Equine arrhythmias Flashcards
Why are arrhythmias in the horse a problem?
hemodynamic: effect on BP, tissue perfusion and CO
saftey: can induce severe/fatal rhytm disturbance, syncope or sudden death
What are causes for arrhythmias in horses?
isolated electrical disorders
OR secondary to:
- structural heart dz
- metab and endoc factors
- systemic infl.
- hypotension, haemorrhage, anaemia, ischaemia
- autonomic influences
- toxicosis
- drugs
What are the goals when investigating arrhythmias?
confirm whether myocardial disease/dysfunction is present
identify specific or underlying causes
Whate are the diagnostic techniques used to investigate arrhythmias?
Clin hx, exam
clin. path.
echocardiography
ECG!!
What clinical pathology can indicate arrhythmias?
Hepatic and renal parameters of haematology and cbc
acid-base and electrolytes
Cardiac tropinin I and T
cardiac isoenzymes (creatine kinase, lactate dehydrogenase)
selenium and glutathione peroxidase
viral serology
blood bact culture (endocarditis)
What is the use of echocardiography in arrhythmia cases?
identifies global myocardial disfunction
rules out concurrent valvular dz or congenital heart dz
What does ECG in horses indicate?
cardiac rate and rhythm only
QRS size and duration doesn’t reflect the shape and size of ventricular myocardium like in small animals
How is the purkinje fibre system different in large animals?
the system is extensive, branches from endocardium to epicardium
depolarising wave is done by purkinje = less cell-cell spread through myocardium
= small wave fronts which are less influenced by myocardial mass than in SA
What does the horse ECG look like?
P is sometimes a double humo
No Q
T wave is vert variable
What is a telemetric ECG?
ECG monitor is attached to horse with surface contact electrodes
ECG is streamed to a distant monitor, instantaneous
can be used during exercise, intensive care, foetal monitoring
What are the physiologic cardiac arrhythmias in horses?
sinus arrhythmia
sinus bradycardia
sinus arrest/block
AV block (1st or 2nd degree)
What are the pathologic cardiac arrhythmias in horses?
atrial arrhythmia
ventricular arrhythmia
conduction disturbances (advanced 2nd degree AV block, 3rd degree AV block)
What is a first degree atrioventricular block?
physiological
delayed conduction through the AV node
causes a prolonged PR interval
slow, slightly variable heart rate
What is a second degree atrioventricular block?
physiological
intermittent block of conduction through the AV node (high vagal tone)
slow HR with pauses at regular intervals
Isolated, normally timed P waves on ECG without a following QRST
4th heart sound audible
What does a first degree AV block look like on an ECG?
longer PR intervals
What does a second degree AV block look like on an ECG?
P waves without QRST at normal time and normal shape
frequency of block is 1in3 and upwards
What is a third degree atrioventricular block?
pathological
complete block of conduction (dissociation)
due to pathology at the AV node (intoxication, uroperitoneum, electrode disturbance)
very slow ventricular rate, syncope and weakness
What is the treatment for 3rd degree AV block in horses?
- vagolytic drugs in emergency, ex: atropine
- corticosteroids? to reduce infl. around the AV node
- pacemaker
What is an atrial premature depolarisation?
premature electrical cycle
occasional APD normal at rest, frequency increases during recovery phase after strenuous exercise but frequent APD during exercise is not normal
How do we investigate and treat APD?
rule out underlying cause
most idiopathic: often managed with coriticosteroids and rest
rarely require anti-arrhythmic therapy as ventricular rate is not increased
What is atrial fibrillation?
most common pathological arrhythmia
supraventrical tachyarrhythmia: uncoordinated atrial depolarisation due to multiple chaotic re-entry waves
What are the types of atrial fibrillation?
Paroxysmal: spontaneously converts to sinus rhythm (24-48h)
Sustained persistent: requires treatment to convert to sinus rhythm
Sustained permanent: does not respond to tx
What are the causes for AF?
Cardiac dz
idiopathic: “lone AF” no underlying dz
What does initiation of AF require?
Large atrial mass (normal horses, atrial enlargement)
Slow SA node rate (high vagal tone)
Variable refractory periods (effect of high vagal tone, also in myocardial dz, electrolyte/pH change, allows re-entry to occur)
What is the clinical significance of AF in horses?
frequently spontaneous and not due to cardiac dz (lone AF doesn’t cause heart failure but can be secondary to heart failure)
atrial contraction contributes 10-15% of ventricular filling, loss only affects CO at high intensity exercise
incidental finding in many types of horses
What are the clinical signs of AF?
Depends on use of horse: none if low level exercise, exercise intolerance/poor performance in high level exercise
irregularly irregular rhythm, variable number with variable pauses, no fourth heart sound
variable pulse quality
variable intensity of heart sounds
exercise-induced pulmonary haemorrhage - epistaxis
What do we investigating when deciding if we need to treat AF?
establish if there is an underlying heart disease (usually spontaneous or an effect not cause of heart dz)
check for signs of heart failure
What are the signs of heart failure in horses?
resting tachycardia (>60bpm)
valve regurg. (murmur) especailly mitral/tricuspid
venous distention
oedema
When do we treat AF and what do we treat with?
Incidental finding = no tx
Competition horse w/ exercise intolerance = quinidine, transvenous electrical cardioversion
Heart failure = palliative furosemide, digoxin, ACE inhibitors
What is the use of Quinidine sulphate?
Prolongs the effective refractory period
Its a Class ! antiarrhythmic: slows the Na+ fast channels
Promotes the electrical homogeneity in the atria
What are some of the (unwanted) effects of Quinidine Sulphate?
Vagolytic (ventricular tachycardia)
Alpha-adrenergic antagonist (hypotension)
Negative inotrope (decreases CO)
Other arrhythmias (possibly fatal)
GI ulceration (diarrhoea, colic)
Nasal oedema
Laminitis
Neuro signs, ataxia
Penile protrusion
What is transvenous electroconversion?
intra-cardiac electrodes placed
shock delivered across atria
95% success rate
32% recurrence rate
What determines the prognosis of a horse with AF?
underlying cardiac dz (don’t tx AF in heart failure cases, they will relapse)
duration of AF prior to tx (less than 3 months 15% recurrence, over 3months 60% recurrence with more side effects)
What is Ventricular premature depolarisations?
Pathologic
1-3 abnormal consecutive beats
can be cause of poor performance if occurs during exercise (heart is emptying faster)
What is the difference between VPD and ventricular tachycardia?
VPD: 1-3 abnormal consecutive heart beats
Ventricular tach: more than 3 consecutive VPDs
What do we need to investigate and how to we treat VPD?
Establish is arrhythmia occurs at rest only, exercise only or both
if idiopathic: corticosteroids and rest
V. arrhythmias more likely to progress into fatal
What are the signs and risks of ventricular tachycardia?
horses with systemic inflammation, electrolyte, acid/base disturbances, hypoxia
risk that rhythm will destabilise to ventricular fibrillation (death)
When should we consider anti-arrhythmic therapy in horses with Ventricular Tachycardia?
clinical signs of low CO
Ventricular rate is greater than 100bpm
polymorphic complexes
R on T phenomenon
What is a polymorphic complex on an ECG?
When there is 2 different types of ectopic complexes, P wave dissociated from QRS
What is an R on T phenomenon on an ECG?
R waves occurring immediately after the preceding T wave
What are the possible treatments for ventricular arrhythmias?
Magnesium sulphate (physiological calcium channel blocker)
Lidocaine
Propranolol (rarely effective in horses)
Procainamide
Quinidine gluconate
What should we know about lidocaine as a tx for ventricular arrhythmias?
causes seizures at high doses
drug of choice under anaesthesia
What are the causes of Ventricular arrhythmias?
isolated electrical disorders
OR
Secondary to
- structural heart dz
- metab and endocrine factors
- systemic infl.
- hypotension, haemorrhage, anaemia, ischaemia
- autonomic influences
- toxicosis
- drugs
What is the pathogenesis causes of primary myocardial dz and dysfunction?
infectious causes (viral, bact. parasite)
nutritional (vit E, selenium deficiency)
cardiomyopathy
neoplasia
immune mediated
toxic
idiopathic
What are the pathogenesis causes of secondary myocardial disease and dysfunction?
systemic inflammation (SIRS)
electrolyte disturbances
acid-base disturbances
hypoxia
catecholamine-induced
vagally-induced
What are examples of viral causes of primary myocarditis in horses?
equine influenza
equine herpes virus
equine viral arteritis
african horse sickness
equine infectious anaemia
What are examples of bacterial causes of primary myicarditis in horses?
s. aureus
clostridium chauvoei
mycobacterium
strep equi equi
actinobacillus
rhodococcus equi
borrelia burgdorferi (lyme dz)
What are examples of parasitic causes of primary myocarditis in horses?
strongyles
onchocerca
toxoplasma
cysticerca
sarcocysta
What is nutritional myodegeneration?
white muscle dz
selenium deficiency from pasture being deficient
oxidative injury to muscle
2 forms: cardiac and skeletal
What is the difference between cardiac and skeletal white muscle dz?
cardiac: neonates, acute/per-acute, severe debilitation or sudden death, resp. signs, arrhythmias
skeletal: slightly older animals, weakness, stiffness and debilitation, signs precipitated by stress
How do we diagnose white muscle dz?
whole blood selenium concentrations
glutathione peroxidase concentrations
What is the treatment for nutritional myodegeneration?
vit E and selenium supplementation, IM injection
What is the post mortem appearance of nutritional myodegeneration?
pale streaky muscles
degeneration and fibrosis of muscles
What are the causes for hypoxia also causing secondary myocardial dz?
blood loss, haemoabdomen
upper airway obstruction
red maple leaf toxicity
What are electrolyte abnormalities causing secondary myocardial dz?
potassium
calcium
magnesium
What are the most important equine arrhythmias?
second degree Av block
A fib
V Tach
When are anti-arrhythmic agents most likely to be used?
for some ventricular arrhythmias and atrial fibrillation
