The ECG Course - EMS Educationems.bcfdmo.com/wp-content/uploads/2018/01/28-AV-Blocks.pdf ·...

Boone County Fire Protection DistrictEMS Education

The ECG Course


Level IG r a t e d

m a t e r i a l


AV Blocks


What Causes AV Block?

Long list of bad things that includes ischemia and..

Old age / disease

Medications or drugs

Electrolyte imbalances


Physiologic BlocksPhysiologic blocks occur for “good” reasons, such as when atrial flutter is conducted partially, resulting in a better heart rate.

Another example of physiologic block is the non-conducted PAC, when the premature P wave occurs very early after the preceding beat, finding the ventricles refractory and unable to produce a QRS.

AV Node attempts to keep the rate <140-150.


1st Degree AV Block

Prolonged PRI (longer than 200 milliseconds)

No dropped beats

Ratio is 1:1


First Degree AVB

Medication causes (digitalis is a common one)

Associated with Inferior Wall MI (about 13% of the time)

May progress—don’t be fooled by the “degree”

Nodal block


1st Degree AV Block


2nd Degree Type I

Type I (Mobitz I, Wenkebach)

Lengthening PRI before “dropped” beat

Only one beat is dropped

Ratio is 3:2 or 4:3 or 5:4 etc.—“1 run game”


Mobitz I

Medication cause (digitalis)

Ischemia cause (Inferior MI common)

Usually benign but may progress

Nodal block


2nd Degree Type I


2nd Degree Type II’s

Infranodal—in His Bundle or below

Left Coronary Artery lesion

Anterior MI

Usually symptomatic

Frequently progresses to Complete AV Block


2nd Degree type II

Type II (Mobitz II)

Constant length PRI before “dropped” beat


Ratio is 3:2 or 4:3 or 5:4 etc.—“1 run game”


Mobitz IIOften, the patient with second degree type II has a pre-existing bundle branch block, usually of the right bundle branch. They sometimes have a hemiblock on the left, as well. (Bi-fascicular block). When the remaining fascicle develops an intermittent block, second degree type II is produced. You could say that second degree AV block type II is an intermittent tri-fascicular block, or an intermittent complete heart block.

Second degree AV block, type II usually represents a necrotic, progressive lesion. It is very likely to progress to complete heart block (third-degree). If it does progress to third degree AV block, the escape rhythm, if there is one, will be idioventricular. The bradycardia caused by the AV block may cause syncopal episodes. Patients with second degree AV block, type II usually are treated emergently with a temporary pacemaker until they can receive a permanent, implanted pacemaker.

One of the biggest worries with 2:1 conduction, regardless of type, is that the conduction ratio automatically cuts the patient’s rate in half. An underlying sinus rate of 70 bpm with 2:1 AV conduction will produce a pulse rate of 35 bpm!


2nd Degree Type II


2nd Degree AV Block 2:1 Conduction

“Only one” PRI before “dropped” beat


Ratio is 2:1

Can’t tell if PRI is constant or lengthening—there is only one so nothing to compare.


2nd Degree 2:1


2nd Degree-High

High-Grade AV Block

“More than one” beat is dropped

Ratio is 3:1 or 4:1 or 5:1 or more

This is NOT a 3rd Degree AV Block


2nd Degree-High


3rd Degree AV BlockAV Dissociation must be present.

Atria are firing at a different rate than the ventricles—P-waves “march out” and the QRS’s also march out—just to different drummers.

Technically, there is no PRI but there is a space between P-waves and QRS complexes —and that space is changing length randomly.

Technically, there are no dropped beats but it may appear that there are.


3rd Degree AV BlockAV Dissociation must be present.

Atria are firing at a different rate than the ventricles—P-waves “march out” and the QRS’s also march out—just to different drummers.


The AV Junction


“Degree” vs Severity

Degree of the AV Block is misleading in terms of estimating severity.

The location of the block is more predictive.


Conduction System Blood Supply

AV Node—right coronary (about 90%)

Bundle Branches—left coronary artery


Conduction System Innervation

SA Node—parasympathetic

AV Node—parasympathetic

Bundle Branches—no parasympathetic


Infranodal AV Blocks

Very few parasympathetic fibers innervate below the Node.

Infranodal block unlikely caused by excessive parasympathetic tone. (usually ischemia)

Atropine will be very unlikely to help.


Location-QRS Width

NOT a hard and fast rule but…

Nodal = QRS is not wide

Infranodal = wide QRS


Location-Cause

Nodal = inferior MI

Infra nodal = anterior MI


Ratio of P’s to QRS’s

Needed in order to accurately identify the rhythm when there are dropped beats (P-wave does not cause a QRS).

First step—make sure the P wave that is not conducted is NOT a PAC.

Step two—if NOT a PAC, then the rhythm is some flavor of 2nd Degree AV Block.


Six Foot Strip

Print out a long rhythm strip—a 12-Lead is only 10 seconds long.

Paper is cheap.

Print it out, lay it out, march it out (the P-waves).

Find out what the P-waves are doing first.


AV Blocks


Blocks

AV Nodal blocks tend to be transient—when their cause is relieved, they resolve.

Blocks below the AV Node (infranodal) tend to be due to a serious lesion and may progress or become life-threatening.


Complete AVB


Complete AVBThird degree AV block is diagnosed when no atrial impulses are conducted to the ventricles. This is one form of AV dissociation.

Third degree AV block can occur in the AV node, bundle of His, or bundle branches. If the block is in the AV node, it will usually have a reliable junctional escape rhythm. When the condition causing the block is successfully resolved, the block will be, too.

Third degree AV block occurring in the His bundle or below will result in an idioventricular escape rhythm. It may occur suddenly, with a block of the His bundle, or the right and left bundle branches simultaneously. Or, third degree AV block may be a result of progressive disease of the fascicles, resulting after a period of second degree AVB, type II.

Third degree AV block can also occur without the benefit of an escape rhythm. This results in ventricular standstill. Sometimes there is a warning, with a progressive AV block occurring. At other times, sudden ventricular standstill can occur!

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