AtrioVentricular BLOCKS (AV Blocks)

AV Heart Blocks AV Heart blocks is the name given to conditions in which electrical conduction at the AV node is somehow affected. We generally speak of three degrees of heart block.

AV blocks occur when there is a Partial or Complete interruption in the electrical conduction flow between the atria and ventricle. The sinus node is functioning appropriately but the impulse is not able to reach it’s destination – the ventricle - in what has been identified as the normal sequence of events.

The AV Blocks Sinus Mechanism with 1st Degree AV Block 2nd Degree AV Block Type I (Mobitz I or Wenckebach) Type II (Mobitz II) 3rd Degree AV Block (Complete Heart Block)

The AV Blocks The AV blocks tend to be the rhythms that confuse folks the most – but really if you take your time figuring out all the steps – it makes sense. we are talking about the electrical system of the heart – not the plumbing (arteries and blood supply) although it does effect the electrical system. Severity is measured in degrees.

AV Blocks In a nutshell….the blocks are categorized as follows: 1st degree heart block: AV conduction is excessively slowed. 2nd degree heart block: AV conduction is incompletely (occasionally) blocked 3rd degree heart block: AV conduction is completely blocked.

About First Degree AV Blocks: 1st degree AV block is caused by a conduction delay through the AV node but all the electrical signals reach the ventricles. Usually a block somewhere above the Bundle of His. Rarely causes a problem. Seen in trained athletes. Normal PRI (0.12 s -0.20 s) or 3-5 small squares on the EKG. PR is lengthened > 0.20 s Frequently seen in coronary artery disease (CAD), cardiac ischemia, Rheumatic Heart Disease; Hyperkalemia; digitalis administration, and all types of myocardial infarction.

1st Degree AV Block Treatment Since no symptoms and usually benign, no treatment is required, but patient should be watched to make sure that the PR is measured and compared to previous for any further lengthening, and report to physician. If this is new for this patient suspect possible MI if not dig toxic.

1st degree block is always based with a sinus rhythm 1st degree block is always based with a sinus rhythm. Every P has a partner (QRS) BUT the PR is lengthened. Greater than what we know to be normal 0.12 -0.20 seconds. In essence, all of the beats originate in the sinus node (atria) and all make it to the ventricle through the conduction pathway, but it takes a little longer. So the conduction pathway is intact – just is slower to conduct the impulse to the ventricle.

1st Degree AV Block Rhythm is regular; Rate is normal; QRS duration-Normal; P wave 1:1 meaning that there is 1 P wave for each QRS complex; PR Interval is prolonged which is greater than 5 small squares.

2nd Degree Block- Mobitz I, Type A, Type I, Wenckebach Another condition whereby a conduction block of some, but not all atrial beats getting through the ventricles. There is a progressive lengthening of the PR interval and then failure of conduction of an atrial beat, this is seen by a dropped QRS complex

What will you see: Looking at the EKG rhythm you will see the following: Rhythm—Regularly irregular Rate-Normal or slow QRS duration will be normal P Wave ratio will be 1: 1 for a few cycles then drop a QRS See a progressive lengthening of the PR interval until a QRS is Dropped

Causes of 2nd Degree AV Block Type I: Most commonly associated with acute inferior MI (probably due to ischemia of the AV node). Usually develops w/in first 24 hr of MI and will not persist beyond 3rd day. Digitalis toxicity (stop Dig) Progressive fatigue of the AV junctional tissues Drug toxicity Lyte imbalances Symptoms: Often no symptoms, but if present are usually related to rate decrease and therefore decrease in cardiac output Treatment: If related to drug administration – stop the drug Usually observe and follow-up with cardiologist

2nd Degree Block Type 2 or Mobitz 2 When electrical excitation sometimes fails to pass through the A-V node or bundle of His, this intermittent occurrence is said to be called second degree heart block. Electrical conduction usually has a constant P-R interval …however in this instance…..atrial contractions are not regularly followed by a ventricular contraction. (more P’s)

2nd Degree Block Type 2 or Mobitz 2 Blockage occurs below the Bundle of His in the Bundle Branches The ventricle fails to respond to atrial stimulation…periodically You may see a P with no QRS QRS may be wide because it is reflective of a block below the ventricle.

What to look for?? More P waves in contrast to the QRS complex. Rhythm that is regular or irregular If there are 2 P waves for each QRS we call that 2:1 Heart Block. Not every P wave makes it to the ventricle. But those that do….do it consistently. Sometimes you may see Type I and Type II together due to medications and degree of heart disease. Meds can be toxins!

May see a widening of the QRS due to the delay of conduction below the bundle of His. (> 0.12 seconds)

Causes of 2nd Degree AV Block Type II : Originate below the bundle of His and organic in origin Associated with Acute anterior wall MI (extensive myocardial damage) Symptoms: Rate dependent usually, decrease in cardiac output/increase in demand Dependent on extent of myocardial damage Treatment Observe patient closely Transcutaneous pacemaker on standby May require permanent pacemaker Atropine is drug of choice for symptomatic bradycardia However, it will increase the rate and not reduce the block. This condition can deteriorate to complete heart block. These are the patients that should have a Life Pak outside their door if they are symptomatic

3rd degree Block 3rd degree or complete heart block occurs when atrial contractions are “normal” but no electrical conduction is conveyed to the ventricles. The ventricles then generate their own signal through an escape mechanism from a focus somewhere within the ventricle. The ventricular escape beats are usually slow. In Complete block-No Atrial impulses pass through the AV node and the ventricles generate their own rhythm.

What do we see??? None of the Atrial impulses make it to the ventricles. The sinus node is ok – but the pathway is totally blocked. It continues to fire – but never makes it through. So the P waves march through across the strip in regular fashion and so do the QRS complexes, but neither communicate to the other.

What do we see?? Because the sinus node or any impulse from above fails to make it to the ventricle – it needs to take over and produce an impulse. This impulse therefore is from the ventricular system. Any impulse from the ventricular system origin is wider in nature hence the QRS duration is greater than 0.12 seconds. Also if you remember – the frequency of the impulse is going to be much slower as well.

P P P waves march across QRS march across

Causes of 3rd Degree AV Block Extensive myocardial damage Inferior wall MI Digitalis toxicity Symptoms Signs of decreased cardiac output Decreased activity tolerance Chest pain Confusion Shortness of breath Treatment Check patients for symptoms Oxygen to increase supply Transcutaneous pacemaker and then Transvenous pacemaker Mortality Rate is 80% The Patient May require CPR at this point if their HR is 10…it is obvioulsy not a perfusing rhythm.

(Complete Heart Block) AV Block Algorithm Rhythm Characteristics Sinus with 1st Degree AV Block 2nd Degree AV Block Type I Wenckebach 2nd Degree AV Block Type II Classic 3rd Degree AV Block (Complete Heart Block) P waves to QRS ratio 1:1 More P’s than QRS’s More P’s than QRS’s Ventricular Regular Irregular Regular or Irregular QRS Regular PR Interval Constant and > .20 Changing progressively gets longer Constant Changing – no pattern long – short-short-long, etc Rate Any sinus rate Varies but atrial rate faster than Varies but atrial rate faster than Ventricular rate Ventricular rate 20 – 40 (Usually) Atrial rate faster This is a chart/algorithm that was developed to help facilitate AV block interpretation. Across the top are the rhythms Down the vertical axis, are the components to identify. So look at P waves and the ratio to QRS complex. If it is 1:1 it has to be Sinus with 1st degree block (just make sure it meets all the other criteria) If more P’s than QRS, then decide if the ventricular rate is regular or irregular. If regular – it could be either 2nd type II or 3rd degree; if Irregular it has to be one of the 2nd degree blocks; then check out the PR – etc. Developed by Judy Haines, RN

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