Basics of ECG Interpretation Flashcards
Why is there a slow rate of rise in SAN action potential?
no sodium channels in center channels
calcium channels are slower
The basis for pacemaker function is….
Slow, spontaneous diastolic depolarization
the unstable baseline
Three conditions for re-entry are…
Closed loop
unidirectional block
transit time > refractory period
The enzyme associated with overdrive suppression is…
Sodium potassium ATPase
The spontaneous diastolic depolarization in SAN is due to …
Opening of funny channels
What is the mechanism of overdrive suppression?
Hyperpolarization due to increased sodium efflux
Refractory period plasticity is related to changes in…
Phosphorylation of potassium channels and increased potassium efflux
The tendency for action potential amplitude and conduction velocity to decreased is known as…
Decrement also conduction
Irregular blocking of impulses leads to impulse division and multi-directional impulse conduction resulting in…
Circus movements
Delayed due to de-innervation and autonomic changes
How much faster than a Purkinje fiber conducting an AP is Usain Bolt?
3 times as fast at 12 meters per second
Purkinje fibers are at 4 meters per second
Where are each of these action potentials coming from?
SA and AV node don’t have the rapid depolarization due to calcium channels instead of sodium.
SA node occurs sooner because conduction system
Atrium repolarizes the fastest
Midmyocardium has the longest duration of action potential
Epicardium has the shortest
Label this image.
Describe the path of the action potential.
What electrical event is measured by the PR interval?
What does the PR segment indicate?
What wave is never visible?
AP starts in SA node - AV node - HIS bundle - bundle branches - purkinje fibers
PR interval - measures the time it takes for AP to travel from SA node to purkinje fibers
PR segment - should be 0 volts, as this is the baseline
What is represented by the QRS complex?
What electrical phases of ventricular myocytes AP are represented by the QRS complex?
QRS
Ventricular depolarization
Ventricular myocytes - phase 0 and 1
Label this image
What part of the AP does the T wave represent, and what action?
When is the U wave visible?
What is the ST segment and what part of the action potential does that represent?
T wave - ventricular repolarization, phase 3
U wave - very slow heart rate can make visible
ST segment - baseline, J point is start, phase 2
Label this image and what each interval/segment is indicative of.
What are the 3 segments/intervals that provide a baseline?
R-R and P-P are indicative of heart rate
*use beginnings of P waves*
QT interval represents the refractory period
T-P segment is another baseline
Baselines are
PR intervals
ST segments
TP segments
Why are the QRS and T waves on an ECG concordant?
Because Depolarization and repolarization move in the opposite direction
Label this image.
Describe how these 12 leads lead to localization.
Which leads are the anterior/septal leads?
Which leads are the inferior leads?
Which leads are the lateral leads?
What lead shows us the cavity of the heart from the perspective of the right shoulder?
12 lead EKGs provide perspective from many different angles that allow localization of electrical events in the heart.
Anterior/Septal Leads
Septal - V1/V2
Anterior - V3/V4
Inferior Leads
II, III, aVF
Lateral Leads
High - I/aVL
Low - V5/V6
View from the right shoulder
aVR
Label this image, and describe the localization of each set of leads.
What arteries are associated with each region?
Lateral leads - Left anterior descending artery (LAD) diagonal or Left circumflex (LCx)
Inferior leads - Right coronary artery and/or LCx
Anterior/septal leads - LAD
Describe the R progression and S progression in precordial leads.
R waves should start small and get larger
S waves should start large and get smaller
This shows that in V1 you are not looking at the left ventricle more right atrium and septum, where as V6 is more left ventricle
What are the 5 steps of ECG Interpretation?
- Rhythm and Rate
- Timing/Intervals (PR, QRS, QT)
- Mean Electrical Axis
- Hypertrophy
- Infarct
What does each small box represent horizontally? Big box?
What does each small box represent vertically? Big box?
Horizontal
small box - .04 seconds
big box - .20 seconds
Vertical
small box - 0.1 mV
big box - 1 mV
What is the heart rate given the following R-R intervals
1 big box
2
3
4
5
6
7
1 big box - 300 b/min
2 - 150 b/min
3 - 100 b/min
4 - 75 b/min
5 - 60 b/min
6 - 50 b/min
7 - 42 b/min
Around 50 b/min
About 150 b/min
Actually 136 with math
About 100
About 75
What expectations do you have when looking at Lead II?
Expections of Lead II
P should be positive
QRS should be positive
may/may not Q wave, S wave
T should be concordant with QRS
Describe the steps to assessing rate and rhythm
(step 1 of interpreting ECGs)
Steps to Examining rhythm
Check rhythm strip (Lead II)
- P waves - present and upright in Lead II
- Are P waves related to QRS - does every QRS have a P and vice versa?
- Regular rhythm - does P-P and R-R match
- Rate
What are the steps to assessing timing/intervals
(the second step of interpreting ECGs)
- is the PR interval normal and fixed
- Determine width of QRS complex
- is QT interval normal
What are the normal values for the following
P wave
QRS Complex
Pathologic Q waves
PR interval
QT interval
RR interval
P wave: 0.08-0.10 sec
QRS complex: 0.06-0.10 sec
Pathologic Q waves: over 25% of R wave (>3mm), >30 msec duration
PR interval: 0.16-0.20 sec (children can be as low as 0.12)
QT interval: 0.36-0.44 sec
RR interval: 1.0-0.6 sec
Why do QTc values exist
What heart rate are calculations for QTc based on?
What is the quick way to estimate QTc?
What is the most common formula used when calculating QTc formally?
Heart rate and refractory period change QT interval, making interpretation at high heart rates difficult
Based on 60 b/min heart rate
Quick: when HR>100 QT should be less than half the preceding RR interval
Bazett’s formula is most commonly used
QTc = QT / square root of RR
What is the most common lead in which QT interval is measured? What other leads would suffice? What leads should be avoided?
What are the normal ranges of QT interval?
What can alter QT interval?
Most common lead: Lead II
Also: Leads I and V5
Not: III, aVL, and V1
Range: 0.36-0.46 msec
Altered QT
drugs
electrolytes, micronutrients
channel mutations
exercise (shortens QT)
What are the concerns associated with prolonged QT?
Prolonged QT
Leads to longer refractory period and increased change of early depolarizations
Increased risk of polymorphic ventricular tachycardia
Short QT interval
Increased risk of paroxysmal A fib or V fib
sudden cardiac death
What are the two methods to assessing axis while interpreting ECGs? (3rd step)
1. Fast way - gets you in a 90 degree vicinity
Check leads I and aVF
If lead I is positive, the mean electrical axis has to be depolarizing in that direction (-90 to 90)
If lead aVF is positive the mean electrical axis has to be depolarizing in that direction (0 to +/-180)
2. Isoelectric point
Find the lead where the QRS complex has a positive and negative excursion that are equal, resulting in an amplitude of 0. This means the mean electrical axis must be perpendicular to this lead. That gives 2 options. To determine which is correct, you can check which other leads have positive amplitudes meaning the mean vector is depolarizing towards those leads.
- Graphing technique with Leads I, II, and III
Net amplitude of QRS complexes graphed
What can you learn through assessment of the mean electrical axis of an ECG?
Insight about conduction through the heart
Vector points away from an infarction
Vector shifts toward bundle branch blocks
Vector points toward hypertrophy
If not in the normal range or mean electrical axis has moved from previous ECGs
Label me and what each quarter indicates?
Where would the vectors point for the following conditions?
- right heart hypertrophy
- right bundle branch block
- right sided infarction
Right heart hypertrophy - right axis deviation
right bundle branch block - right axis deviation
right sided infarction - left axis deviation
Which 2 leads can be used to quickly approximate the mean electrical axis?
What combination of +/- would indicate a normal axis?
What combination would indicate an extreme right axis deviation?
Which combination would indicate a left axis deviation?
Which combination would indicate a right axis deviation?
Leads I and aVF.
Lead I points positively to the right side of the graph. So If lead I has a positive QRS complex then the mean electrical axis is definitely on the right side of the graph (-90 to +90)
Lead aVF points positively to the bottom of the graph. So if this QRS remains positive that means the mean electrical axis is lying somewhere on the bottom half of the graph
(0 to +/-180)
What are the 90 degree relationships between the leads?
I - aVF
II - aVL
III - aVR
What are the steps to assessing for hypertrophy?
Which hypertrophy diagnosis is more reliable with ECG?
(the fourth step)
LVH is moderately accurate, RVH much less
If ECG criteria is met, echo to confirm
Three criteria
- Over 35 y/o
* *2. Sokolov-Lyon criteria** - R wave in aVL>11mm, in aVF>20mm
Sokolov-Lyon Criteria
- add the magnitude of the S wave in V1 to the magnitude of the tallest R wave in V5/6
- must be over 35mm
