EKG Interpretation
Epidemiology of dysrhythmia
In 2003, dysrhythmias caused or contributed to 479,000 deaths. The conduction system is susceptible to damage by
heart disease. Ischemia can cause tissues of the conduction system to be irritable or excitable (extra beats).
Ischemia can also cause tissues to block electrical impulses. This effects hemodynamics and the hearts ability to
perfuse the tissues. Note that some dysrhythmias are benign, but we’re focusing today on the lethal ones.
Basic Electrophysiology
Electrophysiology is the study of the electrical properties of the heart. There is a pattern of electrical impulses through
the conduction system of the heart, whereby electrical signals become mechanical events. However, sometimes the
mechanical event does not happen leading to a dysrhythmia or an arrhythmia.
• dysrhythmia = change in rhythm
• arrhythmia = no rhythm
The SA node sends the impulse down the intranodal pathway to the AV node to the Bundle of His through the bundle
branches to the Perkinje fibers. Think of the AV node as a “relay station”, and the branches as a “freeway system.”
On the EKG we are watching the timing of the electrical impulses to see if it follows a pathway (freeway), or if it got
off the freeway and took a “side street”. If it took a side street, the timing is going to be slooowwwer...just like in real
life!
Nursing Role
• Monitoring and identifying dysrhythmias
• Patient symptoms
• Chest pain
• Shortness of Breath
• Hypotension
• Altered mental status
• Consider the cause of the dysrhythmia Intrinsic Rates of the Heart
• Intervene appropriately for life threatening dysrhythmias SA Node: 60-100 beats per minute
AV Node: 40-60 beats per minute
The 4 Properties of Cardiac Cells Purkinje: 20-40 beats per minute
1. Automaticity - this is the pacing function of the heart and is the role
of the SA Node (preferred), the AV Node and the Purkinje fibers.
The AV node is a relay station and it can block the SA node if it is going too fast, or block the SA completely
if the AV is ischemic. The Purkinje fibers will take over if nothing else is working well...this is pretty bad news
for your patient.
2. Excitability - this is the ability to respond to an electrical impulse and explains why the impulse can get off
the freeway and take the side streets. All cardiac cells are excitable!
3. Conductivity - this is the ability to transmit the electrical impulse
4. Contractility - this is the ability of cardiac cells to shorten in response to an electrical stimulus. This is the
mechanical event we mentioned earlier!
Some meds will increase contractility of the heart such as Digoxin, Dopamine, Dobutamine.
Cardiac Action Potential
Polarization = cell is at rest and ready for an impulse
Depolarization = reversal of electrical charge across cell membrane (this is the Na and K changing places)
Repolarization = recovery of the cell to its original polarized state (Na and K returning to their original positions). The
cell is refractory during this time period. If a cell receives an impulse during this period, it gets irritable leading to a
lethal rhythm and sudden cardiac death. No bueno! More on refractory periods below.
, Refractory Periods...3 Stages
• Absolute refractory period: Cardiac cells will NOT respond to a stimulus AT ALL! No way Jose!
• Relative refractory period: This is a vulnerable period. Some cells have repolarized and the tissue may
respond with a strong impulse.
• Supernormal period: Weaker than normal stimulus could cause depolarization. The cell is “hyper” during this
time and it doesn’t take much to set it off. Stimulation at this time often results in very fast, dangerous rhythms.
Cardiac Conduction System
• SA Node is the pacemaker for the heart. It is located on the upper posterior wall of the RA. It generates a
stimuli at regular intervals (60-100 bpm), and it corresponds with the P WAVE on the ECG.
• Intranodal pathways (between SA and AV nodes)
• AV Node slows the impulse coming from the SA Node (recall that the AV is the “relay station”). The slowing of
the SA node allows the atria time to contract and the ventricles to fill. The ventricles have to be nice and full in
order to have optimal cardiac output! The AV can spontaneously generate an impulse btwn 40-60 bpm and
this is usually an “escape mechanism” or “rescue mechanism”. The AV node’s impulse is portrayed as the PR
interval on the EKG. We want to keep an eye on the length of this line!
• The Bundle of His is the “freeway system” of the heart. It bifurcates into the left and right branches and travels
through the ventricles.
• The Purkinje fibers allow for rapid depolarization of the ventricle. It is seen as the QRS on the ECG. If left to
its own devices, Purkinje would fire at 20-40 bpm...not very good!
ECG Waveforms
The full cardiac cycle consists of a P wave, a PR interval, a QRS complex, an ST segment, a T wave and an
isoelectric line.
• P wave is the start of the cardiac cycle. It results from the electrical firing of the SA node/atrial depolarization.
It is rounded and smooth in appearance, and has a positive deflection (it points up like a little hill). All P waves
on the EKG should look the same. If they don’t something is going on with the AV node.
• The PR Interval relates to the depolarization of the right and left atria, and the impulse delay through the AV
junction (which is your relay station). This is a period of electrical silence and it establishes the isoelectric line.
Your ST segment needs to be at the same level as this line!
• It is measured from the beginning of the P wave to the beginning of the
QRS complex. It’s normal length is 0.12 to 0.20 seconds (3-5 little boxes).
• If the PR interval is long this means there is a blockage in the AV node The Boxes
(most likely d/t ischemia) Each little box signifies TIME
• The QRS Complex is made up of 3 waveforms and these can vary Each LITTLE box = 0.04 seconds
dramatically depending on the view you are utilizing. It represents the Each BIG box = 0.20 seconds
depolarization of the ventricles.
• Q wave is the FIRST downward deflection after the P wave...it can be
pathological indicating damage to the heart muscle. The Q wave is usually not there unless the pt has had
an acute MI in the past. Tiny ones are OK though.
• R wave is the FIRST upward deflection in the QRS. This is always a good thing! On a 12-lead view, we
are looking for R-wave progression...it should get taller and taller as time goes on. If pt has had an acute
MI, the R wave will not progress normally b/c the Q gets in the way.
• S wave is the downward deflection AFTER the R wave.
• The QRS should be no wider than 3 little boxes (0.12 seconds) If it is wider, this means there is a
blockage along the bundle and the impulse got off the freeway and took the side street.
• ST segment is an isoelectric line between the QRS and the T-wave. It should be on the same plane as the PR
interval.
• If it is elevated or depressed this is indicative of injury in the myocardium.
• It begins at the end of the QRS and ends at the beginning of the T wave.
• It represents early repolarization of the ventricle
• T Wave follows the ST segment and represents ventricular repolarization
• It is usually rounded and deflected in the same direction as the QRS.
• A negative T-wave following a positive QRS is suggestive of ischemia
Epidemiology of dysrhythmia
In 2003, dysrhythmias caused or contributed to 479,000 deaths. The conduction system is susceptible to damage by
heart disease. Ischemia can cause tissues of the conduction system to be irritable or excitable (extra beats).
Ischemia can also cause tissues to block electrical impulses. This effects hemodynamics and the hearts ability to
perfuse the tissues. Note that some dysrhythmias are benign, but we’re focusing today on the lethal ones.
Basic Electrophysiology
Electrophysiology is the study of the electrical properties of the heart. There is a pattern of electrical impulses through
the conduction system of the heart, whereby electrical signals become mechanical events. However, sometimes the
mechanical event does not happen leading to a dysrhythmia or an arrhythmia.
• dysrhythmia = change in rhythm
• arrhythmia = no rhythm
The SA node sends the impulse down the intranodal pathway to the AV node to the Bundle of His through the bundle
branches to the Perkinje fibers. Think of the AV node as a “relay station”, and the branches as a “freeway system.”
On the EKG we are watching the timing of the electrical impulses to see if it follows a pathway (freeway), or if it got
off the freeway and took a “side street”. If it took a side street, the timing is going to be slooowwwer...just like in real
life!
Nursing Role
• Monitoring and identifying dysrhythmias
• Patient symptoms
• Chest pain
• Shortness of Breath
• Hypotension
• Altered mental status
• Consider the cause of the dysrhythmia Intrinsic Rates of the Heart
• Intervene appropriately for life threatening dysrhythmias SA Node: 60-100 beats per minute
AV Node: 40-60 beats per minute
The 4 Properties of Cardiac Cells Purkinje: 20-40 beats per minute
1. Automaticity - this is the pacing function of the heart and is the role
of the SA Node (preferred), the AV Node and the Purkinje fibers.
The AV node is a relay station and it can block the SA node if it is going too fast, or block the SA completely
if the AV is ischemic. The Purkinje fibers will take over if nothing else is working well...this is pretty bad news
for your patient.
2. Excitability - this is the ability to respond to an electrical impulse and explains why the impulse can get off
the freeway and take the side streets. All cardiac cells are excitable!
3. Conductivity - this is the ability to transmit the electrical impulse
4. Contractility - this is the ability of cardiac cells to shorten in response to an electrical stimulus. This is the
mechanical event we mentioned earlier!
Some meds will increase contractility of the heart such as Digoxin, Dopamine, Dobutamine.
Cardiac Action Potential
Polarization = cell is at rest and ready for an impulse
Depolarization = reversal of electrical charge across cell membrane (this is the Na and K changing places)
Repolarization = recovery of the cell to its original polarized state (Na and K returning to their original positions). The
cell is refractory during this time period. If a cell receives an impulse during this period, it gets irritable leading to a
lethal rhythm and sudden cardiac death. No bueno! More on refractory periods below.
, Refractory Periods...3 Stages
• Absolute refractory period: Cardiac cells will NOT respond to a stimulus AT ALL! No way Jose!
• Relative refractory period: This is a vulnerable period. Some cells have repolarized and the tissue may
respond with a strong impulse.
• Supernormal period: Weaker than normal stimulus could cause depolarization. The cell is “hyper” during this
time and it doesn’t take much to set it off. Stimulation at this time often results in very fast, dangerous rhythms.
Cardiac Conduction System
• SA Node is the pacemaker for the heart. It is located on the upper posterior wall of the RA. It generates a
stimuli at regular intervals (60-100 bpm), and it corresponds with the P WAVE on the ECG.
• Intranodal pathways (between SA and AV nodes)
• AV Node slows the impulse coming from the SA Node (recall that the AV is the “relay station”). The slowing of
the SA node allows the atria time to contract and the ventricles to fill. The ventricles have to be nice and full in
order to have optimal cardiac output! The AV can spontaneously generate an impulse btwn 40-60 bpm and
this is usually an “escape mechanism” or “rescue mechanism”. The AV node’s impulse is portrayed as the PR
interval on the EKG. We want to keep an eye on the length of this line!
• The Bundle of His is the “freeway system” of the heart. It bifurcates into the left and right branches and travels
through the ventricles.
• The Purkinje fibers allow for rapid depolarization of the ventricle. It is seen as the QRS on the ECG. If left to
its own devices, Purkinje would fire at 20-40 bpm...not very good!
ECG Waveforms
The full cardiac cycle consists of a P wave, a PR interval, a QRS complex, an ST segment, a T wave and an
isoelectric line.
• P wave is the start of the cardiac cycle. It results from the electrical firing of the SA node/atrial depolarization.
It is rounded and smooth in appearance, and has a positive deflection (it points up like a little hill). All P waves
on the EKG should look the same. If they don’t something is going on with the AV node.
• The PR Interval relates to the depolarization of the right and left atria, and the impulse delay through the AV
junction (which is your relay station). This is a period of electrical silence and it establishes the isoelectric line.
Your ST segment needs to be at the same level as this line!
• It is measured from the beginning of the P wave to the beginning of the
QRS complex. It’s normal length is 0.12 to 0.20 seconds (3-5 little boxes).
• If the PR interval is long this means there is a blockage in the AV node The Boxes
(most likely d/t ischemia) Each little box signifies TIME
• The QRS Complex is made up of 3 waveforms and these can vary Each LITTLE box = 0.04 seconds
dramatically depending on the view you are utilizing. It represents the Each BIG box = 0.20 seconds
depolarization of the ventricles.
• Q wave is the FIRST downward deflection after the P wave...it can be
pathological indicating damage to the heart muscle. The Q wave is usually not there unless the pt has had
an acute MI in the past. Tiny ones are OK though.
• R wave is the FIRST upward deflection in the QRS. This is always a good thing! On a 12-lead view, we
are looking for R-wave progression...it should get taller and taller as time goes on. If pt has had an acute
MI, the R wave will not progress normally b/c the Q gets in the way.
• S wave is the downward deflection AFTER the R wave.
• The QRS should be no wider than 3 little boxes (0.12 seconds) If it is wider, this means there is a
blockage along the bundle and the impulse got off the freeway and took the side street.
• ST segment is an isoelectric line between the QRS and the T-wave. It should be on the same plane as the PR
interval.
• If it is elevated or depressed this is indicative of injury in the myocardium.
• It begins at the end of the QRS and ends at the beginning of the T wave.
• It represents early repolarization of the ventricle
• T Wave follows the ST segment and represents ventricular repolarization
• It is usually rounded and deflected in the same direction as the QRS.
• A negative T-wave following a positive QRS is suggestive of ischemia
