SM 135 Bradycardia Flashcards
Describe the location of the SA Node?
SA Node is found in the sulcus terminalis, at the junction of the SVC + R. Atrium, and is relatively epicardial
What major anatomical landmark is near the SA Node?
The junction of the SVC and R. Atrium
Does the SA Node show up on EKG and why?
SA Node does not show up on EKG because it is a very small group of cells
What is the function of the SA Node?
The SA Node is the natural pacemaker of the heart and sets the heart rate
What is a sinus rhythm?
A normal heart rate set by the SA Node
What outflow(s) of the nervous system control the SA Node?
The SA Node and the heart rate are manipulated by Sympathetic Tone
How is the R. Atrium connected to the L. Atrium?
Via Bachmann’s Bundle
How is the SA Node connected to the AV Node?
Internodal tracts
What outflow(s) of the nervous system control the AV Node?
Both Sympathetic and Parasympathetic (Vagal) Tone control the AV Node
Does the AV Node show up on EKG and why?
The AV Node does not show up on EKG because it’s a small group of cells
Where is the AV Node found?
At the junction of the R. Atrium and R. Ventricle
How does conduction change at the AV Node?
Conduction is delayed at the AV Node to allow the Atria to contract and the Ventricles to fill
What are the components of the His-Purkinje system?
Bundle of His, R + L Bundle Branches, Purkinje Fibers
Describe conduction in the His-P system?
“Highway” = rapidly conducting
How does the Left Bundle Branch compare to the Right Bundle Branch?
Both are part of the His-P system, but the Left Bundle Branch is much larger and more branched than the Right Bundle Branch
When does the SA Node depolarize?
Before the P wave - not seen on EKG
When do the Atrial Myocardium and Internodal tracts depolarize?
During the P wave
When does the AV Node depolarize?
During the PR interval, which represents a pause between Atrial and Ventricular contraction; before the His-P system depolarizes
When does the His-P System depolarize?
During the PR interval; after the AV Node
When does the Ventricular Myocardium depolarize?
During the QRS complex
When does the Ventricular Myocardium repolarize?
During the T wave
What heart rate represents Bradyarrhythmia?
Ventricular rate < 60 bpm
What are the symptoms of Bradyarrhythmia?
Syncope, dizziness - may be asymptomatic
Where along the conduction system of the heart do arrhythmia’s usually arise?
Usually arise in the SA and AV node
What intrinsic pathologies underlie Sinus Node dysfunction?
Problems with Impulse Formation and Impulse Propagation to the AV Node, as well as age-associated fibrosis
What does Impulse Formation in the SA Node correspond to on the Pacemaker AP?
Impulse propagation errors arise during Phase 4 of the Pacemaker AP
How do Impulse Formation issues manifest in the SA Node?
Sinus Bradycardia, Chronotropic Incompetence, Tachy-Brady Syndrome, and Sinus Arrest
What can cause Sinus Bradycardia?
Insufficient depolarization frequency
How does Chronotropic Incompetence usually present?
Normal resting HR + inadequate HR response to exercise
How does Phase 4 of the Pacemaker AP set the HR?
The slope of Phase 4 sets the Heart Rate, with cells higher in the SA node having sharper slopes and setting faster Heart Rates
What extrinsic pathologies underlie sinus node syfunction?
Medications such as Beta blockers and Calcium channel blockers, electrolyte abnormalities, hypotension and hypothyroidism
Why is increased Vagal tone to the SA Node usually not pathologic?
Increased vagal tone decreases sinus rate, but usually as a compensatory measure such as athletes with hypertrophied hearts
What is Tachycardia-Bradycardia syndrome?
A burst of tachycardic activity driven by an external stimulus followed by a period of bradycardic activity due to myocardial stunning after the stimulus is removed
What can cause a 1st degree AV block?
Interference at the AV Node due to scarring or high vagal stimulation
How does a 1st degree AV block present?
Delayed PR interval between Atrial and Ventricular depolarization with no dropped beats
What can cause reversible damage to the AV Node?
Increased Vagal Tone - can be countered with drugs
What can cause irreversible damage to the AV Node?
Scar formation - cannot be healed
What can cause a 2nd degree AV block?
Interference at the AV Node due to scarring or high vagal stimulation
How does a 2nd degree Type 1 AV block present?
Progressively longer delays between the P wave and QRS complex with dropped beats
How do a 1st degree and 2nd degree AV block differ?
2nd degree AV blocks have dropped beats and may have varying PR intervals, while 1st degree AV blocks do not drop beats and have constant PR intervals
What can cause increased Vagal Tone?
Athletes and post-surgery; affect the AV Node
How does a 2nd degree Type 2 AV block present?
Fixed delays between the P wave and QRS complex with dropped beats
Which type of 2nd degree AV block requires a pacemakers?
Type 2 2nd degree AV block requires a pacemaker
Which type of 2nd degree AV block is worse?
Type 2 is worse because it is due to damage to the conduction fibers in the His-P system, while Type 1 is often from transient increases in Vagal tone
How do Type 1 and Type 2 2nd degree AV blocks differ?
Type 1 has progressive delays between the P wave an QRS complex that climax cyclically in a dropped beat while Type 2 has constant delays
How does a 3rd degree AV block present?
P waves show up independent of QRS complexes
What causes a 3rd degree AV block?
A total blockage in conduction from the AV Node to the Ventricles
Does a 3rd degree AV block need a pacemaker?
Yes
What types of AV blocks need a pacemaker?
2nd Degree Type 2 + 3rd Degree AV Blocks need pacemakers
Why can QRS complexes form independently of P waves in a 3rd degree AV block?
Despite the total block between the AV Node and the Ventricles, escape pacemakers can provide autorhythmicity at relatively slower rates
What are the escape pacemakers?
The Junctional Escape Rhythm and the Ventricular Escape Rhythm
Which escape pacemaker is faster and why?
The Junctional Escape pacemaker is faster at 40-60bpm, while the Ventricular Escape pacemaker is slower at 20-40bpm; this is because the Junctional Escape pacemaker uses the His-P conduction system while the Ventricular Escape pacemaker relies on cell junctions
Which escape pacemaker uses the His-P system?
The Junctional Escape rhythm, which results in the formation of fast and narrow QRS complexes while the Ventricular Escape pacemakers forms slow and wide QRS complexes
How does the His-P system alter the QRS complex on EKG?
Signals that use the His-P system, such as Sinus rhythm and Junctional Escape Pacemaker, form narrow and fast QRS while those that don’t, such as the Ventricular Escape Pacemaker, form slow and wide QRS
