Arrhythmias in Large Animals

Question 1

Pathogenesis of myocardial disease

and dysfunction in large animals - primary

Answer 1

Infectious causes
Nutritional
cardiomyopathy
neoplasia
immune mediated
toxic
Idiopathic

Question 2

primary myocarditis - viral

Answer 2

equine influenza, equine herpes virus, equine viral arteritis
foot and mouth disease, african horse sickness, equine infectious anaemia

Question 3

Pathogenesis of myocardial disease

and dysfunction in large animals - secondary

Answer 3

most common 
Endotoxaemia
Electrolyte disturbances
Acid-base disturbances
Hypoxia
Catecholamine-induced
Vagally-induced

Question 4

primary myocarditis - bacterial

Answer 4

Staphylococcus aureus, Clostridium chauvoei, Mycobacterium spp.
Strep. equi subsp equi., Actinobacillus spp, Rhodococcus equi

Question 5

primary myocarditis - parasitic

Answer 5

strongyles, onchocerca, toxoplasma, cysticerca, sarcocysta, Borrelia burgdorferi (Lyme’s disease)

Question 6

Nutritional myodegeneration

Answer 6

(white muscle disease)
ruminants and, less commonly, horses grazing selenium deficient pastures
oxidative (ROS) injury to muscle

Question 7

nutritional myodegeneration - cardiac form

Answer 7

Neonates
Acute or peracute
Severe debilitation or sudden death
Respiratory signs, arrhythmias

Question 8

nutritional myodegeneration - skeletal muscle form

Answer 8

Slightly older animals
Weakness, stiffness and debilitation
Signs precipitated by stress

Question 9

nutritional myodegeneration - diagnosis

Answer 9

whole blood selenium concentrations

glutathione peroxidase concentrations

Question 10

nutritional myodegeneration - treatment

Answer 10

Vitamin E and selenium (i.m. injection)

Question 11

nutritional myodegeneration - post mortem

Answer 11

pale streaky muscles

degeneration and fibrosis of muscles

Question 12

cardiomyopathy

Answer 12

Horses - occurs sporadically, causes unknown

cattle - inherited, breed associated

Question 13

cardiac neoplasia

Answer 13

Cattle - right atrial lesions extending into the remainder of the heart and heart base area, adult form enzootic bovine leukosis
Horses - lymphoma and other neoplastic conditions occur sporadically

Question 14

inflammatory lesions + fibrosis

Answer 14

focal or generalised
aetiology unknown
immune-mediated?

Question 15

toxins

Answer 15

halothane (not used in the UK)

antibiotics

Question 16

secondary myocardial disease + dysfunction

Answer 16

Endotoxaemia
Hypoxia
Electrolytes– potassium, calcium, magnesium
Acidosis
Catecholamines

Question 17

clinical pathology

Answer 17

Haematology and general blood biochemistry (esp hepatic and renal)
Acid-base and electrolyte status
Selenium and glutathione peroxidase
Viral serology
Blood bacterial culture
Cardiac Troponin I
Cardiac isoenzymes

Question 18

cardiac isoenzymes

Answer 18

creatine kinase (CK) and lactate dehydrogenase (LDH)
released into the circulation with myocardial cell death
indicators of myocarditis/myocardial necrosis

Question 19

echocardiography

Answer 19

identifies global myocardial dysfunction – frequently unremarkable with focal myocardial disease (i.e. not all that useful)
rules out concurrent – valvular disease, congenital heart disease

Question 20

electrocardiography in large animals

Answer 20

cardiac rate and rhythm only
in large animals the Purkinje fibre system is extensive - branches from endocardium to epicardium
depolarising wave is conducted mainly via Purkinje fibres with much less cell-cell spread through myocardium than in carnivores
this produces small wavefronts which are less influenced by myocardial mass than in SA
therefore, in contrast to SA, the QRS size and duration does not accurately reflect the shape and size of the ventricular myocardium

Question 21

electrocardiography

Answer 21

Conventiona l– Base-apex lead -Positive electrode - left apex, Negative electrode - left base (in front of shoulder), Produces large P wave, clear QRST
Most horses resent the leads on their body less than on their limbs, Limb leads not used
Radiotelemetry
Ambulatory

Question 22

radiotelemetric ECG

Answer 22

ECG monitor is attached to horse with surface contact electrodes
ECG is sent to a distant monitor by
radio – instantaneous, exercise, intensive care

Question 23

ambulatory ECG

Answer 23

ECG is recorded digitally or onto magnetic tape on small monitor for up to 24 hours
horse can be left unattended, removing environmental influences
provides more accurate indication of the frequency of arrhythmias than standard ECG

Question 24

1st degree atrioventricular block

Answer 24

physiological
delayed conduction through the AV node
slow, slightly variable heart rate

Question 25

2nd degree atrioventricular block

Answer 25

physiological intermittent block of conduction through the AV node (high vagal tone) slow heart rate with pauses at regular intervals isolated fourth heart sound heard before block isolated, normally-timed P waves on ECG

Question 26

3rd degree atrioventricular block

Answer 26

always pathological complete block of conduction (dissociation) due to pathology at the AV node very slow ventricular rate, syncope, weakness

Question 27

2nd degree atrioventricular block - ECG appearance

Answer 27

``` P waves without associated QRST P waves are normal shape P waves occur at expected time may have more than 1 blocked P wave frequency of block varies (1 in 3 upwards) ```

Question 28

atrial fibrilation

Answer 28

commonest pathological dysrhythmia initiation of AF requires: •large atrial mass, normal horses, atrial enlargement (valve regurgitation, VSD) •slow SA node rate - high vagal tone •variable refractory periods - effect of high vagal tone, also in myocardial disease, electrolyte/pH change, allows ‘re-entry’ to occur

Question 29

atrial fibrilation - significance

Answer 29

atrial contraction contributes approx 15% of ventricular filling; loss only affects cardiac output at exercise horses with no other cardiac disease atrial fibrillation only causes signs of exercise intolerance if the animal is engaged in vigorous exercise in many types of horse it can be an incidental finding in contrast to SA, equine AF is frequently spontaneous and not due to cardiac disease AF may be paroxysmal but may be persistent

Question 30

atrial fibrilation - clinical signs

Answer 30

exercise intolerance/poor performance/reluctance to exercise irregularly-irregular cardiac rhythm – runs of beats (variable) with variable pauses, no fourth heart sound variable pulse quality variable intensity of heart sounds exercise-induced pulmonary haemorrhage

Question 31

paroxysmal atrial fibrilation

Answer 31

Atrial fibrillation may spontaneously resolve minutes-hours duration exercise-induced, mainly immediately post strenuous exercise horses and cattle with gastrointestinal disease

Question 32

atrial fibrillation - ECG appearance

Answer 32

``` Random electrical activity in atria no P wave f waves random depolarisation of AV node irregularly irregular R-R note: rate is normal ```

Question 33

atrial fibrillation - treatment

Answer 33

establish whether there is underlying heart disease check for signs of heart failure quinidine - if competing horse with exercise intolerance heart failure - palliative, furosemide, digoxin, ACE inhibitors

Question 34

quinidine sulphate

Answer 34

``` prolongs effective refractory period– class 1a antidysrhythmic, slows Na+ fast channels, promotes electrical homogeneity in the atria unwanted effects – vagolytic (ventricular tachycardia), alpha-adrenergic antagonist (hypotension), negative inotrope (decreases cardiac output), gastrointestinal ulceration ```

Question 35

quinidine treatment - protocol

Answer 35

treat with digoxin (5mg/450 kg) on day -2 to +1 do not move from box during treatment quinidine sulphate by stomach tube at 10g/450 kg every 2 hours until conversion occurs or stop if - 6 doses and no conversion, signs of toxicity appear

Question 36

Quinidine sulphate - side-effects

Answer 36

``` Hypotension Decreased cardiac output Supraventricular tachycardia Ventricular arrhythmias Potentially fatal arrhythmias ```

Question 37

quinidine - induced hypotension

Answer 37

due to alpha adrenergic antagonism – Keep the horse calm

Question 38

rapid supraventricular tachycardia

Answer 38

Due to vagolytic effect emergency treatment required if ventricular rate > 100 /min digoxin to slow conduction through the AV node bicarbonate to increase protein binding to reduce effective plasma concentration intravenous fluids to support blood pressure

Question 39

quinidine - induced ventricular arrythmias - treatment

Answer 39

``` Proarrhythmic effect Magnesium sulphate Propanolol Lignocaine NOT PROCAINAMIDE (same as quinidine) ```

Question 40

quinidine sulphate - extra-cardiac side effects

Answer 40

``` diarrhoea colic flatulence nasal oedema penile protrusion ataxia usually dose dependent GI effects are the main cause of treatment failure ```

Question 41

prognosis

Answer 41

Depends on – underlying cardiac disease + duration prior to treatment less than 3 months – recurrence rate 15% greater than three months – recurrence rate 60% higher prevalence of side-effects associated with prolonged duration of treatment

Question 42

normal QRS with abnormal P

Answer 42

APC (atrial premature complexes)

Question 43

abnormal P with no QRS

Answer 43

non-conducted APC

Question 44

abnormal QRS with no P

Answer 44

VPC (ventricular premature complexes)

Question 45

VPC with constant QRS waveform

Answer 45

uniform

Question 46

VPC with variable QRS waveform

Answer 46

multiform

Question 47

< 4 in succession

Answer 47

Isolated APC or VPC

Question 48

> 4 in succession

Answer 48

``` Supraventricular tachycardia (SVT) Ventricular tachycardia (VT) ```

Question 49

APC's

Answer 49

Rarely need specific anti-arrhythmic therapy as ventricular rate not incr Investigate + treat underlying cause Idiopathic - corticosteroids + rest are used for an inflammatory pathogenesis

Question 50

supraventricular tachycardia

Answer 50

rare except with quinidine toxicity

Question 51

Ventricular premature complexes (VPC) and VT

Answer 51

Investigate and treat underlying cause idiopathic - corticosteroids + rest - anti-infl removes undelying cause Ventricular arrhythmias more likely to progress into fatal arrhythmias Establish if the arrhythmia occurs - At rest only + not during exercise, At rest + during exercise or During exercise only

Question 52

ventricular tachycardia

Answer 52

Assess likelihood that the rhythm will destabilise to ventricular fibrillation Consider anti-arrhythmic therapy if – clinical signs of low cardiac output, Ventricular rate > 100, Polymorphic, R-on-t phenomenon

Question 53

Anti-arrhythmics used to treat ventricular arrhythmias

Answer 53

Procainamide – Drug of first choice in conscious horses Quinidine gluconate – USA only, not UK Lignocaine - Seizures, drug of first choice under anaesthesia Propanolol – Rarely effective in horses Magnesium sulphate (physiological calcium channel blocker) – Readily available + inexpensive but variable efficacy