2025 ECG Quiz 4 Flashcards
Conduction Blocks and Pacemakers
What is a Conduction Block
Any obstruction or delay of the normal pathways of electrical conduction
Doesn’t mean stopping all the electricity, can be just slowing it down.
Three types:
* Sinus node block:
immediately after sinus node
fires
- AV block: any block between sinus node and Purkinje fibers
- Bundle branch block: block in one or both ventricular
bundle branches (partial block fascicular or hemi block)
AV Blocks
A partial or complete
interruption of impulse
transmission from the atria
to the ventricles
* Subdivided into 3 types
* First degree
* Second degree
* Third degree
AV Block Etiology
Idiopathic fibrosis of the conduction system
* Approximately 50% of all AV blocks
* Correlates strongly with age
* Lev’s disease (Lenegre-Lev syndrome)
Ischemic heart disease
* CAD = 35% of all AV blocks
* Acute inferior MI = temporary block
* Acute anterior MI = permanent block
Vagal stimulation
Congenital birth defects
Structural heart disease
* AS, AR, MS, MR
* Myocarditis
* Cardiomyopathy
Metabolic
* Hyper/hypokalemia
Medications
* Digoxin, neostigmine, beta blockers, calcium channel blockers, amiodarone, phenytoin
Hypothermia
Diseases and disorders
* Infectious diseases
* Lyme disease (it is reversable)
* Rheumatic diseases
* Neuromuscular disorders
* Infiltrative processes/cardiomyopath
1st Degree AV Block
Normal AV conduction is
slightly prolonged
P waves and QRS are normal
* 1:1 ratio
PR interval is prolonged
* > 0.20 seconds for
diagnosis
Block is most often at level of
AV node
May occur at Bundle of His
2nd Degree AV Block
Not all impulses pass
through AV node to the
ventricles
Ratio of P waves to QRS
complexes is > 1:1
Two types
* Mobitz type I
(Wenckebach) block
* Mobitz type II block
2nd Degree Type I (Mobitz Type I)
AKA: “Wenckenbach”
Progressive PRI lengthening
until a “dropped beat”… typically every 3-4 beats
3:2 (3 ps to 2 qrs)
Leads to “grouped beating”
Block is almost always
located in the AV node
*** This has wrong ECG stripe
2nd Degree Type II (Mobitz Type II)
Conduction is ”all or
nothing”
Far more serious than Type I
Wenckenbach
High risk of becoming a
complete heart block
2nd degree AV blocks: a comparison
3rd Degree AV Block
Complete heart block
No atrial impulses pass through to the ventricles
P waves are normal
PRI varies
QRS is wide/bizzare (sometimes normal)
AV dissociation
* Atria & ventricles beat independently
* No relationship between P waves and
QRS complexes
* Atrial rate 60-100bpm
* Ventricular escape rhythm 30-40bpm
Block may occur at the AV node or below
Localization of AV Blocks
The more distal the block, the higher risk of complete heart block.
Rules of thumb for block localization
* 1st degree AV block = usually AV node
* Mobitz type I = usually AV node
* Mobitz type II = usually Bundle of His or distal to it
* 3rd degree AV block = usually AV node or Bundle of His
* Normal QRS duration = proximal to bifurcation of
Bundle of His
* Prolonged QRS duration = nonspecific (high chance
of being distal to bifurcation, but may be proximal with separate BBB
AV Block: Symptoms
1st degree AV block
* Common but usually asymptomatic even in chronic cases
* Extremely long delay may lead to symptoms
Mobitz type I
* Uncommon, usually asymptomatic
Mobitz type II
* Uncommon, usually asymptomatic, but high risk of developing into a complete heart block
* Symptoms of a high-degree block include irregular heart
rate, palpitations, pre-syncope and syncope
3rd degree AV block
* May cause severe bradycardia leading to reduced cardiac output, symptomatic bradycardia
* Symptoms include lightheadedness, dyspnea, angina, dizziness, pre-syncope and syncope
* Cardiac arrest without escape rhythms
Degenerative Disease is the most common cause of AV Blocks
AV Block: Practical Managment
Acute treatment
No treatment necessary if asymptomatic, however
close observation is recommended
If symptomatic
* Atropine 0.5mg IV
*Transcutaneous/transvenous temporary pacing
* Always consider reversible causes
* HISDEBS & H’s & T’s
* Acute MI or progressing block
Long term treatment
Permanent pacemaker
* 1st degree AV block and Mobitz type I, if symptomatic
* Mobitz type II often require pacemaker, even if asymptomatic
* All 3rd degree AV blocks require pacing
Lyme Disease can be reversed with Antibiotics and Corticosteroids. They can dip between different types of blocks.
Can have a 1st degree block and a Mobitzs Type II
Bundle Branch Blocks
Right Bundle Branch Block (RBBB)
Left Bundle Branch Block (LBBB)
Bundle Branch Block with Repolarization
Causes of Bundle Branch Blocks
Are cases that BBBs only happen when the heart reaches a certain heart rate
LBBB vs RBBB
Practice Makes Perfect
Bunny ears in V1
Deep, broad, notched S waves in V5,V6
RBBB
Practice Makes Perfect
V5 - Notched R waves
V1, V2 - broad, deep S wave
LBBB
Hemiblocks: Incomplete LBBB
Left Anterior Hemiblock
Lead II - negative deflection
Left axis deviation - diagnosed due to Left Anterior Hemiblock when no other indications
Left Anterior Hemiblock Example on 12-Lead
Left Posterior Hemiblock
Like anterior hemiblock, left posterior hemiblock can only be the reason for the axis deviation as the last option of indicators
Hemiblock Criteria and Summary
Criteria for diagnosis
* Normal QRS duration
* No ST segment or T wave changes
* Axis deviation
* True diagnosis is only possible when no other cause of axis deviation is present
Summary
A conduction block of just one fascicle
Left anterior hemiblock
* Tall R waves in left lateral leads, deep S waves in inferior leads
* Left axis deviation, -30 to -90 degrees
More common (possibly due to its length)
Normal and sick hearts
Left posterior hemiblock
* Tall R waves in inferior leads, deep S waves in left lateral leads
* Right axis deviation, 90 to 180 degrees
Sick Hearts… more serious
Bifascicular Block
Bifascicular Block with Left Posterior Hemiblock
Practice
V1 and V2 - RBBB
Look at axis deviation Lead I and aVF
Lead II - negative showing the -30 to -90 deviation
Anterior fascicle with RBBB
Can have an AV block and BBB… look for AV Block, then for BBB, then for deviation
Pacemakers
Conditions commonly treated with pacemakers
* Third degree (complete) AV block
* Lesser degree of AV block or bradycardia, but symptomatic (light headed or hypotensive)
* New onset of a combination of AV
block and bundle branch block during MI
* Recurrent tachycardias
Purpose and Function:
Provide alternate source of electrical impulses to patients with of sinus node
Power source connected to electrodes
Placed subcutaneously
Electrodes are threaded to right atrium and right ventricle through cardiac venous system
Responsive to needs of heart
Programable
Pacemaker Types
If patient has pacemaker, use the bipolar, short burst cautery… not mono
Pacemaker ECG Effects
Pacemaker Dangers
Risk of infection
Pacemaker spikes may induce serious arrythmias
Pacemakers in RV may induce an episode of heart failure in patients with impaired LV function or congestive heart
failure
Transcutaneous Pacing
Transvenous Pacing
