CARDIOLOGY - AV Blocks and Funny Looking Beats (Week 9) Flashcards
Dysrhythmias: AV Blocks/Funny Looking Beats
1) 1st degree AV Block
2) 2nd degree Type I AV Block
3) 2nd degree Type II AV Block
4) 2nd degree AV Block, 2:1 Conduction
5) 3rd degree AV Block
6) Wolff-Parkinson-White Syndrome (AV Re-entrant Tachycardia)
7) Pacemakers (Capture and Pacing)
- Atrial Pacemaker
- Ventricular Pacemaker
- AV Sequential (Dual Chamber) Pacemaker
The AV junction is the electrical link between what?
- atria and ventricles (which allows impulses to travel from atria to ventricles)
- consists of specialized conduction tissue
Describe the conduction pathway in AV blocks.
- impulse originates in SA node and attempts to head down normal conduction pathway BUT there is a partial/complete “interruption” in cardiac electrical conduction between the atria and ventricles
- this delay/interruption can occur in the AV node, Bundle of His, or Bundle Branches
What determines if the cardiac dysrhythmia caused by a block is lethal or not?
depends on the location of the block and the patient’s resulting symptoms
How are AV blocks classified?
By:
1) Site of Block - eg. AV node
2) Degree of Block - eg. Second-degree
3) Category of AV conduction disturbances (i.e. type of block, like Type I)
1st Degree AV Block
- sinus impulse is NOT blocked (all sinus beats are conducted, despite the name)
- impulses are only delayed in AV node
- basically it looks like NSR (or sinus brad/tachy) but with LONGER P-R interval
“if the R is far from the P, then you have 1st degree”
Describe the conduction pathway in 1st Degree AV Block
- Impulse normally originates in SA node and is conducted to AV node BUT delay occurs in AV node which is characterized by a prolonged PR interval
- prolonged PR interval usually between 0.21 - 0.48 seconds (vs. the regular 0.12 -0.20 seconds)
- Ultimately, impulse depolarizes through its normal conduction pathway and terminates in the ventricular muscle
Normal PR interval of 0.12-0.20 seconds would indicate that the impulse conducted through what structures of the heart?
conducted normally through atria, AV node, bundle of his, bundle branches, and purkinje fibers
1st Degree AV Block
Rate
Rhythm
P wave
PR interval
QRS Complex
Rate: can occur at any rate
Rhythm: regular
P wave: regular, upright, and matching; each P wave is followed by a QRS complex (1:1 relationship)
PR interval: >0.20 seconds, prolonged and constant
QRS Complex: <0.12 (>0.12 seconds if BBB is present)
remember it just looks like NSR/sinus brady/tachy but just with longer PR interval!
1st Degree AV Block
Causes
Adverse Effects
Treatment
Causes:
- normal finding in those with no cardiac disease, especially in athletes
- MI
- increased vagal tone
- ischemia or injury to the AV node or AV junction
- Rheumatic heart disease
- Hyperkalemia
- Medications - Quinidine, procrainamide, beta-blockers, CCBs, digitalis, amiodarone
Adverse Effects: usually no ill effects, often asymptomatic
Treatment: treat underlying causes (i.e. discontinue medication, monitor for MI)
2nd Degree Type I AV Block
- aka Wenckebach or Mobitz Type I
- an incomplete block as the AV node has become progressively “sicker” and less able to conduct impulses
- you’ll notice that the P waves are regular as the SA node is firing as normal but the PR intervals get progressively longer until a P wave is not followed by a QRS complex but instead by a pause (dropped beat)
- in the book: note that it says sometimes in the pause (after the dropped beat) that an escape beat MAY occur
“longer, longer, longer drop, and then you have Wenckebach”
Describe the conduction pathway in 2nd Degree Type I AV block.
- Impulse normally originates in SA node and is conducted to the AV node BUT the AV node has become progressively sicker and less able to conduct impulses
- AV node getting sicker is evident in lengthening PR intervals as the impulses generated by the SA node take longer and longer with each beat to conduct through the AV node
- ultimately it is unable to send an impulse down to the ventricles
2nd Degree Type I AV Block (Mobitz Type I; Wenckebach)
Rate
Rhythm
P wave
PR interval
QRS complex
Rate: Atrial rate is usually 60-100; ventricular rate is less than atrial rate due to non-conducted beats
Rhythm: regularly irregular (hallmark of Wenckebach is group of beats and then a pause
P wave: regular, upright and matching; all P waves except the blocked P wave are followed by a QRS complex
PR interval: lengthens with each cycle (you can write this or that it “varies in duration”); gradually longer PR interval until a QRS complex is dropped.
QRS complex: <0.12 seconds (usually narrow)
2nd Degree Type I AV Block (Mobitz Type I/Wenckebach)
Causes
Adverse Effects
Treatment
Causes:
-
MI (typically seen in inferior and posterior MIs)
- Conduction delay within the AV node
- RCA supplies blood to the AV node in 85-90% of people, so RCA occlusions associated with AV block occuring in the AV node can cause this type of block (leading to ischemia)
- This may cause a slower conduction through the AV node (i.e. prolonged PR intervals or dropped beats)
Adverse Effects: usually no ill effects, but monitor for worsening block (usually resolves in 48-72h; and usually asymptomatic because beat is close to normal rate)
Treatment: treat underlying cause (i.e. discontinue meds, monitor for MI)
2nd Degree Type II AV Block
- aka Mobitz Type II
- An incomplete block below the AV node causing inconsistent dropped QRS complexes
- P waves are all regular (but not followed by QRS complexes meaning they are non-conducted beats”
“It some Ps don’t get through, then you have Mobitz II”
How can you tell the difference between 2nd degree Type II AV Block and non-conducted PACs since they both have P waves with no QRS complexes that follow?
- in PACs, the p wave would be premature because the beat is premature
- in 2nd degree Type II (Mobitz II), the p waves are regular, QRS are irregular
Describe the conduction pathway of an electrical impulse in 2nd degree Type II AV block.
- impulse normally originates in SA node and is conducted to the AV node
- the impulse is BLOCKED in either the Bundle of His (uncommon) or Bundle branches (most common)
- PR intervals are constant in conducted beats and occasionally P waves appear with no QRS after it
- ultimately SOME impulses are sent down to the ventricles
2nd Degree Type II AV Block (Mobitz Type II)
Rate
Rhythm
P wave
PR Interval
QRS complex
Rate: atrial rate is greater than ventricular rate; ventricular rate often “slow”
Rhythm: irregularly irregular
P wave: regular, upright, and matching; some P waves are not preceded by a QRS (more P’s than QRS)
PR Interval: normally 0.12 - 0.20 seconds BUT can be slightly prolonged; constant for conducted beats (unlike 2nd degree Type I where it gets longer and longer)
QRS complex: <0.12 seconds
2nd Degree Type II AV Block
Causes
Adverse Effects
Treatment
Causes: MI, heart disease, acute myocarditis
Adverse Effects: If the ventricular rate is wtihin normal limits, then often asymptomatic HOWEVER ventricular rate is often slow, thus decreased cardiac output
- also monitor for progressive 3rd degree block
Treatment: None
2nd Degree AV Block, 2:1 conduction
Two P waves occuring for every one QRS complex
conduction can vary (eg. 3:1, 4:1, etc.)
Describe the conduction pathway in 2nd degree AV Block, 2:1 conduction.
- Impulse normally originates in SA node and is conducted to the AV node
- impulse is blocked in either the Bundle of His (uncommon) or bundle branches (most common)
- PR intervals are constant in onducted beats, and every OTHER p waves appear with no QRS after it
- ultimately some impulses are sent down to the ventricles
2nd Degree AV block, 2:1 conduction
Rate
Rhythm
P wave
PR Interval
QRS complex
Rate: Atrial rate is twice the ventricular rate
Rhythm: regular; atrial regular (P’s plot through on time) and ventricles regular
P wave: regular, upright, and matching; every other P wave is not preceded by a QRS (more P’s than QRS)
PR Interval: normally 0.12-0.20 secs (BUT can be slightly prolonged); constant for conducted beats
QRS complex: usually <0.1.2 seconds
How can you distinguish between 2nd degree AV block 2:1 conduction and 2nd degree Type II AV Block?
Rhythm for Mobitz Type II is irregularly irregular while for 2nd degree AV Block 2:1 conduction the rhythm is regular!
3rd Degree AV Block
A complete block in conduction of impulse between atria and ventricles (they are not talkin to each other) - two pacemakers firing at different rates
“if Ps and QRS’ don’t agree, then you have a 3rd degree”
Describe the conduction pathway of an electrical impulse in 3rd degree AV block.
- impulse normally originates in SA node but a COMPLETE block has occured at either the AV node, Bundle of His or bundle branches
- impulse is unable to be sent down to the ventricles
- therefore a secondary pacemaker (either junctional or ventricular) stimulates the ventricles
- the atria and ventricles beat independently of each other
When interpreting 2nd and 3rd degree AV blocks:
How do you know that the extra P wave with no QRS after it isn’t a non-conducted PAC?
TIMING
- In 2nd and 3rd degree AV blocks, there are more P waves than QRS complexes and all P waves occur on time (regular)
- this happens because the problem is NOT associated with the SA node (SA node is firing regularly), the problem occurs somewhere below the SA node in the conduction system
- in a PAC, the P wave of a non-conducted PAC is early and therefore not on time
Bundle Branch Block (BBB)
interruption in conduction btween either bundle branch causing an unsynchronized depolarization (normal PR interval, wide QRS complex)
Describe the conduction pathway in bundle branch block (BBB)
- atrial depolarize in a normal manner (depending upon the underlying rhythm)
- the impulse then travels down the unblocked branch and stimulates that ventricle but because a block has occurred in the other branch, the impulse must travel from cell-to-cell through the myocardium (rather than the normal conduction pathway) SLOWLY to stimulate the other ventricle
- meaning conduction is slower than normal and QRS appears widened (≥0.12 secs) on ECG
- ultimately the blocked ventricle gets depolarized slightly layer than the normal ventricle, causing two separate depolarizations
- the two depolarizations can be seen on a rhythm strip as a single notched (“rabbit ears”) OR widened WRS
What are the two most common types of depolarizations seen on a rhythm strip for BBB?
AV Re-entract Tachycardia (AVRT)
- aka Wolff-Parkinson-White syndrome (WPW)
- an impulse that follows a normal and abnormal conduction pathway called the Bundle of Kent
In WPW/AVRT, the pre-excitation of the ventricles is seen as a:
delta wave - an intial slurring of the QRS complex and is the result of the ventricles depolarizing via the accessory pathway
Temporary Pacemakers
- those that are NOT implanted
- used to maintain a patient’s heart rate in an emergency situation or until a permanent pacemaker can be surgically implanted
- electrodes can be placed in one of two ways:
- 1. Transvenously (through a vein) - lead wire is threaded through a large vein internally into the right atrium, ventricle, or both to stimualte endocardium
- 2. Transcutaneously (through the skin) - adhesive pads are placed externally on the body and generate an electrical impulse that stimulates cardiac cells to depolarize in a normal manner
Permanent Pacemakers
When are they used, where are they placed, and what are the three indications/conditions for permanent pacing?
- those that ARE implanted
- used when the heart is unable to maintain a normal rate, even with the aid of medications
- pulse generator is usually implanted into subQ tissue of the anterior chest just below right or left clavicle
- patient’s non-dominant side is usually chosen to minimize interference with daily activities
- indications for permanent pacing include:
- symptomatic bradycardia
- 2nd degree Type II AV Block
- 3rd degree AB Block
Pacemaker electrodes can be unipolar or bipolar. Describe the placement of each.
1) Unipolar: one pacing (negative) electrode is located at the distal tip and in contact with the heart
- the pulse generator (located on the outside of the heart) acts as the positive electrode
2) Bipolar: two pacing (positive and negative) electrode located at the distal tip
- most tranvenous pacemakers have bipolar electrodes
What are the 4 types of permanent pacemakers?
1) Atrial pacemaker
2) Ventricular pacemaker
3) AV sequential pacemaker/Dual Chamber Pacemaker
4) Biventricular pacemaker
AV Sequential pacemaker/Dual Chamber Pacemaker
Lead wire insertion location
Impulse conduction
ECG findings
Insertion: Lead wires are inserted in both right atria and right ventricle
Impulse: generated from the two lead wires and stimulate both RA and RV in a normal sequential manner
Findings:
- Discharge is represented by vertical lines (pacer spike/spike)
- pacer spikes are normally followed with P waves and QRS (two lines!)
Pacemaker malfunctions: Failure to pace
- aka failure to fire
- pacemaker failed to deliver an electrical stimulus OR when it fails to deliver the correct number of electrical stimulations to per minutes
- on the ECG: recognized by the absence of pacer spikes and return of the underlying rhythm for which the pacemaker was implanted
- Causes: battery failure, lead wire fracture, displacement of tip, electromagnetic interference, and/or sensitivity setting is too high
- Patient may show S/S of syncope, chest pain, bradycardia, and hypotension
Pacemaker malfunctions: Failure to capture
- capture is successful depolarization of the atria and/or ventricles by an artifical pacemaker
- failure to capture is recognized by the presence of pacer spikes NOT followed by P waves (if the electrode is located in the right atria; i.e. atrial pacemaker) OR QRS complexes (if the electrode is located in the right ventricle; ventricular pacemaker)
- pacemaker is working by firing impulses but not talking to the heart
- Causes: battery failure, lead wire fracture, displacement of pacing lead, perforation of myocardium, and/or sensitivity settings too low, medications
- Patient may display S/S of syncope, chest pain, bradycardia, and hypotension
