Conduction Blocks and Pacemakers

What is a conduction block?

Any obstruction or delay of the flow of electricity along the normal conduction pathways

Types of conduction blocks

1. Sinus exit block

2. AV block

3. Bundle branch block

Types of AV blocks

1. 1st degree

2. 2nd degree

→ Mobitz type I

→ Mobitz type II

3. 3rd degree

Types of bundle branch blocks

1. Right

2. Left

3. Right + Left

4. Partial

→ Fascicular or hemiblock

What is a 1st-degree AV block?

Conduction delay (not a true block) at the AV node or Bundle of His

What is the diagnostic criteria for 1st-degree AV block?

Prolonged PR interval

→ Greater than 0.2 seconds

What is the P:QRS ratio in 1st-degree AV block?

1:1

1st-degree AV block is commonly seen in normal hearts during

Sleep

What characterizes 2nd-degree AV blocks?

Dropped ventricular beats

What is the P:QRS ratio in 2nd-degree AV blocks?

Greater than 1:1

What happens to the P-P interval in 2nd-degree AV blocks?

Remains consistent since the sinus node functions normally

Mobitz type I is also known as

Wenckebach AV Block

Mobitz type I is characterized by

Variable block within the AV node

→ Increases with each impulse

What happens to the PR interval in Mobitz type I?

It progressively lengthens until a QRS is dropped

How often does a dropped beat typically occur in Mobitz type I?

Usually every 3rd or 4th beat

What is the key diagnostic criterion for Mobitz type I (Wenckebach) block?

Progressive lengthening of the PR interval until a QRS complex is dropped

True or false: Mobitz type I block generally does not require a pacemaker

True

Where is the block usually located in Mobitz type II?

Below the AV node, typically at the bundle of His

What is the key diagnostic criterion for Mobitz type II block?

Presence of dropped QRS without progressive lengthening of PR Interval

Why is Mobitz type II considered more serious than type I?

Greater risk of evolving to complete heart block

What treatment is indicated for Mobitz type II?

Pacemaker

What happens to the PR interval in Mobitz type II?

Remains normal and consistent

→ No progressive lengthening

P:QRS ratio in Mobitz type II

Ratio of P:QRS will constantly vary

What characterizes a 3rd-degree AV block?

Zero atrial impulses conducted to the ventricles

→ Complete heart block

What happens to atrial and ventricular activity in 3rd-degree AV block?

Driven by separate pacemakers

→ AV dissociation

What is the typical rate of the sinus pacemaker?

60-100 bpm

What is the typical rate of the ventricular pacemaker?

30-45 bpm

What type of escape rhythm is commonly seen in 3rd-degree AV block?

Idioventricular escape rhythm

Diagnostic criteria for 3rd-degree AV block

1. Presence of AV dissociation

2. Ventricular rate < Sinus/atrial rate

Is the QRS complex wide or narrow in:

1. 1st degree block

2. 2nd degree block

3. 3rd degree block

1. Narrow

2. Narrow

3. Wide

What happens to the QRS complex when bundle branches conduct normally?

QRS is narrow

What is the normal QRS axis range?

0° and +90°

What is a bundle branch block?

Conduction block (or slowing) in either the right or left bundle branch (or both)

How are bundle branch blocks diagnosed on EKG?

Looking at:

1. Width

2. Axis

3. Configuration

What happens to the QRS duration in right bundle branch block (RBBB)?

Widens (≥ 0.12 sec)

What characteristic pattern is seen in RBBB?

RSR′ (rabbit ears) in V1 and V2

→ Can also be seen in V3 and III

What causes the R′ wave in RBBB?

Delayed right ventricular depolarization

What repolarization changes may be seen in RBBB?

Inverted T waves and ST depression

→ Abnormal depolarization is followed by abnormal repolarization

In RBBB, late RV depolarization causes reciprocal __________ in __________ leads?

1. Wide and deep S waves (looks like a W)

2. Lateral leads (I, aVL, V5, and V6)

→ Can also be seen in V4

V1 and V2 locations

V1 → Right of septum

V2 → Left of septum

What happens to the QRS duration in left bundle branch block (LBBB)?

Widens (≥ 0.12 sec)

What is seen in lateral leads in LBBB?

Wide/notched R waves

→ Looks like an M, but not true rabbit ears

Why is there a prolonged rise in the R wave in LBBB?

Due to delayed LV depolarization

Are Q waves seen in LBBB?

No, because septal activation is reversed

What repolarization changes may occur in LBBB?

Inverted T waves and ST depression

→ Abnormal depolarization is followed by abnormal

repolarization

In LBBB, late LV depolarization causes reciprocal __________ in __________ leads?

1. Wide and deep S waves (looks like a W)

2. V1 and V2

Hemiblocks involve which conduction system branch?

Left bundle branch and its fascicles

Do hemiblocks prolong the QRS complex?

No

Do hemiblocks cause repolarization changes (T wave inversions or ST depression)?

No

What is the main effect of hemiblocks on an EKG?

Axis deviation

Under what condition can hemiblocks be reliably diagnosed?

Only in the absence of other causes of axis deviation, such as ventricular hypertrophy

In left anterior hemiblock, all conduction travels through

Posterior fascicle

How does the depolarization vector progress in left anterior hemiblock?

1. Inferior-to-superior

2. Right-to-left

What axis deviation is seen in left anterior hemiblock?

Left axis deviation

→ Between -30° and -90°

How is the QRS axis redirected in left anterior hemiblock?

Upward and leftward

What is seen in the lateral leads in left anterior hemiblock?

Tall R waves

What is seen in the inferior leads in left anterior hemiblock?

Deep S waves

What confirms that left axis deviation is beyond -30° on ECG?

Negative QRS in lead II

True or False: To diagnose a hemiblock, there cannot be other causes of left axis deviation

True

In left posterior hemiblock, all conduction travels through

Anterior fascicle

How does the depolarization vector progress in left posterior hemiblock?

1. Superior-to-inferior

2. Left-to-right

What axis deviation is seen in left posterior hemiblock?

Right axis deviation

→ Between +90° and +180°

How is the QRS axis directed in left posterior hemiblock?

Downward and rightward

What is seen in lateral leads in left posterior hemiblock?

Deep S waves

What is seen in inferior leads in left posterior hemiblock?

Tall R waves

What confirms that right axis deviation is beyond +90° on ECG?

Negative QRS in lead I

What are the two types of bifascicular block?

1. RBBB + Anterior hemiblock

2. RBBB + Posterior hemiblock

What serious risk is associated with a bifascicular block?

Increased risk of complete heart block

What happens to the QRS complex in a bifascicular block?

It becomes wide (due to RBBB)

What characteristic pattern is seen in V1 and V2 in a bifascicular block?

RSR′ pattern (due to RBBB)

What axis deviation is seen with RBBB + Anterior hemiblock?

Left axis deviation (-30° to -90°)

What axis deviation is seen with RBBB + posterior hemiblock?

Right axis deviation

What results from a block of both RBBB and LBBB?

Complete heart block

Pacemakers are generally used for

1. 3rd Degree (complete) AV block

2. Symptomatic bradycardia (Sick Sinus Syndrome)

Where are pacemaker electrodes usually threaded?

Right side of the heart

→ RA or RV (single chamber)

→ RA + RV (dual chamber)

What is the most popular type of pacemaker?

Demand pacemaker

When does a demand pacemaker fire?

When the heart rate falls below a set threshold

Modern pacemakers can increase HR in response to _________ or ___________

1. Motion

2. Increased respiration

What is another name for a dual-chamber pacemaker?

Sequential pacemaker

Spike followed by P wave, normal PR interval, normal QRS

Atrial pacemaker

No P wave, spike followed by wide, bizarre QRS (like LBBB)

Ventricular pacemaker

Spike followed by P wave, and spike followed by wide, bizarre QRS (like LBBB)

Dual-chamber pacemaker

Risks of pacemakers

1. Induce VTach of Vfib

→ R-on-T phenomenon

2. Precipitate heart failure

→ Ventricular dyssynchrony (R before L) → Reduced pumping efficiency/function

Normal pacemakers are contraindicated for patients with

Heart failure

For patients with reduced LV function and/or native LBBB ____________ pacemakers can be used

Biventricular

→ Add a 3rd electrode to the lateral vein of the LV

Biventricular pacemakers are also known as

Cardiac resynchronization therapy

Biventricular pacemakers can improve

1. EF

2. Symptoms of heart failure

Leadless pacemakers

Promote synchronized contraction on both sides