Electrocardiography Flashcards
5 important pieces of information that ECG provide
- Anatomical orientation of the heart
- Relative sizes of chambers
- Disturbances in rhythm and conduction
- Extent, location, progress of ischemic damage
- Influence of certain drugs
What is Arrythmia?
the disturbance in rhythm and conduction
ECG can provide information about ___ in case of a myocardial infraction.
the ischemic damage
How ECG works
: during the recording of the ECG we use surface electrodes as they can detect the electrical activity of the heart.
There are 2 conditions needed for ECG to function correctly
-> What are these 2 conditions?
- a large number of cells is needed
- synchronized electrical activity needs to be achieved.
=> allow us to record the small electrical currents from the heart with surface electrodes.
What types of electrodes we need to use during recording?
ECG must use extracellular electrodes during recording
ECG must use extracellular electrodes during recording. So there are two extracellular electrodes applied and connected to a voltmeter.
-> When a depolarization wave is incoming through a cell or a fiber.
-> What do we need to check?
the charge distribution
When a depolarization wave is incoming through a cell or a fiber.
The first part where the depolarization is incoming is depolarized
=> What does this mean?
it will be positive intracellularly and negative intracellularly.
When a depolarization wave is incoming through a cell or a fiber.
- The first part where the depolarization is incoming is depolarized which means it will be positive intracellularly and negative intracellularly.
=> What will happen in the second part?
The second part will still be at rest so it will be positive extracellularly and negative intracellularly.
(. So if we check the extracellular charge distribution a dipole can be created which points from negative to positive charges. A positive deflection will be recorded.)
Now if the whole fiber gets depolarized both parts will be depolarized
-> What does it mean?
we will have extracellularly negative charge and positive charge intracellularly on both parts of the fibers.
-> No potential difference in this case as there are negative charges everywhere. The zero signal will be recorded and deflection drops down.
What will happen after depolarization?
repolarization occurs
After depolarization repolarization occurs.
-> What does repolarization wave do?
The repolarization wave goes through the first part of the fiber and it will be repolarized
=> meaning it will be positive extracellularly and negative intracellularly.
After depolarization repolarization occurs
-> What will happen in the second part?
The second part will still be depolarized as repolarization wave didn’t reach it yet so it will be positive intracellularly and negative extracellularly.
(A dipole will be created from the extracellular charges and it will point towards the positive charges so to the negative end of the volt meter. A negative deflection will be created.)
After depolarization repolarization occurs.
=> Where does this mechanism occur?
That is the mechanism that occurs in axons and atrial fibers.
Extracellular recording 3: ventricular fibers
=> What will happen this time?
This time the repolarization will come from the other direction so the positive end of the electrode will be repolarized first and will have a positive extracellular charge while the other end will still be depolarized and have a negative extracellular charge.
=> The dipole will point towards the positive charge causing a positive deflection this time.
Why is there a difference in the direction of repolarization wave of ventricular fibers and repolarization wave of atrial fibers and axons?
The depolarization wave starts by going through the SA-node->Atrium->AV-nod->bundle branches of HIS-> endocardium-> epicardium.
However the duration of the action potential is not the same.
The endocardial action potential duration is ___ (shorter/longer) than the epicardial action potential duration.
longer
The endocardial action potential duration is longer than the epicardial action potential duration.
-> what is the consequence?
The consequence of this is that the repolarization will start from the epicardial surface and goes towards the endocardial surface. That’s why the direction of repolarization and depolarization are opposites.
Following the electrical activity of the heart is done by __
by using multiple leads.
6 leads are used in the frontal plane and 6 leads are used in the transverse plane (total is 12 leads used).
In the frontal and transverse planes, what type of leads do we use?
In the frontal we use limb leads and in transverse we use precordial leads.
What are the types of limb leads?
Limb leads can be 3 bipolar and 3 pseudo-polar and the precordial are all unipolar.
What are the 3 requirements for ECG?
- Supine, relaxed position
- Recording speed: 25 mm/s
- Sensitivity: 10 mm/mV
Bipolar Lead
-> How to measure potential difference?
Potential difference is measured between 2 given points
Bipolar Lead
-> How to measure potential difference?
Potential difference is measured between 2 given points
Describe Eithoven Leads
These 3 bipolar leads are placed on the right shoulder, left shoulder and pubic area. Lead 1 is between the right and left shoulder. Lead 2 is between the right and pubic area. Lead 3 is between the left shoulder and the pubic area. This will create an equilateral triangle with three apexes called the Einthoven triangle.
P wave which represents depolarization.
-> What do we have at this moment?
At this moment we will have an
extracellularly negative charge and the rest of the atria will be not depolarized so we will have a positive charge
Using the triaxial system we can check the dipole created from P-wave, what does this show?
positive deflection.
=> Lead 1 will have the largest positive deflection lead 2 will have a smaller one and lead 3 will have a very small positive deflection.
=> This is know by check the perpendicular heart projection of the heart vectors.
What does T-wave represent?
atrial repolarization but it cannot really be viewed on the ECG as its small signal is obscured by the large ventricular depolarization signal.
What does this QRS complex I show?
we are at rest so extracellular charges are all positive and no difference in charges no dipole or deflection shown:
What does this QRS complex II show?
the intraventricular septum is activated and will be negative extracellularly and the other parts of the ventricles will still have a positive extracellular charge as they aren’t activated yet.
QRS complex 2: the intraventricular septum is activated and will be negative extracellularly and the other parts of the ventricles will still have a positive extracellular charge as they aren’t activated yet.
=> Describe this in terms of leads?
When the perpendicular projections of heart vectors are checked we can see a large negative deflection in lead 1 smaller in lead 2. In lead 3 we see a small positive deflection.
What does this QRS complex 3 show?
the whole endocardial heart is depolarized but the epicardial surface is still not activated.
QRS complex 3: the whole endocardial heart is depolarized but the epicardial surface is still not activated.
=> describe this in terms of leads?
When we check the perpendicular heart projection we can see 2 large positive deflection in leads 1 and 2 and a small positive deflection In lead 3.
What does this QRS complex 4 show?
QRS complex 4: at this moment all the parts of the ventricle are depolarized except the basolateral part as it’s the last one to depolarize.
QRS complex 4: at this moment all the parts of the ventricle are depolarized except the basolateral part as it’s the last one to depolarize.
=> describe this in terms of leads
When we check the heart vector again, we will see a positive deflect in lead 1 and small negative in lead 2. Lead 3 will have a large negative deflection.
What does this QRS complex 5 show? Describe this in terms of leads
- QRS complex 5: all the ventricle has been depolarized so we have extracellular negative charge all over leading to no deflection and we go back to the zero point
What does T-wave show?
- T-wave: shows the ventricular repolarization.
T-wave: shows the ventricular repolarization.
=> Describe it in terms of leads
If we check the heart vector we can see a positive deflection in all leads.
=> This is because the repolarization started from the epicardial surface towards the endocardial that’s why the T-wave is positive.
Wave, Segments and Intervals
=> What are the relationships between them?
- segments are between waves
- intervals contain waves and segments.
What does PQ interval show?
the atrioventricular conduction time.
What is QT interval?
can be called electrical systole because its closely related to an average ventricular
action potential.
What does RR-interval show?
shows the duration of one cardiac cycle.
=> This will help us calculate the heart rate which will be 60/RR-interval.
What does Einthoven’s Law state?
potential difference measured in the second lead= the sum of the potential difference of the 1st and 3rd lead at a given moment.
Voltage of second lead will be equal to ___
the potential in the left leg minus the potential in the right arm.
