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    A-V BLOCKS

    Doina Kulick, MD, MS, FACP

    2007

    Learning objectives

    At the end of this class

    you will be able to diagnose

    the following ECG problems:

    1st Degree AV Block

    Type I (Wenckebach) 2nd Degree AV Block

    Type II (Mobitz) 2nd Degree AV Block

    Complete (3rd Degree) AV Block

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    PR-interval is prolonged over 0.20 s (thedepolarization spreads normally from the sinus node to the atria, butusually upon reaching the AV node the conduction is delayed)

    All P waves conduct to the ventricles

    1st Degree AV Block

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    What is wrong with this ECG?

    Previous ECG:1st Degree Heart Block and LBBBSinus @ 60 BPMPR = 0.24 s = 1st degree AV block (>0.2 seconds)QRS = 0.16 s = prolonged (>0.12 seconds)QT = 0.48 sThis ECG has a plethora of interval abnormalities. The PR, QRS and QT intervalare all prolonged.Prolongation of the PR interval is caused by abnormalities in the AV node. Sinceevery P wave is followed by a QRS we know that the signal is getting through theAV node. This is therefore a 1st degree heart block. In order to call a AV node block2nd or 3rd degree there must be some signals which don't pass through to theventricles (P wave not followed by a QRS)

    Each P wave is folowed by a QRS complex, thus the rhythm is of suprventriculareorigin. In this setting the prolonged QRS accompanied by broad R waves I,V6 andbroad S waves in V1 defines a left bundle branch block.QT interval is 0.48. As mentioned this is prolonged for a heart rate in the normalrange. Using the rule of thumb we also see that the QT is > 1/2 the RR interval. Ifyou initially thought the QT was less than the above measurements, note whichleads you used to make this measurement. Lead II for example is hard toappreciate the QT interval, while III provides a more accurate measure since the Twave is more pronounced.

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    Causes of 1st degree AV block:

    May be normal (young or athletic individuals due toexcessive vagal tone)

    Acute inferior or right ventricular MI due toincreased vagal tone or AV node ischemia

    Ischemic heart disease Digitalis toxicity Excessive inhibitory vagal tone Drugs: Ca channel blockers, Beta blockers

    Electrolyte imbalance: hyperkalemia Myocarditis

    2nd Degree AV Block

    In 2nd degree A-V block not every atrial impulse is conducted to theventricles ( not every P wave is followed by QRS)

    There are two types of 2nd degree A-V blocks:

    -Mobitz type I second-degree A-V block (Wenckebach Block)

    -Mobitz type II second-degree A-V block

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    Mobitz type I, 2nd degree A-V block (Wenckebach Block)

    P

    -PR interval gets longer until a nonconducted P wave occurs

    -The block can be 2:1, 3:2, 4:3 (like in this ECG)

    -The RR interval of the pause is less thanthe two preceding RR

    intervals, and the RR interval after the pause is greater thanthe RRinterval before the pause. These are the classicrules of Wenckebach(atypical forms can occur)

    - Almost alwayslocated in the AV node, which means that the QRS duration isusually narrow, unless there is preexisting bundle branch disease.

    - PR intervals are constant until a nonconducted P wave occurs

    -The RR interval of the pause is equal tothe two preceding RR intervals

    - Almost alwayslocated infranodal (the bundle branches), which meansthat the QRS duration is wide

    -Block can be 2:1, 3:2, 3:1, 4:3, 4:2,..

    -Variable conduction will result in irregular ventricular (R-R) rhythm

    Mobitz II, 2nd degree A-V block

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    In Type II block several consecutive P waves may be blocked asillustrated above (4:2)

    Causes of Mobitz type I, (Wenckebach Block) :- Underlying structural abnormalities of the node due to fibrosis or ischemia(usually inferior MI)

    - An increase in vagal tone that causes a reduction in the rate of impulseconduction

    - Drugs that impair or slow nodal conduction including digoxin, beta blockers,and the calcium channel blocking agents

    - Can be a normal variant in young or athletic individuals due to excessive

    vagal tone

    The prognosis is generally good good

    Causes of Mobitz type II:-Mobitz types II block is almost always below the AV node and hasoraganic causes (ischemia, usually anterior Mi)

    The prognosis is worse than type I

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    ECG #2 , - Is this Mobitz I or II?

    ECG#2

    The atrial rhythm is sinus, and 2:1 atrioventricular (AV) conduction ispresent. The PR intervals of the conducted beats are normal at about 0.19s. The QRS complexes are narrow and normal-appearing. The 2:1 AVconduction could represent either type I second-degree (Wenckebach) ortype II second-degree AV block. When type I second-degree AV block ispresent, the PR intervals of the conducted complexes are often prolongedand the QRS complexes narrow and normal-appearing, whereas in type IIsecond-degree AV block the PR intervals of the conducted complexesare generally normal and the QRS complexes broad, indicating bundlebranch disease. In this tracing, the PR intervals are normal. The site ofblock cannot be known with certainty from this tracing, and manipulationof the AV conduction ratio by atropine or exercise, which facilitate AVconduction, might be required. Type I second-degree AV block generallydoes not require cardiac pacing because it is occurring within the AVnode; if the block is within the His bundle (which might requireelectrophysiologic study to document), however, permanent cardiacpacing is indicated.

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    AMI and 2nd A-V blocks:

    Mobitz type I block usually arises in the AV node. Most of thetime is transient. Treatment of symptomatic or severebradycardia with Mobitz type I block could begin by usingatropine (cautiously); a temporary pacemaker is occasionallynecessary if the ventricular response is inadequate andprolonged.

    Mobitz type II second-degree A-V block is due to disease in theinfranodal conduction system. A temporary (and oftenpermanent ) pacemaker is necessary with this type of block

    because of the higher likelihood of progression to completeatrioventricular block, and the escape pacemaker with type IIblock originates in the ventricle and may not generate anadequate ventricular rate. Do not try atropine, will not work , andcan be dangerous in the setting of acute MI.

    3rd Degree A-V Block

    -Usually see complete AV dissociation because the atria and ventricles are eachcontrolled by separate pacemakers: independent beating of the atria and

    ventricles

    -P waves are present and occur at a rate faster than the ventricular rate; QRScomplexes are present and occur at a regular rate, usually

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    3rd AV block

    If the block is above the His bundle the QRScomplexes are narrow, ventricular rate 40-60/min

    3rd AV block

    Wide QRS rhythm suggests a ventricular pacemakersituated bellow the bundle of His; ventricular rate lessthan 40/min

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    Causes of 3rd AV block:

    Transient and reversible 3rd degree AV block is usually associated with

    normal QRS and HR 45-60- Acute inferior or right ventricular MI (increased vagal, ischemic AV node)- Ischemic heart disease- Excessive vagal tone- Digitalis toxicity- Ca channel blockers, Beta blockers- Electrolyte imbalance- Myocarditis

    Permanent or chronic 3rd degree is usually associated with wide QRS, HR-30-40 (Commonly the result of complete block of both BB)

    - Anterior MI- Chronic degenerative changes (Lenegres, Levs diseases)- Usually NOT a result of increased vagal stimulation, or toxicity

    Treatment:

    - external pacing and atropine (sometime works ifnarrow QRS complexes), for acute, symptomaticepisodes

    - permanent pacing for chronic complete heartblock