15 and 16 - Electrocardiogram I and II Flashcards
While you are rounding in outpatient IM, a patient taking a beta blocker for high blood pressure is seen. Noting his slow heart rate, you know the mechanism of this effect is:
a, Increasing the rate of decline of K+ permeability during phase 4 of SA nodal cells.
b. Having a positive inotropic effect on myocardial cells.
c. Increasing the slope of the SA nodal cell (“pacemaker”) prepotential.
d. Decreasing the rate of influx of sodium and calcium during phase 4 of SA nodal cells.
d. Decreasing the rate of influx of sodium and calcium during phase 4 of SA nodal cells.
Fast sodium channel blockers would be expected to have the LEAST effect on which cells:
a. SA nodal cells
b. Atrial myocytes
c. Ventricular myocytes
d. Purkinje fiber cells
a. SA nodal cells
What is an ECG?
A body surface recording of a change in electrical potential which is a projection of the net potential changes occurring in the heart
The summation of all the depolarization and repolarization currents occurring in cardiac tissue
Describe the horizontal axis of ECG chart paper
- It is divided into 1 mm sections
- One small square is equal to 1/24 of a second, or 0.04 seconds
Describe the vertical axis of ECG chart paper
- A 10 mm deflection is equal to 1 mV
- At rest, the line will be at the vertical midpoint of the paper
How many small vertical lines will pass under the pen of an ECG machine in 2 second?
24
A 10 mm vertical deflection represents ____ mV
1
What is an isoelectric line?
The vertical midpoint of the ECG paper (0 mV)
- If it goes up, it is a positive deflection
- If it goes down, it is a negative deflection
What is a P-wave?
It represents atrial depolarization (sum of all phase 0’s in the atrial tissues)
What is the P-R interval?
The amount of time from the beginning of the P wave (beginning of atrial depolarization) to the beginning of the QRS complex (the beginning of ventricular depolarization)
What is the normal duration of the P-R interval?
0.12 to 0.20 seconds
What does a shorter than normal duration of the P-R interval suggest?
The existence of a bundle of Kent, which is an embryological pathway around the AV node that should no exist
What is a first degree block?
Conduction through the AV node is reduced, and the PR interval is excessively long, but all P waves still result in a QRS
What is a second degree block?
Not all P waves will result in a QRS complex because some did not go all the way through the AV node
What is a third degree block?
There are NO P waves conducted though the AV node because there is a complete block
What is the P-R segment?
When the recording returns to baseline after the P-wave
Depolarization is now slowly going through the AV node
What is the QRS complex?
A sequential depolarization of the ventricular cells (sum of all phase 0’s in the ventricular cells)
What is the duration of the QRS complex?
About 0.08 seconds
What happens if the QRS complex is over 120 ms (0.12 seconds)
There is a block in the right or left bundle
What is the Q-wave of the QRS complex?
Any negative deflection that precedes the R wave
What is the R-wave of the QRS complex?
Any positive deflection in the QRS complex
What is the S-wave of the QRS complex?
Any negative deflection that follows the R-wave
What is the S-T segment?
It represents the long time period through phase 2 (non-polar plateau)
What happens to the S-T segment with any acute injury?
Phase 2 will occur at some voltage (positive or negative) other than zero
What accounts for this voltage difference?
It is due to the lack of current flow between the normal areas and the abnormal areas of the myocardium
What is the T wave?
The ventricular re-polarization (summation of all phase 3’s in ventricular cells)
What is the U wave?
It may represent re-polarization of the papillary muscles or the purkinje conduction system
What represents all the phase 0’s of atrial muscle cells on an ECG?
The P wave
What represents all the phase 0’s of ventricular muscle cells on an ECG?
The QRS complex
What represents all the phase 3’s in ventricular muscle cells on an ECG?
The T wave
What is a normal duration of the P-R interval?
About 140 ms or 0.14 seconds
What is a normal duration of the QRS complex?
80-120 ms or 0.08 to 0.12 seconds
What are the three unipolar limb leads?
- AVR
- AVL
- AVF
What do the unipolar limb leads do?
They allow us to compare voltage differentials between a single point on the body and a grounded reference point
Give an example of how the unipolar limb leads work, using AVR as an example
- Place a positive lead on the right arm (AVR) and a negative lead on the left arm (AVL) and left leg (AVF)
- The right lead (AVR) will then BISECT the angle created by the intersection of leads I and II… It will be perpendicular to lead III
Is it the same for AVF and AVL?
Yes…
When AVF is the positive lead, it will bisect lead II and III
When AVL is the positive lead, it will bisect lead I and III
What are the three sets of bipolar limb leads?
- Lead I: LA (+) and RA (-)
- Lead II: RA (-) and LL (+)
- Lead III LA (-) and LL (+)
What do bipolar limb leads do?
They measure the potential differences between two points on the ody
What do the three bipolar limb leads form?
They form an equilateral triangle called Einthoven’s triangle
What plane do all of these leads measure?
These all measure ECG in the frontal plane
How may unipolar chest leads are there?
Six
What are the names of the unipolar chest leads?
V1 V2 V3 V4 V5 V6
Where do we place V1?
At the 4th intercostal space on the right sternal border
Where do we place V2?
At the 4th intercostal space on the left sternal border
Where do we place V3?
Midway between V2 and V4 on the line joining the two points
Where do we place V4?
At the 5th intercostal space, midclavicular line on the left side
Where do we place V5?
At the anterior axillary line at the same level as V4
Where do we place V6?
At the midaxillary line at the same level as V4 and V5
What is the purpose of the unipolar chest leads?
They allow us to analyze ECGs in the horizontal plane
How can you predict if the P-wave and QRS complex will be positive or negative in the limb leads?
You can look at the average vector of ATRIAL depolarization and the average vector for VENTRICULAR depolarization
Describe the process of predicting the direction of the P-wave and QRS complex
- Look a the direction that the vector points
- If it points towards the positive pole, the deflection of the P-wave and QRS complex will be positive (and vice versa for negative)
- If the vector is parallel to the lead, the resultant vector will be large
There are three ways to determine the heart rate from an ECG. What are they?
1 - Formula
3 - Division by 300
4 - Marked paper
What is the formula method of determining the heart rate from an ECG?
- Select two points of consecutive R waves
- Count the number of large boxes (this is n)
Heart rate = 1/n x chart speed
What is the division by 300 method of determining the heart rate from an ECG?
- Memorize the numbers… 300, 150, 100, 75, 60, 50
- Each number corresponds to the number of large boxes between R waves
- If there is just one box between the waves, it is 300 bpm
- If there are four boxes between them, it is 75 bpm
What is the marked paper method of determining the heart rate from an ECG?
- Most hospitals use paper marked at three second intervals
- Count the QRS complexes in a 6 second interval and multiply by 10
What would a normal sinus rhythm look like (bpm)?
The resting heart rate would be between 60-100 bpm
What is tachycardia?
When the resting heart rate is greater than 100 bpm
What is bradycardia?
When the resting heart rate is lower than 60 bpm
What is the hexaxial reference system in the frontal plane?
- It is a tool that utilizes the equilateral triangle formed by leads I, II and III and the unipolar chest leads
- The center point of this reference tool is the AV node
- The axis of each lead is moved so that it intersects with the AV node at its midpoint
What degrees of the circle will you find each of the leads included?
- AVF: +90 (bottom of circle)
- Lead I: +180 (left of circle)
- AVR: -150 (left upper)
- Lead II: -120 (left upper)
- Lead III: -60 (right upper)
- AVL: -30 (right upper)
What is meant by a “normal axis”?
The vector lies between 0 and 90 degrees (bottom right)
What is meant by “right axis deviation”?
The vector lies between 90 and 180 degrees (bottom left)
What is meant by “left axis deviation”?
The vector lies between 0 and -90 degrees (top right)
There are four steps in the process of determining the approximate mean electrical axis in the frontal plane. What is the first step?
First, you need to select any two leads that are 90 degrees apart from each other
Example: Lead I and AVF are convenient because they are horizontal and vertical, respectively
What do you do after you have selected two leads that are 90 degrees apart from each other?
For this example (Lead I and AVF), you would evaluate the QRS in lead I
If the QRS in lead I is deflected in the positive direction, the vector will point to the left because that is the positive pole
After you have ealuated the QRS in lead I, what do you do?
Evaluate the QRS in AVF
If the QRS in AVF is deflected in the positive direction, the vector will point down because that is the positive pole
What do you do after you have determined the direction of both vectors?
Use vector addition to determine the resultant mean axis
What is a possible pathological reason that the PR interval would be shorter than normal?
A first degree block in the conduction through the AV node
What is a possible pathological reason that the PR interval would be shorter than normal?
It can be a normal, harmless variant, but it can also indicate…
- Wolff–Parkinson–White syndrome
- Lown–Ganong–Levine syndrome
What is a possible pathological reason that you would see a very large and wide QRS complex appearing in a lead with mostly normal QRS complexes?
Uhhh… Ask Leslie?
What is a possible pathological reason that the QRS complex would consistently appear large and wide?
1 - A bundle branch block (conduction block)
2 - Hyperkalemia
What happens to the heart rate in the case of increased sympathetic tone?
Increased heart rate
fight or flight = fast HR
What happens to the heart rate in the case of increased activation of beta-one receptors on the SA node?
Increase heart rate in SA node
chronotropic effect
What happens to the heart rate in the case of decreased sympathetic tone?
Decreased heart rate
What happens to the heart rate in the case of decreased activation of beta-1 receptors on the SA node
Decrease heart rate in the SA node
works against the chronotropic effect
What happens to the heart rate in the case of increased vagal tone
Decreased heart rate
Vagus works with parasympathetics
What happens to the heart rate in the case of increased activation of muscarinic receptors on the SA node?
Decreased heart rate (M2 receptors)
opposite for M3 receptors
What happens to the heart rate in the case of decreased vegal tone?
Increased heart rate
Less parasympathetics at work
What happens to the heart rate in the case of decreased activation of muscarinic receptors on the SA node?
Increased heart rate (M2 receptors)
opposite for M3 receptors
What happens to the conduction velocity through the AV node in the case of increased sympathetic tone?
Conduction velocity increases
What happens to the conduction velocity through the AV node in the case of increased activation of beta-1 receptors on the SA node?
Conduction velocity increases
What happens to the conduction velocity through the AV node in the case of decreased sympathetic tone?
Conduction velocity decreases
What happens to the conduction velocity through the AV node in the case of decreased activation of beta-1 receptors on the SA node?
Conduction velocity decreases
What happens to the conduction velocity through the AV node in the case of increased vagal tone?
Decreased conduction velocity (could even lead to a conduction block)
What happens to the conduction velocity through the AV node in the case of increased activation of muscarinic receptors on the SA node?
Reduced conduction velocity
What happens to the conduction velocity through the AV node in the case of decreased vagal tone?
Increased conduction velocity
What happens to the conduction velocity through the AV node in the case of decreased activation of muscarinic receptors on the SA node?
Increased conduction velocity
What is the definition of an agonist and an antagonist?
Agonist: a chemical that binds to a receptor and activates the receptor to produce a biological response
Antagonist: a substance that interferes with or inhibits the physiological action of another
