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