during the resting state, the inside of the cell is more electrically negative relative to the
outside of the cell due to the differences in ions concentrations. - This negatively
charged resting stated is referred to as the polarized state
when the cardiac cell receives an electrical stimulus...
basic cardiac rhythm interpretation
TEST 2024
during the resting state, the inside of the cell is more electrically negative relative to the
outside of the cell due to the differences in ions concentrations. - This negatively
charged resting stated is referred to as the polarized state
when the cardiac cell receives an electrical stimulus, sodium and calcium shift into the
cell and potassium shifts out causing the cell to become more positively charged. - This
alteration in ion charge is called depolarization, which normally results in myocardial
contraction.
Repolarization refers to recovery of the cell to its resting state. - Repolarization normally
results in myocardial relaxation
The action potential is a five phase cycle that produces changes in the cell membrane's
electrical charge - depolarization [phase 0]-the cell is almost impermeable to sodium
unless a stimulus occurs. This stimulus can be from the SA node, defibrillation,
hypoxemia, respiratory acidosis, and sodium bicarbonate.
in depolarization, more sodium moves into the cell through the fast sodium channels
and creates a fast response action potential. - the inside of the cell becomes positively
charged.
early repolarization [phase 1]- sodium channels close - plateau phase [phase 2] -
calcium channels open. the influx of calcium maintains the positive charge
[depolarization] a little longer.
repolarization [phase 3] - potassium shifts back into the cell to create the original
electrochemical gradient - resting membrane potential [phase 4] - repolarization is
completed, the original electrochemical gradient is in place, and the cell is ready to be
depolarized again
beginning in the phase 0 and ending at the midpoint of phase 3, the cell cannot respond
to another stimulus regardless of the strength of the stimulus. this is called the absolute
refractory period. - It is completely resistant to stimuli because the sodium channel is
still inactivated by the previous stimulation.
beginning at the midpoint of phase 3 and lasting until the beginning of phase 4, the cell
recovers sufficiently to allow a stronger than normal electrical stimulus to trigger
depolarization. This period is called the relative refractory period. - phase 4 represents a
,time in which a weaker than normal stimulus can produce depolarization. This is called
the supranormal period.
examples of serious abnormal cardiac rhythms that can result from depolarization
during the relative refractory and supranormal periods are supraventricular and
ventricular tachycardia. - certain cardiac cell have a unique characteristic called
automaticity is the ability to spontaneously depolarize; i.e., they can create a repetitive
impulse without requiring external stimulation.
Such cells are know as pacemaker cells, the differ from regular cardiac cells in that they
have a constant sodium influx; thus, they slowly depolarize at a steady rate until the
threshold is reached and an action potential created - through the characteristic of
conductivity, impulses generated by pacemaker cells are then transmitted to the
surrounding myocardium
the three potential pacemakers include: sinus or SA node, atrioventricular AV junction,
and the ventricular purkinje fibers. - the sinus or SA node generates an impulse at 60 to
100 beats per minute and is located in the upper right atrium near where the superior
vena cave comes into the heart
the atrioventricular junction generates an impulse at 40 to 60 beats per minute and is
located in the base of the right atrium near the tricuspid valve and is available if the SA
node pacemaker fails - the ventricular purkinje fibers, generate an impulse of 20 to 40
beats per minute and are located in the ventricular walls as a network and is available if
SA node and AV junction pacemakers fail.
with the normal conduction pathway the impulse originates in the SA node and travels
through special conductive [internodal] pathways to the AV node. - the impluse is
delayed at the AV node for about 0.1 second to allow the atria to depolarize and
contract
the impulse then continues on down the conduction pathway to the bundle of His - from
the bundle of His, the impulse rapidly travels through the right and left bundle branches
located in the septum and into a network of special conductive fibers called purkinje
fibers, which carry impulses directly to the ventricular muscle cells.
the ECG is a graphic representation of the electrical, not mechanical, activity of the
heart. - the flat base line is the isoelectric line and it represents the absence of electrical
activity.
deflections that rise above the isoelectric line are positive waves and those that fall
below the line are negative. - a 12-lead ECG provides 12 different views of the electrical
output of the heart
, the P wave indicates atrial depolarization, stimulated by the firing of the SA node. - the
PR interval depicts conduction of the impulse from the SA node through the internodal
pathway to the AV node and downward to the ventricles
the QRS complex reflects ventricular depolarization. Atrial repolarization is also
occurring during this period but is is hidden by the QRS complex - the normal QRS is
spiked in appearance and its shape varies between individuals and may consists of 1 to
3 waves
Q wave - a negative deflection that precedes a positive deflection. it begins at the end of
the PR interval. there can be only one with each QRS complex or it may not be present
- R wave - one or more positive deflections. if there is more than one , they are labeled
as R and R [R prime].
S wave - a negative deflection that follows the R wave. if there is no R wave in the
complex, it is labeled a QS wave - the ST segment is a line that begins directly after the
QRS complex and ends at the beginning of the T wave. It is normally located at the
isoelectric line
the ST segment represents the completion of ventricular depolarization and the
beginning of ventricular repolarization - the T wave directly follow the ST segment and
appears as a rounded slightly asymmetrical positive deflection when the QRS complex
is positive. It represents repolarization of the ventricles
the T wave is often referred to as the resting phase of the cardiac cycle. - the upstroke
of the T wave occurs during the absolute refractory period
the downstroke of the T wave represents a relative refractory period during which time a
sufficiently strong electrical stimulus can trigger depolarization. The heart is vulnerable
during the relative refractory period. - an abnormal impulse [dysrhythmia] of sufficient
electrical strength that triggers prematurely, on the down-stroke of the T wave, can
result in development of a potentially life-threatening dysrhythmia such as, V-tach or V-
fib AKA the R on T phenomenon.
the T wave become flatten or inverted in the presence of myocardial ischemia. - a
patient with elevated serum potassium level [hyperkalemia] develops a tall peaked T
wave
the ST segment can become depressed with myocardial ischemia and elevated with
myocardial infarction - the QT interval represents ventricular depolarizaton and
repolarization. it is measured from the beginning of the QRS complex to the end of the T
wave
all cardiac monitoring systems use four basic components: an oscilloscope display
system [ECG machine or cardiac monitor], a monitor cable, lead wires, and electrodes -
electric impluses sent out from the heart are detectable on the skin throughout the body.
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