ECG Ch6 Blocks

What are heart blocks and where can they occur?

• Blocks retard or prevent the conduction of depolarization.

• They can occur in the:

  • SA node

  • AV node

  • Larger divisions of the ventricular conduction system

• The same patient may have more than one type of block.

• Must check for all of them on each reading.

What happens during an SA node block?

• An unhealthy SA node stops its pacing for at least one cycle.

• Usually transient.

• Pacing resumes at the same rate and timing as prior to the block.

• May induce an escape beat from an automaticity focus.

What is sick sinus syndrome?

• Arrhythmias caused by:

  1. SA node dysfunction

  2. Unresponsive supraventricular (atrial and junctional) automaticity foci

     • They can’t employ their normal escape mechanism to induce pacing.

• Very common in the elderly with heart disease.

• Usually associated with sinus bradycardia.

• May present as recurrent episodes of sinus block or sinus arrest without supraventricular escape beats or rhythms.

• Sometimes can include intermittent episodes of SVT or even atrial fibrillation or flutter.

What are AV blocks and what are the different types?

• Either retard or eliminate (or both) conduction from the atria to the ventricles.

• Types:

  • 1° AV Block

  • 2° AV Block

    • Wenckebach or Type I

    • Mobitz or Type II

  • 3° AV Block

What are the EKG characteristics of a 1° AV block?

• Retards AV node conduction.

• Prolongs the PR interval more than .2 seconds (1 large square) on EKG.

• The PR interval includes the P wave and the baseline that follows, up to the point where the QRS complex begins.

• A normal PR interval is less than .2 seconds.

• PR prolongation is consistent in every cycle.

• The P-QRS-T sequence is normal in every cycle.

What are the characteristics of a 2° AV block — Wenckebach (Type I)?

• Occurs in the AV node.

• Some atrial depolarizations conduct to the ventricles while some are blocked.

• Produces a series of cycles with progressive blocking of AV node conduction until the final P wave is totally blocked, eliminating the QRS complex.

• Note the P:QRS ratio:

  • 5:4

  • 3:2

  • 4:3

• There will always be one less QRS complex than the number of P waves.

What are the characteristics of a 2° AV block — Mobitz (Type II)?

• Occurs below the AV node.

• Usually produces a series of cycles consisting of normal P-QRS-T cycle/cycles preceded by a P wave or P waves that fail to conduct through the AV node → no QRS response.

• Must note the P:QRS ratio, such as:

  • 3:1

  • 4:1

  • 5:2

• Slow ventricular rates may produce loss of consciousness due to hemodynamic instability.

What is a 2:1 AV block?

• Has a characteristic P wave with a conducted QRS followed by a lone P wave.

• Can either be:

  • Wenckebach → at AV node

  • Mobitz → His bundle or bundle branches

• Unable to tell which one from EKG alone

What should you check on every EKG to identify AV blocks?

1. PR Interval

• Increased consistently in 1° AV block

• Progressively increases in each cycle with Wenckebach

• Totally variable in 3° AV block

2. P without QRS response

• Wenckebach and Mobitz 2° AV blocks

• 3° AV block: independent atrial and ventricular rates

What are the characteristics of a 3° (complete) AV block?

• Conduction of supraventricular depolarizations to the ventricles is TOTALLY blocked.

• An automaticity focus escapes to pace the ventricles at its inherent rate.

• Causes AV dissociation:

  • P waves occur at a regular atrial rate.

  • QRS complexes occur at a separate rate.

  • QRS complexes are totally disconnected from the P waves.

• Also called parasystole in the lecture.

How does the location of a 3° AV block determine which automaticity focus paces the ventricles?

• Complete block of upper AV node:

  • Allows junctional foci to escape.

• Complete block of entire AV node or in Bundle of His:

  • Allows only a ventricular focus to pace.

• Below the His Bundle, all paths completely blocked:

  • Allows only a ventricular focus to pace.

• Blocks distal to the AV node → hemodynamic instability and compromised cerebral blood flow → SYNCOPE.

• Continuous surveillance and maintenance of airway is necessary.

• These patients need a pacemaker

What are the EKG characteristics of a bundle branch block (BBB)?

• Caused by a block of conduction in the right or left bundle branch.

• The blocked bundle branch delays conduction to the ventricle it supplies.

• One ventricle depolarizes more slowly than the other → two “joined QRS” complexes.

• Two R waves are seen:

  • R

  • R′ (R-prime)

• Results in RSR′ waves.

• R′ represents depolarization of the blocked ventricle.

• QRS is wider than .12 sec (3 small boxes) → diagnostic.

How do you determine whether a bundle branch block is right or left?

• Check the right and left chest leads.

• Right side of heart:

  • V1 and V2

• Left side of heart:

  • V5 and V6

• R, R′ morphologies are distinct for right and left BBB, respectively.

• Diagnosis can be made by appearance alone.

What is the key EKG difference between 1°, Wenckebach, Mobitz II, and 3° AV block?

• 1° AV block: PR > .20 sec and consistently prolonged; every P still has a QRS.

• 2° Type I — Wenckebach: PR progressively increases → dropped QRS.

• 2° Type II — Mobitz: P waves suddenly fail to produce a QRS; note the P:QRS ratio.

• 3° AV block: P waves and QRS complexes are totally independent → AV dissociation.