Electrophysiology....Where to start.

Electrophysiology

Where to start?

Cardiac Electrophysiology is the study of the heart’s electrical system. The process of learning electrophysiology can be very confusing. Knowing where to start studying is key. 

 Understanding how the heart initiates an electrical impulse and conducts that impulse throughout the heart is the foundation of electrophysiology. This "foundation" allows for a better understanding of arrhythmias and their treatment.

 Needless to say, it allows for a better understanding of what's going on in the EP Lab and more important, why.

The bottom line.....Figure this out and you just about have EP whipped!

The Cardiac Action Potential

“The Foundation”

“The cardiac action potential is one of the most despised and misunderstood topics in electrophysiologic testing. It is also a leading cause of the mystique surrounding electrophysiology testing."  R. Fogrose, M.D.

I'll try to make this as painless as possible.

The following are the basics of how and why heart cells contract.

Cardiac Cells

Electrically-charged.....

Ions are electrically-charged particles contained in fluid that fills and surrounds the cardiac cells.

A positively-charged (+) ion has lost an electron.

                       

     A negatively-charged (-) ion has gained an electron.

The cardiac myocyte (Heart Cell) is a specialized muscle cell and is only found in the heart. The most important function of heart cells is to contract rhythmically and systematically.  The contraction of the heart as a whole is as a direct result of the contraction of all of the tiny cells of the heart muscle called Myocytes.

Myocytes branch to form a “Y” and interlock so that when one cell is stimulated to contract, so are the adjacent cells

 Each Myocyte has a nucleus and multiple myofibrils – parallel strands of tissue that run the length of the cell. Myocytes are separated by discs that have low electrical impedance, which allows fast conduction of electrical impulse.

(Fig 1) "Y" shaped Myocytes

Each cell in our body is surrounded by a thin cell membrane. Different ions can move across the cell membrane through the special ion channels (think of the channels as gates or gated channels). The channels can freely let one type of ion go through the membrane and block passage of other types of ions.

Because ions are charged molecules, an electrical gradient is also established across (between outside and inside) the cell membrane, transforming each cell into a tiny battery. The resulting voltage difference across the cell membrane is called the Transmembrane Potential.

The Transmembrane Potential is negative inside then outside, to be more exact it has a resting membrane potential of approximately (- 0.1 V or -100mV).

Depolarization

The gated channels, in response to a stimulus (electrical, mechanical, or chemical), open and allow positive charged sodium ions to rush into the cell, causing a rapid positively directed change in the transmembrane potential. 

When these stereotypical voltage changes are graphed against time, the result is the cardiac action potential.





(Fig 2) The 5 Phases of the Cardiac Action Potential

Phase 0 is the immediate depolarization that sends the voltage past the zero millivolt level, making it positive. This is due to the sudden increase in membrane permeability to sodium ions and decrease in potassium permeability. Once the high sodium permeability decreases, slight repolarization occurs. The moment when the voltage declines makes up Phase 1.

The membrane potential then reaches a steady point at around zero millivolts. This is called the plateau of the action potential, and it makes up the gist of Phase 2 as well. There is a reason for this moment of steadiness in the voltage. The inward flow of calcium ions is equal to that of the outward flow of potassium ions.

So, why doesn’t the voltage just remain at zero? Well, because of the falling membrane potential, the calcium permeability declines while the potassium permeability increases. This initiates repolarization once again, and it makes up Phase 3. The voltage decreases to its original value where it will remain steady until the next action potential is generated (Phase 4).

Effective Refractory Period

Once an action potential is initiated, there is a period of time comprising phases 0, 1, 2, and part of phase 3 that a new action potential cannot be initiated. This is termed the effective refractory period (ERP) or the absolute refractory period (ARP) of the cell. During the ERP, stimulation of the cell by an adjacent cell undergoing depolarization does not produce new, propagated action potentials.

The ERP acts as a protective mechanism in the heart by preventing multiple, compounded action potentials from occurring (i.e., it limits the frequency of depolarization and therefore heart rate). This is important because at very high heart rates, the heart would be unable to adequately fill with blood and therefore ventricular ejection would be reduced.

Automaticity

Automaticity is the ability of the Myocytes to depolarize spontaneously, i.e. without external electrical stimulation from the nervous system.

This spontaneous depolarization is due to the plasma membranes within the within certain area of the heart that have reduced permeability to potassium (K+), but still allow passive transfer of calcium ions, allowing a net charge to build until it spontaneously depolarizes.  

That’s it!...For now!          I hope that wasn't to painful. 

Next will be normal conduction.