Friday, October 28, 2011

EKG Rhythm Strips 13- Heart Blocks

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Answers
01.
Sinus Rhythm With 1st Degree AV Block

Sinus Rhythm With 1st Degree AV Block.  This block is characterized by a PR interval that is greater than .20 sec.  The rhythm is usually regular.  There is one P wave for each QRS complex (no dropped beats).  What is the actual PR interval in this strip? 

02.
3rd Degree Heart Block

3rd Degree Heart Block  or Complete Heart Block is identified by P waves and QRS complexes that do not occur together.  Recall that in a sinus rhythm there should be one P wave for each QRS complex and that the PR interval should be < .20 seconds.   In most cases, structually there is a block below the level of the AV node so this prevents electrical impulses by the SA node from reaching the ventricles. Thus, an escape mechanism comes into play and the ventricular rate will be dependent upon a junctional focus (40-60) or ventricular focus (20-40).
It is well worth mentioning the pacemaker rule for the heart.  The Pacemaker Rule:  the fastest rate controls the heart.
1. This is usually the SA node unless an irritable foucs is faster: called irritability
2. If an upper pacemaker fails, then the lower pacemakers (junctional or ventricular) assumes control: escape beat or rhythm

03.
Sinus Rhythm With 1st Degree AV Block

Sinus Rhythm With 1st Degree AV Block.  Again the PR interval is over > 0.20 sec.  The rhythm is regular except for the PVCs  The rate is 75.  The PR interval is ___?  The QRS is < .12.   QT .44.     What does the PR interval represent?   There are also two unifocal PVCs present.

04.
2nd Degree Heart Block Type II (Mobitz II)
2nd Degree Heart Block Type II (Mobitz II).  When you look at the rhythm strip you see that there are P waves that are not immediately followed by QRS complexes.  These P waves are referred to as nonconducted P waves or nonconducted beats.  These nonconducted beats are atrial in orgin but they are not conducted down the conduction pathway of the heart to the ventricles.  Usually there is a block below the level of the AV node that prevents them from reaching the ventricles.  When you see nonconducted beats:  Think no cardiac output- no blood flow. 

05.
2nd Degree Heart Block Type II (Mobitz II)
2nd Degree Heart Block Type II (Mobitz II).  As above,  there are nonconducted P waves but then there are some P waves that are followed by QRS complexes that appear sinus in origin.   On these conducted beats there is one P wave for each QRS complex.  The PR interval is constant, usually less than 0.20 sec. and the R-R interval is also constant.  This is important to remember when trying to distinguish  a Mobitz I from Mobitz II block.  On a Mobitz II block the PR interval on the conducted beats will be the same whereas on a Mobitz I block the PR interval progressively lengthens.

06.
2nd Degree Heart Block Type I (Mobitz I or Wenchebach)
2nd Degree Heart Block Type I (Mobitz I or Wenchebach).  The rhythm is irregular.  There is no one P wave for each QRS complex and the PR interval progressively lengthens.  Usually this occurs in cycles on the rhythm strip where you will have 3 or 4 beats that is followed by a nonconducted P wave.  Usually this block occurs at the level of the AV node.   What coronary artery profuses the AV node?  What kind of MI would you likely see this block in?  Inferior?  Anterior?  Lateral? 

07.
3rd Degree Heart Block
3rd Degree Heart Block.  In looking at this rhythm strip, you immediately notice that there are P waves that do not match up with the QRS complexes.  This should clue you in to the fact that the Atrium and Ventricles are not beating in synchronization.  Calculate the atrial rate and the ventricular rate.   Are they the same?  If you measure the P-P interval you will see that there are some P waves that are buried within the QRS complexes. 

08.
2nd Degree Heart Block Type II (Mobitz II)
2nd Degree Heart Block Type II (Mobitz II).  As you look at this rhythm strip, you see that there are more P waves than QRS complexes.  There are actually 3 P waves for each QRS complexes so you might see this referred to as a 3:1 conduction.  What is the atrial rate? (Measure the number of small boxes between the P waves and divide that number into 1500)  What is the ventricular rate?   With a slow rate, there is an insufficient amount of blood being pushed into the systemic circulation by the ventricles so the patient may be symptomatic (dizzy, light headed, feel weak) or unstable (short of breath,  cold and clammy, have chest pain, hypotensive).  If the patient is only symptomatic- think medications.  If the patient is unstable- think electrical interventions: pacing.  Use Atropine cautiously in Mobitz II and complete heart blocks as it may worsen the block.  The AHA now recommends catecholamine infusions as an acceptable substitute for pacing.  What are the catecholamines infusions and what is the dosage range?  What is the dosage of Atropine?  What is the maximum cumulative dose of Atropine?

09.
2nd Degree Heart Block Type I (Mobitz I or Wenchebach)
2nd Degree Heart Block Type I (Mobitz I or Wenchebach).  There is progressive lengthening of the PR interval in the conducted beats and this is followed by a nonconducted P wave.  This is rarely a serious block.  

10.
3rd Degree Heart Block
3rd Degree Heart Block.  Now that you know how to measure the P-P interval and the R-R interval, determine the atrial rate and the ventricular rate.  This block occurs below the level of the AV node somewhere near the septum. You might see this kind of block with involvement of the LAD artery.  What kind of MI would you see with LAD involvement?  Inferior?  Anterior?  Lateral?   Let us say that our patient is unstable and that the MD decides to try transvenous pacing on the patient with this heart rhythm.  What is the correct places to place the comb pads?   What is the difference between a demand pacing mode (synchronous) and non-demand (asynchronous) mode.   What is meant by the term pacing threshold?  How would you assess for mechanical capture?   : )

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