Myocardial Disease in Large Animals: Arrhythmias

Question 1

What are the primary causes of myocarditis in large animals?

Answer 1

Viral
Bacterial
Parasitic
Nutritional
Cardiomyopathy
Neoplasia
Immune-mediated
Toxic
Idiopathic

Question 2

What are the viral causes of primary myocarditis in the horse?

Answer 2

EHV
EVA
Equine influenza

African horse sickness
EIA

Question 3

What are the bacterial causes of primary myocarditis in horses?

Answer 3

Staphylococcus aureus
Clostridium chauveoi
Mycobacterium spp.
Strep equi equi
Actinobacillus spp.
Rhodococcus equi

Question 4

What are the parasitic causes of primary myocarditis in horses?

Answer 4

Strongyles
Toxoplasma
Borella burgdorferi (Lyme’s)
Sarcocysta

Question 5

What are the nutritional causes of primary myocarditis in the horse?

Answer 5

White muscle disease

- Ruminants, and less commonly horses grazing on selenium deficient pastures causing oxidative injury to muscle

Question 6

What are the clinical signs of the cardiac form of white muscle disease?

Answer 6

Neonates
Acute
Severe debilitation or sudden death
Resp signs, arrhythmias

Question 7

What are the clinical signs of the skeletal form of white muscle disease?

Answer 7

Older animals
Weakness, stiffness
Signs precipitated by stress

Question 8

How is white muscle disease diagnosed and treated? How does it appear on PM?

Answer 8

Diagnosis:
Selenium concentration in blood

Treatment:
Vit E/Selenium IM injection

PM:
Pale streaky muscles with fibrosis

Question 9

What damaging conditions does endotoxaemia result in?

Answer 9

Hypoxia
Electrolyte imbalances
Acidosis

Question 10

What is acid-base and electrolyte status important for?

Answer 10

Premature ventricular contraction

Question 11

What nutritional parameters for you need to take in a blood test for myocardial disease?

Answer 11

Selenium

Glutathione peroxidase

Question 12

What does an increase in cardiac troponin 1 indicate?

Answer 12

Damage to myocardium

Question 13

What are the 2 cardiac isoenzymes which are released into the circulation with myocardial cell death?

Answer 13

Creatine kinase

Lactate dehydrogenase

Question 14

What is different about the purkinje system in LA compared to SAs?

Answer 14

Branches from endocaridum to epicardium - only endo in SA

Question 15

What is the conventional ECG configuration?

Answer 15

Base-apex lead

Question 16

Which degree of AV block is always pathological in LAs?

Answer 16

Third degree

Question 17

What is 2nd degree AV block?

Answer 17

P waves without associated QRST

Question 18

In 2nd degree AV block one at 60bpm what happens?

Answer 18

Stops as vagal tone is reduced through excitement and exercise

Question 19

What does atrial fibrillation look like on ECG?

Answer 19

No P waves
F waves
Random depolarisation of AV node
Irregularly irregular R-R
Rate is normal

Question 20

What is the significance of AF in horses?

Answer 20

Atrial contraction contributes approx 15% of ventricular filling; loss (in AF) only affects cardiac output at exercise

In horses with no other cardiac disease, atrial fibrillation only causes signs of exercise intolerance if the animal is engaged in vigorous exercise
- racehorses, eventers, some hunters

In many types of horse it can be an incidental finding
- breeding stock, hacks, some show jumpers

In contrast to SA < equine AF is frequently spontaneous and not due to cardiac disease

AF may be paroxysmal but persistent

Question 21

What are the clinical signs of AF in horses?

Answer 21

Depends on use

• No clinical signs
• Exercise intolerance/poor performance

Variable pulse quality

EIPH

Question 22

What is paroxysmal atrial fibrillation?

Answer 22

Frequently transient i.e. resolves spontaneously

Minute-hours duration

Exercise-induced, mainly immediately post strenuous exercise

Horses and cattle with GI disease

Question 23

What does initiation of AF require?

Answer 23

Large atrial mass
Slow SA node rate
Variable refractory periods

Question 24

How does high vagal tone result in AF?

Answer 24

Slow SA node rate - longer to repolarise and contract

Question 25

When is treatment of AF required?

Answer 25

Exercise intolerance in competition horses

Heart failure

Question 26

What is the main treatment of horses with AF?

Answer 26

Quinidine sulphate

Question 27

What 2 additional drugs could you give when treating with quinidine for AF?

Answer 27

Digoxin - beforehand to prevent vagolytic effects

Bicarbonate

Question 28

How does quinidine sulphate treat AF? Side effects?

Answer 28

Prolongs the effective refractory period

• class 1a antidysrhythmic
• slows Na+ fast channels
• promotes electrical homogeneity in the atria

Question 29

What are the side effects of quinidine sulphate?

Answer 29

• Vagolytic (ventricular tachycardia)
• Alpha-adrenergic antagonist (hypotension)
• Negative inotrope (decreases cardiac output)
• Gastrointestinal ulceration

Question 30

Why might an APC not be conducted?

Answer 30

AV node may be refractory when the P wave arrives

Question 31

Out of APC and VPC which is more likely to causes an irregular rhythm and why?

Answer 31

VPC - ventricular rate is altered

Question 32

What does normal QRS with abnormal P wave suggest?Abnormal P wave with no QRS?

Answer 32

• APC

- Non-conducted APC

Question 33

What does abnormal QRS with no P wave suggest?

Answer 33

VPC

Question 34

What is the drug of first choice used to treat horses with ventricular arrhythmias?

Answer 34

Procainamide

Question 35

How would you treat APCs in horses?

Answer 35

Investigate underlying disease
Corticosteroids (small areas of fibrosis)
Rest

Question 36

How would you treat tachycardia in horses?

Answer 36

Digoxin - slow conduction

Bicarbonate to increase protein binding and reduce plasma concentration