is determined by:

• Sympathetic NS

• Parasympathetic NS

Sympathetic Nervous System

• Causes release of Epinephrine

  • aka: Adrenalin

Sympathetic NS (Epinephrine) increases:

• heart rate

• BP

• force of vent contraction

• cardiac output

  • Adrenergic response

Parasympathetic NS

• affects SA and AV nodes via the Vagus nerve

• stimulation results in release of acetylcholine

The parasympathetic nerve system (acetylcholine) decreases:

• heart rate

• SA node discharge rate

• AV node conduction rate

• Cholenergic response

Types of Cardiac Cells:

1. Specialized (Pacemaker) cells

2. Myocardial cells

Specialized (Pacemaker) cells

• located in pathway of the electric conduction system within the heart

• controls the Electrical activity of the heart

Primary function of Pacemaker cells:

• Automaticity

  • function of generation of electrical impulses

  • initiate electrical impulse without stimulation

• Conductivity

  • function of conduction of electrical impulses

  • carry impulses

Myocardial cells

• contain contractile filaments that slide together

• make up the myocardium

  • thin layer in atrial wall

  • thick layer in ventricle wall

• controls the Mechanical activity of the heart

Does Mechanical activity occur before or after Electrical activity?

Mechanical activity occurs after / in response to Electrical activity

4 properties of Cardiac Cells:

1. Automaticity

2. Conductivity

3. Irritability / Excitability

4. Contractility

Automaticity

• found in:

  • SA node

  • Bundle of His

  • Purkinje fibers

• the ability of cardiac pacemaker cells to spontaneously initiate electrical impulse without stimulation

  • heart can begin and maintain rhythmic activity without aid of NS

Conductivity

• found in all cardiac cells

• ability of cardiac cells to transfer an electrical impulse to a neighboring cardiac cell

Irritability / Excitability

• found in all cardiac cells

• ability of cardiac cells to respond to to an electrical stimulus

Contractility

• found in myocardial cells

• ability of card cells to shorten, causing muscle contraction in response to electrical stimulus

The Conduction System

• The system of pathways of conductive cells through the heart

• initiates and maintains the rhythmic contractions of the heart

• responsible for electrical activity

  • specialized cells

The Conduction System consists of:

• Sinoatrial (SA) Node

• Internodal Tracts

• Atrioventricular (AV) Node

• AV Junction

• Bundle of His

• Bundle Branches → R & L

• Purkinje Fibers

Sinoatrial (SA) Node

• a cluster of specialized cells

• highest location

• pacemaker of the heart

• intrinsic rate: 60-100 BPM

  • rate of normal heart

Sinoatrial (SA) Node location:

• in upper posterior portion of Right Atrium (RA)

• at the entrance of the SVC

Bradycardia

Slow heart rate

Tachycardia

Fast heart rate

Impulse of the conduction system starts in right atrium:

As impulse leaves SA node, it causes depolarization of adjacent myocardial cells, resulting in their contraction

Internodal Tracts

• path of conduction cells that carry impulses

• carries electrical impulse through the atria

• tract located in left atrium (LA) called:

  • “Brachmann’s Bundle”

Atrioventricular (AV) Node

• located in lower portion of the Right Atrium (RA)

  • near the base of the Tricuspid Valve

• the “gatekeeper”

  • delays the electrical impulse for 0.1 of a sec

    • allows complete contraction of atria

      • A-kick

    • allows complete filling of ventricles

• no intrinsic rate

  • has no pacemaker cells

AV Junction

Technically the AV node and Bundle of His

Bundle of His (Common Bundle)

• located in upper portion of Interventricular Septum (IVS)

• comes off AV node

  • links AV node to Bundle Branches (BB’s)

• intrinsic rate: 40-60 BPM

Bundle Branches → R & L

• No intrinsic rate

  • has no pacemaker cells

Right Bundle Branch (RBB)

• runs along Rt side of IVS

• delivers electrical impulse to RV

Left Bundle Branch (LBB)

• runs along Lt side of IVS

• divides into 2 fascicles

  • left anterior fascicle

  • left posterior fascicle

• delivers electrical impulse to LV

Purkinje Fibers

• terminal portion of conduction system

• smaller branches off the BB

• forms a web of fibers through the myocardium

• intrinsic rate: 20-40 BPM

Escape Beat / Escape Rhythm

• next highest intrinsic rate pacemaker site can “take over/kick in” if preceding rate slows below its inherent rate

  • ie: if SA node stops, bundle of His can take over, next would be purkinje fibers

• all sites provide different EKG waveforms and rates

Two types of Cardiac Activity:

1. Electrical activity

2. Mechanical activity

Electrical activity

• depolarization / repolarization of cells

  • electrical events expected to result in mechanical activity

Mechanical activity

contraction of myocardial cells

Interaction of mechanical and electrical activity:

• Electrical proceeds Mechanical activity

  • cannot have mechanical without electrical

  • can have electric without mechanical

    • “pulseless electrical activity”

The Electrolytes

• ions that are able to conduct an electric current

  • Calcium (Ca⁺⁺)

    • necessary for contraction of myocardium

  • Potassium (K⁺)

    • required for myocardium relaxation

  • Sodium (Na⁺)

    • essential in maintaining fluid balance

• movement of ions across the cell membrane creates the electrical activity reflected in the EKG waveform

Action Potential

• Describes 5 phase cycle of de-/re-polarization

• 0 - Depolarization

  • sodium ions flow from out to inside cell membrane

• 1 - Brief Repolarization

  • potassium ions flow from in to outside

• 2 - Excitation Contraction Coupling

  • calcium ions flow from out to inside

  • activates contraction

• 3 - Rapid Repolarization

  • potassium ions flow from in to outside

• 4 - Resting phase

  • ions are even both in and outside the membrane

Absolute Refractory Period

• the “effective” period

• during phase 1 and 2

• cardiac cells will not respond to another stimulus

Relative Refractory Period

• the “vulnerable” period

• during phase 4

• cardiac cells can be stimulated to depolarize

  • result in arrhythmia /dysrhythmia