Lecture 2: Physiological Basis of the ECG Flashcards
How is a electrocardiogram conducted?
9 electrons are placed on the surface of the skin at various locations, obtaining 12 different readings (called leads). Each records the voltage difference between itself and another electrode on the body. If there is a difference–> deflection. If not, no deflection.
What exactly does a ECG measure?
It measures the change in electrical activity of cardiac muscle by measuring the extracellular potential .
What causes a deflection on the ECG?
- When the cardiac tissue has a different membrane potential than the rest of the heart.
- Current flow between regions.
Do the electrodes record voltage differences in either the ventricles or the atria OR between the atria and ventricles?
In EITHER the ventricles or the atria; because there is insulation between the two.
What are the X and Y axis of a ECG?
Y axis–> changes in mV.
X axis–> time
Each electrode of an ECG will look at the _______.
Voltage changes in the heart from different directions.
With an ECG, we measure the intracellular/extracellular potential.
EXTRACELLULAR
What cells are the first to repolarize in cardiac muscle?
The last to depolarize
At rest, the cells are (-) compared to the (+) outside environment.
As cells depolarize, what happens?
The cells become (+) compared to the negative (-) outside environment.
Depolarization is occuring towards the positive electrode, thus, there will be a positive deflection representing depolarization
What does the ECG look like once ALL cells are depolarized?
There is no voltage difference between the two electrodes; thus, we go the isoelectricpoint point.
Image B
What will repolarization look like on the ECG?
Repolarization will reflect an upward reflection (+) the first cells to repolarize are the ones that depolarize last. See before image.
The average current flow occur with ______ towards the base of the heart and with _______ towards the apex.
The average current flow occur with negativity towards the base of the heart and with positivity towards the apex.
Describe and label a normal ECG
- P wave- atrial depolarization
- PR interval-time it takes the SA node to send message to the AV node (normal= .12-.20 seconds)
3. QRS complex- ventricular depolarization from R–>L ventricle; apex–> base; interior to exterior (normal= .05- .1 second)
- ST segment–> phase 2 of AP delays repolarization; still a part of ventricular depolarization. Ventricles are in absolute refractory period because it is ejecting blood.
- T wave- ventricular repolarization from L–> R, base to apex; although still a part of ventrilar depolarization because not all cells have depolarized so they will and continue to eject the remaining blood.
What leads are the P wave upright, inverted and variable?
P waves goes towards a leads, making it ____.
- Upright in: leads 1, 2, V4, V6 and AVF.
- Inverted in: AVR.
- Variable in all other leads.
- P waves go towards a lead, making it positive.
What is a normal PR interval?
.12 –> .20 seconds.
Around 0.16 seconds
What does it mean if our PR interval is more than .2 seconds?
AV block.
Describe the QRS complex
-What is the normal time?
The QRS complex is the period of ventricular depolarization.
Normal time:.05 seconds–> .1 second.
- Blood is currently in the L ventricle. In this stage, isovolumetric ventricle contraction is occuring: no change in volume; thus, all valves are closed. Ventricular muscle is building up pressure to allow ejection through the semilunar valves.
Describe the P wave
P wave- atrial depolarization
-Represents phase 0 of AP in the atria muscle.
At the end of the P wave, all cells are depolarized and we are now at our isoelectric point, but the AP is still occuring because the atria is still contracting.
We do not see repolarization of the atria because it is masked by the atrial T wave.
Describe the QRS complex, ST interval and T wave
What phase of ventricular m. AP does this correspond with?
QRS- ventricular depolarization
- Q (phase 0) of the AP in the ventricular muscle
- At the ST segment, all of the cells are depolarized; thus here is a plateua because mv=0.
- T wave (phase 3) is the repolarization (and some depolarization) of the ventricles. At the end, we are at isoelectric point and everything is back to baseline.
What is a U wave?
U waves are not typically seen. If they are; they follow the T wave.
It can represent many things such as delayed depolarization of the purkinje fibers, prolonged repolarization of mid-myocardial M cells, etc.
At what point are most ventricular myocutes contracting?
At the S-T segment.
When will contraction begin compared to the AP?
Contraction will begin at the begining of the AP, but will not peak until well afterwards.
When is peak atrial contraction?
PR segment.
When is Na+ most permeable?
P wave. Phase 0
What is a segment?
The duration of a single event on the ECG
What is a interval?
The duration of two or more events.
Duration and voltage are tracked on the ECG.
Horizontally, what do 5 large boxes represent.
Vertically, what do 2 large boxes represent?
Horizontally, 5 large boxes are one second. Each small box is .04 seconds.
Vertically, 2 large boxes is 1mV.
What is the PR interval?
From the begining of the P wave–> begining of the QRS complex. Usually lasts 0.16 ms.
What is the QT interval?
begining of the QRS interval until the T wave.
Usually .35 ms.
What is the PR segment?
End of atrial depolarization until the begining of QRS
Where is the ST segment?
End of QRS –> begining of the T wave.
The ST segment likes to move when there is a problem. What does this tells us?
Ventricles are not reaching a consistent period where they all contract at the same ti
Isoelectric point is not reached.
T/F: Electrode placement does not matter for voltage.
False
It DOES
What are the normal voltages for QRS, P wave and T wave?
QRS- 1.0- 1.5 mV from the top of the R wave to the bottom of the S wave
P wave- 0.1- 0.3 mV
T wave- 0.2 -0.3 mV
How can we determine rate?
Count from R wave–> R wave (depolarization of ventricles to the next depolarization of ventricles)
After the first R wave that falls on a black line; label 300–> 150–>100–> 75–> 60–> 50–> 43 bpm
We often look at lead 2 to consider.
Why do we conduct 12 leads?
Each lead shows voltage differences in the myocardium from different perspectives by different electrodes.
What are our standard bipolar limb leads?
Draw them out.
These are the only 3 leads that only use 2 electrodes.
Lead 1: goes from RA–> LA
Lead 2: goes from RA–> LL
Lead 3: goes from LA–> LL.
Vectors are all moving towards + electrode so they should be upward deflections.
What are our augmented limb leads?
Draw them out.
- aVF (foot)–> compares the average of the RA+LA to the LL.
- aVR–> augmented vector, right–> Compares the average of LL+ LA to RA.
- aVL (LA) augemented vector, left–> compares the average of LL+RA to LA.
What are the chest/prechordial leads?
Which ones usually have a negative QRS?
Whivh ones usually have a positive QRS?
V1-V6.
Negative QRS- V1 and V2 (R is small, S is deep)
Positive ORS–> V4, V5, V6.
If not seeing this, something is wrong.
Next; what do we do with with bipolar limb leads and augmented limb leads?
DO IT.
Superimpose them on a chart
As the heart depolarizes, the leads see different things.
If the AP is going to the + electrode–> _______ deflection
If the AP is going to the - electrode–> _____ deflection
If the AP is going to the + electrode–> POSITIVE deflection
If the AP is going to the - electrode–> NEGATIVE deflection
Are the chest/prechordial lead assigned an axis?
No. They are not. Instead, they are assigned REGIONS of the heart and each lead is a positive electrode.
Inferior part of the heart is best detected by what leads?
II
III
aVF
Septal part of the heart is best detected by what leads?
V1
V2
Anterior part of the heart is best detected by what leads?
V2
V3
V4
Lateral part of the heart is best detected by what leads?
V4
V5
V6
I
aVL
What leads are associated with the lateral circumflex artery?
Lead 1
aVL
V5
V6
What leads are associated with the inferior part of the RCA?
Lead II
Lead III
aVF
What leads are associated with the septal/left anterior descending artery?
V1
V2
What leads are associated with the anterior/left anterior descending?
V3
V4
What does the axis of deviation tell us and how to we figure it out?
How do we figure out the axis of deviation?
—-Tells us the average direction of the AP in the ventricles and helps us to determine conditions in the heart—–
Consider only the QRS complex of lead 1 and aVF.
- Use thumb trick: put both thumbs out and up; L thumb represents LEAD 1; R thumb represents aVF.
- The average of the areas of the QRS complexes in both should be +. Thus, average them. If the average areas correspond with a downward deflection, move that finger down. If not, keep it up. Whichever thumb is up the axis you are deviated.
- Both thumbs up= normal axis deviation
- Left thumb up and right thumb down= left axis deviated.
Right thumb up and left thumb down= right axis deviated
THIS IS THE DIRECTION OF POLARIZATION
- Now to find the degree of polarization
A. Find lead at the isolectric point (area of QRS curve cancels out)
B. Axis of deviation would be the degrees that are perpendicular to the isoelectric point on the graph.
3.
If leads I and aVF are ______, then there is a normal mean electrical axis.
Positive.
How do we become extreme R.A.D?
If both lead I and lead aVF are negative.
Generally, depolarization goes from _________–>________
Upper left–> lower right.
What does a right axis deviation and left axis deviation signify?
RAD–> right ventricle hypertrophy
LAD–> left ventricle hypertrophy
P wave begins before/after atrial contraction.
before
QRS complex begins before/after ventricular contraction.
T wave begins before/after ventricular relaxation.
Before
Before
