Myocardial Disease in Large Animals: Arrhythmias Flashcards
What are the primary causes of myocarditis in large animals?
Viral Bacterial Parasitic Nutritional Cardiomyopathy Neoplasia Immune-mediated Toxic Idiopathic
What are the viral causes of primary myocarditis in the horse?
EHV
EVA
Equine influenza
African horse sickness
EIA
What are the bacterial causes of primary myocarditis in horses?
Staphylococcus aureus Clostridium chauveoi Mycobacterium spp. Strep equi equi Actinobacillus spp. Rhodococcus equi
What are the parasitic causes of primary myocarditis in horses?
Strongyles
Toxoplasma
Borella burgdorferi (Lyme’s)
Sarcocysta
What are the nutritional causes of primary myocarditis in the horse?
White muscle disease
- Ruminants, and less commonly horses grazing on selenium deficient pastures causing oxidative injury to muscle
What are the clinical signs of the cardiac form of white muscle disease?
Neonates
Acute
Severe debilitation or sudden death
Resp signs, arrhythmias
What are the clinical signs of the skeletal form of white muscle disease?
Older animals
Weakness, stiffness
Signs precipitated by stress
How is white muscle disease diagnosed and treated? How does it appear on PM?
Diagnosis:
Selenium concentration in blood
Treatment:
Vit E/Selenium IM injection
PM:
Pale streaky muscles with fibrosis
What damaging conditions does endotoxaemia result in?
Hypoxia
Electrolyte imbalances
Acidosis
What is acid-base and electrolyte status important for?
Premature ventricular contraction
What nutritional parameters for you need to take in a blood test for myocardial disease?
Selenium
Glutathione peroxidase
What does an increase in cardiac troponin 1 indicate?
Damage to myocardium
What are the 2 cardiac isoenzymes which are released into the circulation with myocardial cell death?
Creatine kinase
Lactate dehydrogenase
What is different about the purkinje system in LA compared to SAs?
Branches from endocaridum to epicardium - only endo in SA
What is the conventional ECG configuration?
Base-apex lead
Which degree of AV block is always pathological in LAs?
Third degree
What is 2nd degree AV block?
P waves without associated QRST
In 2nd degree AV block one at 60bpm what happens?
Stops as vagal tone is reduced through excitement and exercise
What does atrial fibrillation look like on ECG?
No P waves F waves Random depolarisation of AV node Irregularly irregular R-R Rate is normal
What is the significance of AF in horses?
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
What are the clinical signs of AF in horses?
Depends on use
- No clinical signs
- Exercise intolerance/poor performance
Variable pulse quality
EIPH
What is paroxysmal atrial fibrillation?
Frequently transient i.e. resolves spontaneously
Minute-hours duration
Exercise-induced, mainly immediately post strenuous exercise
Horses and cattle with GI disease
What does initiation of AF require?
Large atrial mass
Slow SA node rate
Variable refractory periods
How does high vagal tone result in AF?
Slow SA node rate - longer to repolarise and contract
When is treatment of AF required?
Exercise intolerance in competition horses
Heart failure
What is the main treatment of horses with AF?
Quinidine sulphate
What 2 additional drugs could you give when treating with quinidine for AF?
Digoxin - beforehand to prevent vagolytic effects
Bicarbonate
How does quinidine sulphate treat AF? Side effects?
Prolongs the effective refractory period
- class 1a antidysrhythmic
- slows Na+ fast channels
- promotes electrical homogeneity in the atria
What are the side effects of quinidine sulphate?
- Vagolytic (ventricular tachycardia)
- Alpha-adrenergic antagonist (hypotension)
- Negative inotrope (decreases cardiac output)
- Gastrointestinal ulceration
Why might an APC not be conducted?
AV node may be refractory when the P wave arrives
Out of APC and VPC which is more likely to causes an irregular rhythm and why?
VPC - ventricular rate is altered
What does normal QRS with abnormal P wave suggest?Abnormal P wave with no QRS?
- APC
- Non-conducted APC
What does abnormal QRS with no P wave suggest?
VPC
What is the drug of first choice used to treat horses with ventricular arrhythmias?
Procainamide
How would you treat APCs in horses?
Investigate underlying disease
Corticosteroids (small areas of fibrosis)
Rest
How would you treat tachycardia in horses?
Digoxin - slow conduction
Bicarbonate to increase protein binding and reduce plasma concentration
