Flashcards in Cardiac Conduction and ECG Deck (39):
What happens after the pacemaker cells in the SA node generate an action potential?
• It propogates rapidly through the gap junctions
• From SA node, high in right atrium, depolarization (through gap junctions) continues through right and left atria
• This generates the P wave
As far as action potentials go, what is the difference between Purkinje cells and contractile myocytes
• Faster action potential propagation and longer duration in purkinje cells than contractile myocytes
How are the right and left bundles from the AV node different?
• Right bundle is one entity and services the whole right atrium
• Left bundle splits into left andterior and left posterior
Where does the depolarization wave spread after the AV node deploarizes?
• Through the bundle of His and into the left and right bundle branches
• The bundles then divide into fibers made up of Purkinje cells
• Purkinje fibers radiate toward the contractile cardiac myocyets and induce contraction
• The depolarization spreads through myocytes due to gap junctions
Where in the heart is the AV node?
• Between the fibrous tricuspid and mitral valve rings that separate the atria from the ventricles
What is the P wave on the ECG?
• Atrial depolarization
What does the QRS complex reflect?
• Depolarization in the ventricles
Which direction is the T-wave?
• It moves in the same direction as the QRS complex even though in an individual myocyte depolarization and repolarization are in opposite directions
Why is atrial repolarization not generally seen in the ECG?
• Occurs during the QRS complex and it's relatively minor in the scheme of electric propagation
What does the T wave reflect?
• Repolarization of the ventricles
What number/phase of the ventricular action potential matches up with the QRS complex?
• Phase 0, or upstroke, and phase 1
What does the T wave reflect?
• Repolarization, phase 3 of the action potential
• Rapid decrease in voltage as potassium efflux continues and Calcium influx dies off
What does the isoelectric ST segment reflect?
• Phase 2 of the action potential, or the long plateau with little change in voltage
• Calcium influx and potassium efflux are pretty balanced at this point
What is the normal time course of QRS through T on an ECG?
• 400 ms
What number/phase of the ventricular action potential matches up with the T wave?
• Repolarization or phase 3
What number/phase of the ventricular action potential matches up with the ST wave?
• Plateu phase or phase 2
The R wave of the QRS complex reflects what?
• The upstroke, or phase 0 of the ventricular action potential
• Due to the fast sodium current
Which depolarizes first, the endocardium or the epicardium?
• Endocardium depolarizes earlier than epicardium
• However, there is a transmural repolarization gradient and epicardial cells repolarize earlier than endocardial cells because they have a shorter action potential
Which REPOLARIZES first, the endocardium or epicardium?
• Epicardium repolarizes first, even though they depolarize second
What does the discordance of repoloarization mean in the ECG?
• The T wave should be in the same direction as the QRS complex
• If there is discordance, this reflects pathology, like ischemia or ventricular hypertrophy
What does the PR interval reflect?
• Conduction time across the AV node
How many leads are in an ECG?
• 12 separate leads
• Shape of QRS is dependent on lead placement, in particular the postive electrode
Using the red boxes, you can determine that which complexes or intervals are abnormal?
• PR interval
○ Normal is 0.12-0.2 seconds
○ Normal is 0.08 - 0.1 seconds
○ Normal is 0.06 - 0.1 seconds
• Q-Tc interval
○ Normal is less than 0.44 seconds
What do the square red boxes in the ECG represent?
• "y-axis" is 10mm = 1mV
• "x-axis" every line is 0.04 seconds.
What does the QT interval reflect?
• Total duration of depolarization and repolarization
When will you see a positive deflection on an ECG?
• Positive deflection is UPWARD
• This happens when activation wave is moving towards a sensing electrode
What makes a deflection larger or smaller?
• Muscle mass. The more mass, the larger the deflection
Where is the SA node and where does the depolarization wave move?
• SA node is high in right atrium and depolarization sweeps downward and leftward
• Thus, a lead with a positive electrode near the right arms normally has negative QRS
• Lead with positive electrode near left leg has positive QRS
What are the 4 steps of deploarization in the heart?
• One - upper portion of septum is depolarized from left to right
• Two - depolarization downward in the septum to apex
• Three - depolarization moves from endocardium to epicardium
• four - depolarization moves upward from apex in the free walls of both ventricles
• FINAL - depolarization of bass of ventricles
Where can blocks in the conduction occur?
• SA block and sinus arrest - A
• AV block - B
• Right bundle branch block - C
• Left bundle branch block - C
• Posterior fascicular block - D
• Anterior fascicular block - D
What's up with the AV block?
• Three types
○ First degree
§ Conduction delayed but all P waves conduct to ventricles
○ 2nd degree
§ Some P waves conduct but other do not
○ 3rd degree
§ None of the P waves conduct and ventricular pacemaker takes over
What's up with SA node abnormalities?
• 'sick sinus syndrome'
• Slow sinus rates or takeover by other pacemakers which may be fast or slow
What happens when the right bundle is blocked?
• QRS widening with delayed conduction to the right ventricle
What happens when the left bundle fascicles are blocked?
• Shifts in the direction of depolarizaiton but no QRS widening
What happens when the left bundle is blocked?
• QRS widening with delayed conduction to the left ventricle
Where can abnormal reentry pathways be present?
• In the atria, ventricles or the junctional tissue
What happens in triggered activity?
• Deals with abnormal afterpolarizations
• Can be triggered by the preceding action potential
• Looks like the triggering of an action potential before full repolarization has happened
• Can also be longer than normal after repolarization, leading to a different arrhythmia
○ These are seen usually with a long QT interval
What are ectopic foci?
• Occur when a focus of myocardium outside the conduction system acquires automaticity and if the rate of depolarization exceeds that of the sinus node an abnormal rhythm occurs
• Can be isolated ectopic beats or sustained tachyarrhythmias
What is the most common mechanism of serious tachycardias?
• Abnormal reentry pathways due to slowed or unidirectionally blocked conduction pathways