ELECTRICAL COMPONENTS

  1. SA node    2. AV node   3.Bundle of His   4. Bundle branches   5. Purkinje fibres

1. SA node

SA node has “intrinsic automaticity”, meaning it has specialized nerve tissue that is able to create and transport an electrical impulse.
Electrical impulse generated -> initiates depolarization –> wave moves across Rt and Lt atria by internodal pathway and cell to cell conduction -> contraction.
Atrial septum serves as electrical insulator/barrier (connective tissue that does not conduct electrical impulses).
Bachman’s Bundle tunnels thru septum to continue electrical wave across to Lt atrium.
Electrical wave takes approx. 3/100 sec to cross atria and arrive at the AV node.
SA node fires at a rate of 60-100 times per minute.

2. AV node

AV node has similar cells to SA node and also has “intrinsic automaticity”.
Usually it doesn’t initiate impulse but is able to serve as a backup pacemaker if the SA node fails.
AV node fires at a rate of 40-60 times per minute.

3. Bundle of His (BOH)

An electrical connection between the atria and ventricles.
AV junction consists of the AV node and the  BOH.
AV node with BOH slow impulse conduction to allow atria time to contract prior to ventricular contraction (provides time for atrial kick).

SA node, atria and AV junction are 3 main supraventricular sites.

In atrial conduction only one wave is propagated thru atria whereas ventricular conduction is very fast with 3 waves conducted simultaneously.

4. Bundle branches

Bundle branches are like the Autobahn that allows the impulse to split into at least 3 simultaneous waves, reducing the distance to travel across ventricles.

The impulse must have traveled thru the BOH to arrive at the Bundle branches, it’s the only entry point to the Autobahn

5. Purkinje fibers

Specialized cells tailored for fast conductivity.
The speed of contraction translates directly into force of contraction and increased SV & CO.
For a rapid wave of depolarization to cover the ventricles the impulse must originate above the ventricles (BOH or above) (supraventricular) which produces a narrow QRS.

If the impulse originates from an ectopic impulse from the Right ventricle, instead of 3 simultaneous waves only one wave depolarizes right then left ventricles which takes more time and produces a wide QRS.

Names of ECG rhythms begin with origin of impulse (i.e. sinus rhythm).

PHYSIOLOGY

Cardiac cells are mechanical and electrical. 

A specialized nerve tissue (SA node) creates and transports an electrical impulse ->  initiates depolarization -> leads to contraction (mechanical response to impulse).
Impulse by SA node travels thru muscle fibres by internodal pathway and cell to cell conduction leading to excitability (electrically stimulated) of cells.
At rest, or in polarized state, the inside cell is negative compared to outside.  
Normally there is increased K+ inside and Na+ outside the cell.
When cell or group of cells are stimulated, each cell membrane changes its permeability and Na moves rapidly into cell -> becomes positive (depolarized).

Depolarization is when positive ions move into cardiac cells and negative out.  
Repolarization is a slower movement of ions across membrane that restores  the cell to a polarized state.
The polarity needs to be reversed through the Na/K pump for the next AP to be able to take place.

Action Potential (AP) is a nerve impulse that occurs when there is a shift of polarity in a cell’s membrane.  There is a voltage change that happens when ions move across the membrane and the inside of the cell becomes negative.  There are millions of APs.
AP in atria is P wave and in ventricle is the QRS.

ECG is a graph of electrical activity in a moment in time.  It ONLY shows electrical activity!

Resting membrane potential is measured by the differences of ion concentrations inside and outside of the cell (potassium, sodium, calcium, chloride).  The typical value is -90 mV.
Threshold membrane potential the minimum voltage change that must occur for an action potential to be generated by a neuron.  The typical value is – 55 mV.

Phases of cardiac AP:

phase 0 – rapid depolarization (positive ions move into cell).
phases 1 – Na+ and K+ channels opening and closing (less Na+ goes in, few K+ out).
phase 2 – balance of charge moving in and out. Ca2+ and Cl- move into cell, K+ moves out.
phase 3 – rapid repolarization. K+ channels stay open -> K+ moves out.
phase 4 – polarized cell/ resting state.

 Antidysrhythmia drugs have a direct effect on various phases of AP.

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