ECG Waveform Explained: Labeled Components
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ECG/EKG Waveform
How to read and interpret electrocardiograms (ECG/EKGs) is an important skill to have.
In order to successfully read an EKG, you must first understand the basics of an EKG waveform.
The main components of an EKG wave include the following:
P wave
PR segment
QRS complex
ST segment
T wave
U wave
TP segment
There are also intervals that will be discussed:
PR interval
QT interval
ECG Waveform: We will explain the above labeled components of an EKG in this lecture.
This lecture will walk you through the different parts of an EKG wave step-by-step!
We will also correlate each part of the EKG to the cardiac cycle, blood flow through the heart, and the cardiac conduction system!
This lecture will also be a great review of the anatomy and physiology of the heart!
So if you want to master EKG interpretation, then you have come to the right spot!
Let’s get right into it!
Cardiac Cycle: Diastole vs Systole
The best way to learn the different components of an EKG is to walk through one cardiac cycle and apply it to the EKG waveform as we go.
Which is exactly what we will do step-by-step!
For more information on the cardiac cycle make sure to check out the following EZmed lecture:
The cardiac cycle has 2 main phases:
Diastole
Systole
Diastole occurs when the heart is at rest (particularly the ventricles), which allows the cardiac chambers to fill with blood.
Systole occurs when the heart is contracting (particularly the ventricles), which pumps blood forward into the main pulmonary artery and aorta.
Diastole vs Systole:
Diastole occurs when the heart is at rest, allowing blood to fill the cardiac chambers.
Systole occurs when the heart is contracting and pumping blood forward into the main pulmonary artery and aorta.
SVC = Superior Vena Cava; IVC = Inferior Vena Cava; RA = Right Atrium; RV = Right Ventricle; LA = Left Atrium; LV = Left Ventricle; PA = Pulmonary Artery (main); PV = Pulmonary Veins
EKG Wave Components - PQRSTU
Let’s first label the main parts of an EKG waveform, and then we will walk through each component step-by-step and apply it to the cardiac cycle.
An EKG contains 3 main components:
Waves
Segments
Intervals
1. Waves
A wave is a positive or negative deflection from baseline.
A wave indicates a change in voltage or an electrical event.
The main EKG waves include:
P wave
Q wave
R wave
S wave
T wave
U wave
Together the Q wave, R wave, and S wave form the QRS complex.
The U wave is not always visible on a normal EKG, but would be located after the T wave if present and is usually in the same direction as the T wave.
The waves are easy to remember because they go in alphabetical order starting with “P” and ending with “U”.
Each wave will be discussed below.
Labeled ECG: Waves include the P wave, Q wave, R wave, S wave, T wave, and U wave. Together the Q wave, R wave, and S wave form the QRS complex.
2. Segments
A segment is a region between 2 waves.
The main EKG segments include:
PR segment
ST segment
TP segment
Each segment will be discussed below.
Labeled ECG: Segments include the PR segment, ST segment, and TP segment
3. Intervals
An interval is a duration of time that includes one segment and one or more waves.
The main EKG intervals include:
PR interval
QT interval
Each interval will be discussed below.
Labeled ECG: Intervals include the PR interval and QT interval
TP Segment - Early Diastole
Let’s now walk through the cardiac cycle and apply it to an EKG waveform, starting with diastole and the TP segment.
As previously mentioned, diastole is when the heart is at rest allowing for the cardiac chambers to fill with blood.
Deoxygenated blood from the systemic vasculature enters the right atrium via the superior vena cava (SVC) and inferior vena cava (IVC).
Oxygenated blood from the pulmonary vasculature enters the left atrium via the pulmonary veins.
As blood enters the right and left atria, the atrial pressures begin to increase.
The ventricles are in a low pressure state during diastole because they are relaxed and not contracting.
Since the ventricles are in a low pressure state, the atrial pressure will eventually surpass that of the ventricles as the atria fill with blood.
Once the right atrial pressure is greater than the right ventricular pressure, the tricuspid valve will open.
Once the left atrial pressure is greater than the left ventricular pressure, the mitral valve will open.
Remember the tricuspid valve is located between the right atrium and right ventricle, and the mitral valve is located between the left atrium and left ventricle.
Due to the atrial pressure being greater than the ventricular pressure, as well as a suction mechanism and the valves being open, blood from the right atrium will enter the right ventricle and blood from the left atrium will enter the left ventricle.
**Remember to check out the following lectures if you need more information or a refresher on what was discussed above!
EKG Application
The phase described above is early diastole - In which the atria fill with blood and atrial pressure increases, thereby opening the tricuspid and mitral valves, followed by blood flowing from the atria to the ventricles.
There is no conduction or electrical activity taking place in the heart during early diastole.
Early diastole is an isoelectric period, which means there is no electrical activity, action potential, or changes in voltage across cell membranes.
The flow of blood is passive and is simply from differences in atrial and ventricular pressures.
Since there is no electrical activity, early diastole will appear as a “flat line” on EKG.
The TP segment is the flat line that represents early diastole.
Remember we said a segment is the region between 2 waves.
As the name suggests, the TP segment is the region between the T wave (ventricular repolarization - to be discussed later) and the P wave (atrial depolarization - to be discussed next).
TP Segment = Early Diastole:
During early diastole blood enters the right atrium (RA) via the superior and inferior vena cava, and blood enters the left atrium (LA) via the pulmonary veins.
As the pressures in the atria increase, the tricuspid and mitral valves open.
The ventricles are in a low pressure state during diastole, so blood in the atria will flow into the ventricles as a result (RV = Right Ventricle; LV = Left Ventricle)
This process is passive/isoelectric and appears as the TP segment on EKG.
P wave - Atrial Depolarization
Let’s continue through the cardiac cycle and discuss the P wave next.
As blood flows from the atria to the ventricles as explained above, the ventricular pressure starts to increase.
As a result, the atria must increase their pressure in order to continue to pump the remaining atrial blood into the ventricles.
The right and left atria increase their pressure by contracting.
In order for the atria to contract they must depolarize, which brings us to the conduction system of the heart.
In a normal functioning heart, the SA node (sinoatrial node) is the primary pacemaker that generates a cardiac action potential or electrical impulse.
The SA node is made up of pacemaker cells that have the ability to generate their own spontaneous action potentials.
The SA node is located in the back of the right atrium near the opening of the superior vena cava.
Once the SA node generates a cardiac action potential, the electrical impulse will then travel from the SA node through the atria.
The action potential travels to the left atrium via Bachmann’s bundle and through the right atrium via the internodal pathways.
As the action potential travels through the atria, the atria depolarize.
The depolarization of atrial myocytes (muscle cells) leads to atrial contraction.
Atrial contraction will increase atrial pressure, which will allow for further blood to flow from the atria into the ventricles.
The process of atrial depolarization and contraction occurs during mid-late diastole.
EKG Application
The phase described above is mid-late diastole - In which the SA node generates a spontaneous action potential that travels through the atria, leading to atrial depolarization and contraction.
The atrial contraction increases atrial pressure, which allows for further blood flow from the atria to the ventricles.
Unlike early diastole in which blood flow is passive from pressure differences, late diastole involves atrial depolarization and electrical activity.
In order for cells to depolarize, there must be a change in voltage across cell membranes.
Changes in voltage or electrical activity appear as positive or negative deflections on an EKG, known as waves.
Since atrial depolarization involves changes in voltage across cell membranes, we will see a wave on EKG (P wave).
The P wave on EKG represents the atrial depolarization and contraction that occurs during mid-late diastole.
P wave = Atrial Depolarization:
The ventricles begin to fill with blood as blood passively flows from the atria to the ventricles (see previous section). As a result, ventricular pressure starts to increase.
During mid-late diastole, the atria depolarize and contract to increase their pressure, which allows for further blood to flow from the atria to the ventricles.
The action potential generated by the SA node depolarizes the right and left atria.
Atrial depolarization and contraction appear as the P wave on EKG.
PR Segment - Late Diastole
Let’s continue through the cardiac cycle and discuss the PR segment on EKG next.
As previously mentioned, the SA node generates a spontaneous action potential and sends its electrical impulse through the atria.
This leads to atrial depolarization and contraction, represented by the P wave on EKG.
The electrical impulse then converges on another node, called the AV node (atrioventricular node).
Similar to the SA node, the AV node is made up of pacemaker cells as well.
The AV node is located at the base of the right atrium.
The AV node slows down the conduction velocity of the electrical impulse to allow time for the atria to contract before depolarizing the ventricles.
If the electrical impulse did not slow down at the AV node, then that atria and ventricles would contract at the same time which would be detrimental.
The delay in conduction velocity at the AV node allows the atria to finish contracting and pumping blood from the atria to the ventricles.
This process occurs during late diastole.
EKG Application
The phase described above is late diastole - In which the AV node slows down the conduction velocity of the electrical impulse to allow time for the atria to finish contracting and pumping blood into the ventricles.
The pause in conduction that takes place at the AV node leads to another “flat line” on EKG, represented by the PR segment.
Although the atria remain depolarized during this time, there are no new voltage changes across cell membranes.
As a result, the PR segment is another isoelectric period represented by a “flat line” on EKG and there are no designated waves.
Remember segments are located between waves!
Therefore, the PR segment is the period between the end of the P wave (atrial depolarization as discussed above) and the start of the QRS complex (ventricular depolarization discussed next).
The PR segment mainly represents the period of time in which the electrical impulse is delayed at the AV node before traveling to the ventricles and depolarizing them.
PR Segment = Late Diastole:
The action potential generated by the SA node travels through the atria and enters the AV node.
Conduction velocity is slowed through the AV node to allow time for the atria to contract before the ventricles are depolarized.
The pause in conduction at the AV node appears as the PR segment on EKG.
PR Interval
Do not confuse the PR segment with the PR interval (which includes the P wave).
Remember intervals include one segment and one or more waves!
The PR interval is the duration from the onset of the P wave (atrial depolarization) to the onset of the QRS complex (ventricular depolarization).
In other words, the PR interval is the PR segment + the P wave.
As a reminder the PR segment is the region between the end of the P wave and the start of the QRS complex.
The PR interval represents the time between atrial depolarization and ventricular depolarization as the electrical impulse travels from the SA node through the atria and AV node.
In other words, the PR interval primarily reflects atrial depolarization and the conduction from the SA node through the AV node.
Prolongation of the PR interval may suggest the presence of an AV nodal heart block.
PR Interval vs PR Segment:
The PR segment is the region between the end of the P wave and the start of the QRS complex (blue).
The PR interval is the PR segment + P wave, or the duration from the start of the P wave to the start of the QRS complex (yellow)
QRS Complex = Ventricular Depolarization
Let’s continue through the cardiac cycle and discuss the QRS complex next.
As previously mentioned, the action potential generated by the SA node will travel through the atria and converge onto the AV node, where conduction is slowed down to allow time for the atria to contract.
The electrical impulse will then exit the AV node and travel through the bundle of His, followed by the right and left bundle branches, and lastly the Purkinje fibers.
The action potential depolarizes the ventricles as it travels through the His-Purkinje system.
The right bundle branch mainly depolarizes the right ventricle, and the left bundle branch mainly depolarizes the left ventricle.
Ventricular depolarization leads to ventricular contraction and the start of systole.
Remember systole is the cardiac phase in which the heart (particularly the ventricles) contracts to pump blood forward into the main pulmonary artery and aorta.
EKG Application
The phase described above is the start of systole - In which the action potential exits the AV node and travels through the bundle of His, bundle branches, and Purkinje fibers which leads to ventricular depolarization and contraction.
Ventricular depolarization leads to a change in voltage across the cell membranes of ventricular myocytes (muscle cells).
Since there is electrical activity and changes in voltage present, we will see another wave on EKG.
The wave is known as the QRS complex, and it represents ventricular depolarization and contraction.
The QRS complex is made up of the Q wave, R wave, and S wave.
QRS Complex = Ventricular Depolarization:
The action potential exits the AV node and travels through the bundle of His, followed by the right and left bundle branches, and lastly the Purkinje fibers.
The ventricles depolarize and contract as the action potential travels through the His-Purkinje system.
Ventricular depolarization appears as the QRS complex on EKG.
ST Segment = Systole
Let’s continue through the cardiac cycle and discuss the ST segment on EKG next.
As previously mentioned, the action potential will travel through the bundle of His, bundle branches, and Purkinje fibers.
The ventricles depolarize and contract as the action potential travels through the His-Purkinje system.
Ventricular pressure begins to increase as the ventricles contract.
Moreover, atrial repolarization occurs during ventricular depolarization which reduces atrial pressure.
Once the right ventricular pressure surpasses the right atrial pressure, the tricuspid valve closes.
Once the left ventricular pressure surpasses the left atrial pressure, the mitral valve closes.
All 4 valves are closed at this point.
As the ventricles continue to contract during ventricular depolarization, their pressure increases even more.
Once the right ventricular pressure surpasses the pressure of the main pulmonary artery, the pulmonic valve will open.
This will allow for blood to pump from the right ventricle to the pulmonary vasculature via the main pulmonary artery.
Once the left ventricular pressure surpasses the pressure of the aorta, the aortic valve will open.
This will allow for blood to pump from the left ventricle to the systemic vasculature via the aorta.
This process occurs during systole.
EKG Application
The phase described above is systole - In which ventricular depolarization leads to ventricular contraction, thereby increasing ventricular pressure and causing the tricuspid/mitral valves to close and the pulmonic/aortic valves to open.
The right ventricle is then able to pump blood into the main pulmonary artery, and the left ventricle is able to pump blood into the aorta.
Since the ventricles remain depolarized and there are no new voltage changes present, this will appear as a “flat line” on EKG.
In other words, it is another isoelectric period in which the ventricles remain depolarized, in contraction, and continue to pump blood forward.
This isoelectric period is the ST segment on EKG.
Remember segments are regions between waves.
The ST segment is the region between the end of the QRS complex (ventricular depolarization) and the start of the T wave (ventricular repolarization discussed next).
IMPORTANT: Changes to the ST segment becomes clinically relevant in diagnosing ventricular ischemia or hypoxia, especially STEMIs (ST-elevation myocardial infarctions or heart attacks) in which the ST segment can become elevated or depressed.
ST Segment = Systole:
The heart is in another isoelectric period while the ventricles remain depolarized to allow time for ventricular contraction and for blood to pump out of the heart.
This appears as the ST segment on EKG.
T wave = Ventricular Repolarization
Let’s continue through the cardiac cycle and discuss the T wave next.
As previously mentioned, the ventricles are depolarized and contracting which allows for blood to pump out of the right and left ventricles and into the main pulmonary artery and aorta respectively.
The ventricular pressure begins to decrease as blood is pumped out of the ventricles.
Furthermore, the ventricular myocytes (muscle cells) begin to repolarize and relax.
This leads to ventricular repolarization and relaxation, which decreases the ventricular pressures even more.
Once the right ventricular pressure falls below the main pulmonary artery pressure, the pulmonic valve will close.
Once the left ventricular pressure falls below the aortic pressure, the aortic valve will close.
This marks the end of systole.
EKG Application
The phase described above is the end of systole - In which ventricles empty with blood and ventricular repolarization occurs, thereby decreasing ventricular pressure and causing the pulmonic and aortic valves to close.
As the ventricles repolarize, the ventricular myocytes become more negative and there is another change in voltage across the cell membranes as a result.
This will result in another wave on EKG, known as the T wave.
The T wave represents ventricular repolarization and subsequent ventricular relaxation.
Of note, you will notice there was not a designated wave for atrial repolarization. This is because atrial repolarization occurs during ventricular depolarization, so it gets buried in the QRS complex.
T wave = Ventricular Repolarization
The ventricular myocytes repolarize and relax.
The T wave on EKG reflects ventricular repolarization.
QT Interval
The QT interval is another important interval to know on EKG.
Remember intervals include one segment and one or more waves!
The QT interval is the duration from the onset of the QRS complex (onset of the Q wave) to the end of the T wave.
In other words, the QT interval is the ST segment + the QRS complex + the T wave.
As a reminder the ST segment is the region between the end of the QRS complex and the start of the T wave.
The QT interval represents the entire duration of ventricular systole, which includes ventricular depolarization (QRS complex) and ventricular repolarization (T wave).
The electrical impulse is traveling through the His-Purkinje system during the QT interval.
QT Interval: The QT interval is the ST segment + QRS complex + T wave, or the duration from the start of the QRS complex to the end of the T wave (orange)
The QT interval represents ventricular systole, which includes ventricular depolarization (QRS complex) and ventricular repolarization (T wave).
TP Segment = Early Diastole
We are now back to where we started.
The ventricles have repolarized (T wave) and are relaxed again.
The ventricular pressures are low, allowing for blood to fill the cardiac chambers as discussed above.
EKG Application
We are now back to where we started on the EKG.
Ventricular repolarization has occurred (T wave), and we are in another isoelectric period until the atria depolarize (P wave) and contract again.
The region between the end of the T wave and the start of the P wave is known as the TP segment.
TP Segment = Early Diastole
The ventricles have repolarized and are relaxed again.
The heart is back in diastole, which will allow the cardiac chambers to fill with blood again.
This appears as the TP segment on EKG.
Summary
Hopefully this gave you a clear understanding of the different components of an EKG waveform, and how they correlate with the cardiac cycle and conduction system.
Simply remember the following:
P wave - Atrial depolarization and contraction
QRS complex - Ventricular depolarization and contraction
T wave - Ventricular repolarization and relaxation
PR segment - Region between the end of the P wave and the start of the QRS complex - Primarily reflects the time in which the electrical impulse travels through the AV node and the atria are still depolarized and contracting during late diastole.
ST segment - Region between the end of the QRS complex and the start of the T wave - Primarily reflects the time in which the ventricles are depolarized, contracting, and pumping blood forward during systole.
TP segment - Region between the end of the T wave and the start of the P wave - Primarily reflects early diastole in which the heart is relaxed and filling with blood.
PR Interval - Duration of atrial depolarization (start of the P wave to the start of the QRS complex).
QT Interval - Duration of ventricular systole, including ventricular depolarization and ventricular repolarization (start of QRS complex to the end of the T wave).
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