Week 1- Electrophysiology of the Heart Flashcards
What is the main function of the electrical conduction system?
- to create an electrical impulse and transmit to the rest of the myocardium
- this is an electrochemical process that creates electrical energy, picked up by the electrodes in an ECG
What are some things to keep in mind about the conduction system?
- Actually inside the heart walls
- Atrial myocytes are innervated by direct contact from one cell to another
- Internodal pathways transmit the impulse from the SA node to the AV node
- Purkinje system is the final component- innervates myocardial cells
What is the pacemaker function?
- Dictates the rate at which the heart will cycle through its pumping action to circulate the blood
- Creates an organized beating of all cardiac cells in a specialized sequence
- Sets the pace of all other cells that will follow
What is the SA Node?
- The main area that acts as the heart’s pacemaker
- Controls the beat based on information it receives from the nervous, circulatory, and endocrine system
What is the pacemaker setting for the SA Node?
- 60-100 BPM
- 60-80 BPM
What is the pacemaker setting for the atrial cells?
- 55-60 BPM
What is the pacemaker setting for the AV Node?
- 45-50 BPM
- 40-6 BPM
What is the pacemaker setting for the Bundle of His?
- 40-45 BPM
What is the pacemaker setting for the Bundle branch?
- 40-45 BPM
What is the pacemaker setting for the Purkinje Cells?
- 20-40 BPM
What is the pacemaker setting for Purkinje system?
- 20-40 BPM
What is the pacemaker setting for Myocardial Cells?
- 30-35 BPM
Where is the SA Node located?
- Found in the upper part of the wall of the right atrium at its junction with the superior vena cava
- Electrical impulses are generated here
- The farther away the impulse is generated from the SA node the slower it becomes
What is the main purpose of the Internodal Pathways?
- Main purpose is to transmit the pacing impulse from the SA node to the AV node
Where are the Internodal pathways found?
- they are found in the walls of the right atrium and interatrial septum
What are the three pathways of the internodal?
- Anterior
- Middle
- Posterior
What is the Bachmann bundle?
- small tract of specialized cells that transmits impulses through the interatrial septum, preferred path for electrical activity for left atrium
What does the AV node do?
- Controls HR (electrical relay station)
- Slows down conduction from atria to ventricles long enough for atrial contraction- then allows the signal to pass into the ventricles
- Helps maintain output of heart at the maximum level
- Always supplied by right coronary artery
Where is the AV node located?
- In the wall of the right atrium next to the opening of the coronary sinus, near the coronary sinus and the septal leaflet of the tricuspid valve
What is the Bundle of His?
- Collection of heart muscle cells specialized for electrical conduction
- Transmits impulses from the AV node to purkinje fibers then to the ventricles
- The ONLY route of communication between the atria and ventricles
Where is the Bundle of His located?
- Starts at the AV node
- Found partially in right atrium, and interventricular septum
What is the Left Bundle Branch? (LBB)
- First area to depolarize
Where is the LBB located?
- Begins at the end of bundle of His
- Travels through interventricular system
- Ends at beginning of left anterior and left posterior fascicles
What is the Right Bundle Branch?
- Gives rise to fibers that innervate RV and right face of interventricular septum
Where is the RBB located?
- Begins at the bundle of His
- Terminates in Purkinje fibers
What is the Purkinje System?
- Carry contraction impulses from the left and right bundle branches to the ventricles
- Initiates ventricular depolarization cycle (contraction, pumping blood outside of your body)
Electrophysiology
- Electrical stimulus= muscle tissue contraction
- Cardiac muscle is unique as it can produce it’s own impulse- pacemaker cells
- Common characteristics of pacemaker cells
What is automaticity?
- Generate their own electrical impulses
What is excitability?
- Irritability
- Ability to respond to a change
What is conductivity?
- Pass a charge on to the next cell
What is contractility?
- Ability to shorten
- Contraction
What are the phases of normal electrical activity of the heart?
- Cardiac conduction is essentially split into 2 phases: systole (contraction) and diastole (relaxation)
- Polarization (relaxed)
- Depolarization (contract)
- Repolarization (getting ready to have another contraction)
What is polarization?
- State of readiness
- Muscle is relaxed and ready to receive electrical impulses
- Potassium inside, Sodium outside, Calcium outside
What is depolarization?
- Contraction
- Electrical impulse transmitted
- 1- opening of sodium channels to allow Sodium to move inside the cell
- 2- Potassium moves to the outside
- Ca++ moves inside and stays longer
REFRACTORY PERIOD
What is Repolarization?
- Recovery phase
- Cells returning to a ready state
- After a delay (absolute refractory) termination of action potential occurs as potassium channels open allowing K+ to leave the cell
What is the action potential- electrical activity of a single cell?
- Phase 4- polarized- resting membrane potential
- Phase 0- depolarization (Na inside)
- Phase 1- early repolarization (K out)
- Phase 2- Plateau phase (Ca moves in) (2nd depolarization)
- Phase 3- Rapid Repolarization
- For contraction sodium in, potassium out
- For resting potential you need equal sides (Ca+ in)
What is action potential?
- Stimulation of the cardiac nerve to transmit an impulse to the cardiac cells
- Start of a heart beat
What are the steps to follow for action potential?
- when an adequate stimulus is applied, the stimuli gated Na+ channels open and Na+ rapidly diffuses into the cell. This causes depolarization.
- As the threshold potential is reached, voltage gated Na+ channels open
- As more Na+ enters the cell through the voltage gated Na+ channels, the membrane depolarizes even further
- The magnitude of the action potential peaks when the voltage gated Na+ channels close (+30mv)
- Repolarization begins when voltage gated K+ channels open allowing outward diffusion of K+ calcium goes into the cell
- After a brief period of hyperpolarization, the resting potential is restored by the sodium/ potassium pump and the ion channels return to their resting state
What is refractory period?
- Brief period during which the cells will resist stimulation
- Last approx. 0.5 ms after the membrane reaches the threshold potential
What is the absolute refractory?
- Will not respond to any stimulus, no matter how strong
What is relative refractory?
- Few ms after the absolute- the membrane is repolarizing and restoring the membrane potential
- During this time, the membrane will only respond to very strong stimuli
What is the conduction pathways?
- Sinoatrial node
- Internodal and interatrial pathways
- Atrioventricular node
- Bundle of his
- Right and left bundle branches
- Perkinje’s fibers
- Ventricular muscle
What is the first step?
- SA node activity and atrial activation begin
- Time= 0 msec
What is the second step?
- Stimulus reaches the AV node
What is the third step?
- 100 msec delay at the AV node
- Atrial Contraction begins
What is the fourth step?
- Impulse travels along the septum wall within the AV bundle and branches to the Purkinje fibers
What is the fifth step?
- Impulse is distrubted and relayed through the ventricular myocardium
- Atrial contraction complete
- Ventricular contraction starts
What does atrial depolarization on the ECG look like?
- The P wave on the ECG represents atrial depolarization (Atrial Contraction)
- First wave on the ECG
What does ventricular depolarization look like on the ECG?
- QRS complex on the ECG (ventricular contraction)
- The action potential here is moving from the AV node through the bundle of His and into the Purkinje Fibers
What ventricular repolarization look like on the ECG?
- The T wave on the ECG represents ventricular repolarization
- Ventricles relax during this phase
What does repolarization of Purkinje Fibers look like on ECG?
- U wave on the ECG
- This is not always visble on the ECG
- Very small wave
- Depends on the rate of re-stimulation of the cardiac cells
What is the basic principles of electrode placement?
- May be necessary to shave body hair
- Rub the electrode site briskly with an alcohol swab before application
- Gently scrape the site with the plastic backing of the electrode
- Attach the electrodes to the ECG cables before placement
Lead Placement
- Remember these leads are considered Limb Leads- they should preferably be placed on the limbs
- If not possible, use chest/ torso
What is in lead II?
- White electrode= RA
- Red electrode= LL
- Black electrode= LA
- Green electrode= RL
What happens when the electrodes are positioned correctly?
- When positioned correctly on the chest, these leads form a triangle around the heart called the Einthoven Triangle
What are the 4 limb leads?
- The 4 limb leads are bipolar
- They will switch between - and + depending on which view you are looking at
What are the 6 chest leads?
- The 6 chest leads are unipolar
- They can only look in one direction
What are leads I, II, and II?
- They are bipolar leads and contain a positive and negative pole
Where is lead I formed?
- formed between the right and left arm electrodes.
- The left arm electrode is the positive terminal
Where is lead II formed?
- formed between the right arm and left leg
- The left leg electrode is positive terminal
