ECG Part 3 - Common Cardiac Arrhythmias

Question 1

why do bradyarrhythmias often occur?

Answer 1

occur due to slow/absent sinoatrial node depolarization or atrioventricular node conduction

Question 2

what is sinus bradycardia? what causes it? how is it treated?

Answer 2

depression of sinoatrial node depolarization

causes - high vagal tone, anesthetic/analgesic administration

treatment - treat underlying cause, give atropine if high vagal tone is suspected

Question 3

what is a respiratory sinus arrhythmia?

Answer 3

arrhythmia normal to see in dogs & horses but rare in cats

sinoatrial node discharge varies based on the respiratory cycle, so increased heart rate on inspiration & decreased heart rate on expiration

Question 4

what is the difference between intermittent & complete sinoatrial node block/arrest? what causes it?

Answer 4

intermittent = block

complete = arrest

cause - sinoatrial node fails to depolarize

Question 5

what happens in sinus arrest caused by sick sinus syndrome?

Answer 5

with sinus arrest, the atrioventricular node takes over as the auxiliary/emergency pacemaker, so the heart rate is significantly slower than the normal sinus rhythm

no p waves are seen on ECG

in less severe cases - the sinoatrial node depolarization is slowed down & causes sinus bradycardia

Question 6

how is sick sinus syndrome treated?

Answer 6

some cases will respond to treatment

parasympatholytic (atropine) or beta-adrenergic agonists (terbutaline or theophylline)

artificial pacemaker implantation needed for non-responsive clinical cases

Question 7

what is atrioventricular block? what causes atrioventricular block?

Answer 7

atrial & ventricular contractions are not synchronized, on ecg the PP & PR intervals are regular but they are not the same & the PR interval varies due to 2 sites of impulse generation (SA node & AV node)

atrial action potential isn’t propagated to the ventricles, so the atria beat at a rate determined by the SA node while the ventricles beat at a rate of spontaneous atrioventricular node depolarization which is slower than the sinus rate

Question 8

what is the mildest form of AV block?

Answer 8

1° AV block - SA node impulse is more slowly propagated by the AV node & the PR interval is prolonged

Question 9

what type of av block is shown in the photo? how can you tell?

Answer 9

1° AV block -

Question 10

what type of av block is shown in the photo? how can you tell?

Answer 10

2° AV block (mobitz I or wenckebach)

progressively prolonged PR interval before a blocked P wave

Question 11

what type of av block is shown in the photo? how can you tell?

Answer 11

2° AV block (mobitz II) - ratio of P to QRS, look at the ratio (more commonly pathologic)

Question 12

what type of av block is shown in the photo? how can you tell?

Answer 12

2° AV block (2:1 block) - compare P to QRS, this ratio is 2:1

Question 13

what type of av block is shown in the photo? how can you tell?

Answer 13

3° AV block with junctional escape beats

ventricles beat at an escape rate, p waves/min are faster & QRS/min are slower & both are independent of each other

Question 14

what is mobitz type I 2° AV block? what causes it? how is it treated?

Answer 14

progressively prolonged PR interval before a blocked P wave typically

causes - usually high vagal tone

treatment - usually none, common in fit horses (40%) that will go away with exercise

Question 15

what is mobitz type II 2° AV block? what causes it?

Answer 15

progressively prolonged PR interval before the blocked P wave, describe it by a ratio of P to QRS, if > 3:1, considered to be high grade/advanced

causes - pathologic, not related to vagal tone

Question 16

what is 3° AV block?

Answer 16

aka complete heart block

no impulses from the SA node are conducted through the AV node

ventricles beat at an escape rate of spontaneous AV depolarization

P waves/min are faster while QRS/min are slower & both are independent of each other

Question 17

what are the 2 main reasons why we see tachyarrhythmias?

Answer 17

1. more rapid SA node depolarization - increased sympathetic tone
2. ectopic pacemaker - region of abnormal cardiac muscle that depolarizes spontaneously independent of the SA node in the atria/ventricles/SA node/AV node, or conduction pathway

Question 18

what is supraventricular tachycardia a generalized term for?

Answer 18

sinus, atrial, & junctional tachycardia

Question 19

what is sinus tachycardia? how does it look on ecg?

Answer 19

SA node depolarizes at an elevated rate often due to increased sympathetic tone such as excitement, exercise, & physiologic stress

normal ECG with a fast heart rate

Question 20

what defines atrial tachycardia?

Answer 20

multiple atrial premature contractiosn

Question 21

what is atrial tachycardia?

Answer 21

progression of atrial premature complexes

ectopic pacemaker in the atrium depolarizes independent of the SA node to produce the APC

singular APCs are random only conducted through the AV node if the node & ventricular myocardial cells are out of the refractory period

Question 22

what is atrial flutter?

Answer 22

very rapid atrial tachycardia where the AV node conducts some but not all impulses & the ventricular rate is protected by the slowed transmission of the action potential through the AV node

Question 23

what is atrial fibrillation?

Answer 23

continuous random atrial depolarization

Question 24

what is the mechanism of atrial fibrillation?

Answer 24

no effective coordinated atrial depolarization, so no atrial contraction

AV node protects the ventricles by slowed depolarization & only conducting some atrial action potentials

Question 25

what does atrial fibrillation look like on an ECG?

Answer 25

fibrillating baseline with no discernible p waves (called f waves instead)

QRS complexes are rapid & irregular based on when the beats get through the AV node, but the QRS complexes are still narrow & tall due to normal conduction through the AV pathway

Question 26

what is the most common pathologic arrhythmia in horses? what causes it?

Answer 26

atrial fibrillation

can be secondary to underlying heart disease causing atrial enlargement or lone afib that occurs without any underlying cardiac disease

Question 27

what is junctional tachycardia?

Answer 27

ectopic pacemaker in the AV node or the first part of the AV bundle

Question 28

what are ventricular premature complexes?

Answer 28

ectopic foci in the ventricle depolarizes without atrial depolarization, so it isn’t related to p waves

not conducted through the normal bundle branches & purkinje fibers, so the shape of the complexes are wide & bizarre

Question 29

what defines ventricular tachycardia?

Answer 29

more than 3 sequential VPCs

Question 30

T/F: VPCs may occur singly with a normal heart rate or in groups of 2 or more

Answer 30

true

Question 31

what does it mean if all VPCs look the same on an ECG? what if they all look different?

Answer 31

the same - likely from one focus

different appearance/polarity - suggestive of multiple ectopic foci

Question 32

what is another name for benign ventricular tachycardia?

Answer 32

accelerated idioventricular rhythm

Question 33

what are the 2 forms of vtach?

Answer 33

benign vtach & malignant vtach

Question 34

what does benign vtach look like on an ECG? what causes it? how is it treated?

Answer 34

rate is typically less than 200 bpm

causes - commonly occurs secondarily to systemic disease such as trauma, GDV, & endotoxemia

only treat if causing hemodynamic instability!!!

Question 35

what does malignant vtach look like on ECG? what animals do we see it in?

Answer 35

rate higher than 200 bpm

commonly seen in dogs with severe underlying cardiac disease & horses with myocarditis (DCM, arrhythmogenic right ventricular cardiomyopathy)

Question 36

what is the risk that malignant vtach poses?

Answer 36

predisposes an animal to develop ventricular fibrillation causing a fatal arrhythmia & often results in sudden death