PATHOLOGY - Equine Dysrhythmias Flashcards
What is the normal heart rate at rest for horses?
28 - 40bpm
What is the maximum heart rate for horses at maximum exercise?
220 - 240bpm
What are the four normal heart sounds in horses?
S4
S1
S2
S3
What causes S4 heart sounds in horses?
S4 heart sounds are a ‘le’ sound caused by atrial contraction moving blood into the ventricle
What causes S1 heart sounds in horses?
S1 heart sounds are a ‘lub’ sound caused by the closure of the atrioventricular (AV) valves
What causes S2 heart sounds in horses?
S2 heart sounds are a ‘dub’ sound caused by the closure of the aortic and pulmonary valves
What causes S3 heart sounds in horses?
S3 heart sounds are a ‘shh’ sound caused by ventricular filling
Identify the waves in this normal equine ECG
Why do horses have a biphasic P wave?
Horses have a biphasic P wave due to their large atria which results in slightly ashynchronous atrial depolarisation
What are the five steps to interpreting an ECG?
- Determine the heart rate
- Determine if the rhythm is regular or irregular
- Are there P waves present? Are there always P waves preceding QRS complexes and QRS complexes following P waves
- Determine the size of the QRS complex
- Determine if the P waves are positive in lead II
Which lead do you use to read equine ECGs?
Read equine ECGs at lead 1
Why do horses commonly present with non-pathological dysrhythmias?
Horses often present with non-pathological dysrhythmias due to their high vagal tone
What are the five most common non-pathological dysrhythmias in horses?
Atrial premature contractions
Second degree atrioventricular (AV) heart block
Sinus pause
Sinus arrhythmia
Ventricular premature contraction
What are atrial premature contractions?
Atrial premature contractions are where the electrical impulse that triggers the atrial contraction originates in the atrial myocardium rather than the usual sinoatrial node
What is the key characteristic of an atrial premature contraction on an equine ECG?
Normal heart beat that occurs earlier than the next expected heart beat
What are atrioventricular (AV) heart blocks?
Atrioventricular (AV) heart blocks is a term used to describe dysfunctional conduction of electrical impulses through the atrioventricular (AV) node
What is a second degree atrioventricular (AV) heart block?
A second degree atrioventricular (AV) block is an intermittent blockage in the electrical impulse as it travels through the atrioventricular (AV) node
What is the characteristic feature of a second degree atrioventricular (AV) block on an equine ECG?
Intermittent P waves that are not followed by a QRS complex
What is a mobitz type I second degree atrioventricular (AV) block?
A mobitz type I second degree atrioventricular (AV) block is there is progressive prolongation of the P-R interval prior to the atrioventricular block
What is a mobitz type II second degree atrioventricular (AV) block?
A mobitz type II second degree atrioventricular (AV) block is where there is a consistent P-R interval prior to the atrioventricular block
How do you determine if a second degree atrioventricular (AV) block is physiological or pathological in the horse?
Physiological second degree atrioventricular (AV) blocks in the horse are due to high vagal tone and this should disappear during exercise when the sympathetic tone is increased
What is an advanced second degree atrioventricular (AV) heart block?
An advanced second degree atrioventricular (AV) heart block is a pathological arrhythmia that occurs even during exercise
What is a third degree atrioventricular (AV) heart block?
Always pathological in horses
A third degree atrioventricular (AV) heart block is a complete blockage of the electrical impulse as it travels through the atrioventricular (AV) node
What are the characteristic features of a third degree atrioventricular heart block on an equine ECG?
P waves that are unrelated to the junctional and ventricular escape beats
Which diagnostic test can be done to determine if an atrioventricular (AV) block has a cardiac aetiology or is due to high vagal tone?
Atropine response test
What are three possible causes of an advanced second degree atrioventricular (AV) heart block and third degree atrioventricular (AV) heart block in horses?
Electrolyte imbalances
Digitalis toxicity
Atrioventricular (AV) nodal disease
What is digitalis toxicity?
Digitalis toxicity is digoxin toxicity which can occur is horses due to ingesting foxglove plants
How do you treat advanced second degree and third degree atrioventricular (AV) heart blocks in horses?
Correct any electrolyte imbalances
Supportive care for digitalis toxicity
Implant a pacemaker if there is an atrioventricular nodal disease
What is the prognosis for a third degree atrioventricular (AV) heart block?
Poor prognosis. The horse will likely collapse thus making them unsafe for riding
What is a sinus pause?
A sinus pause is a brief cessation of electrical impulse formation by the sinoatrial (SA) node
How do you determine if a sinus pause is physiological or pathological in the horse?
A horse with a physiological sinus pause should have a heart rate within the normal range (usually towards the slower end), and the sinus pause is due to high vagal tone so should disappear during exercise when the sympathetic tone is increased
What are ventricular premature contractions?
Ventricular premature contractions is where the electrical impulse originates in the ventricle, rather than the usual sinoatrial node - triggering the ventricles to contract prematurely
What are the characteristic features of ventricular premature contractions on an equine ECG?
- Wide and abnormal QRS complex morpholgy
- No P waves preceding the abnormal QRS complexes
- Often the abnormal QRS complexes are followed by an abnormal T wave (sometimes orientated in the opposite direction to normal)
- These abnormal QRS complexes will interrupt the normal heart rhythm
What are monomorphic premature ventricular contractions?
Monomorphic premature ventricular contractions have an identical morphology, indicating the electrical impulses are coming from one site within the ventricles
What are polymorphic premature ventricular contractions?
Polymorphic premature ventricular contractions have a varying morphology, indicating the electrical impulses are coming from more than one site within the ventricles
How do you differentiate between physiological and pathological premature ventricular contractions in horses?
If the premature ventricular contractions are frequent, polymorphic and occur during exercise, this suggests they are pathological
What can you use to monitor the frequency of premature ventricular contractions in horses?
24 hour holter ECG
What are seven possible underlying causes of pathological premature ventricular contractions in horses?
Myocardial inflammation
Myocardial degeneration
Myocardial necrosis
Myocardial fibrosis
Myocardial hypoxia
Endotoxaemia
Electrolyte imbalances
What investigative tests should you do for a horse presenting with pathological premature ventricular contractions?
Serum electrolyte levels
Look for markers of cardiac damage (cardiac troponin I)
Look for evidence of infection
How do you treat pathological premature ventricular contractions in horses?
Correct any electrolyte imbalances
Corticosteroids if there is evidence of an inflammatory aetiology
Prolonged rest
Antiarryhthmic drugs
What is the main indicator for the administration of antiarryhthmic drugs for premature ventricular contractions?
Frequent premature ventricular contractions with a heart rate over 100bpm
Will also be beneficial to consider referral and further investigation at this point
Which antiarrhythmic drug can be used to manage pathological premature ventricular contractions?
Phenytoin
What is atrial fibrillation?
Atrial fibrillation is where there are uncoordinated supraventricular ectopic electrical impulses. These electrical impulses bombarde the atrioventricular (AV) node, and since the AV node acts as a filter, only intermittent atrial electrical impulses will be conducted through the atrioventricular (AV) node, causing an irregular rhythm
What are the characteristic features of atrial fibrillation on an ECG?
No P waves
Fibillatory (F) waves
Narrow, normal QRS complexes with an irregular rhythm
Why are horses at an increased risk of developing atrial fibrillation?
Horses are at an increased risk of developing atrial fibrillation due to their high vagal tone and their large atrial size
How does high vagal tone increase the risk of atrial fibrillation?
Which cardiac diseases can increase the risk of atrial fibrillation in horses?
Atrial enlargment due to mitral or tricuspid valve regurgitation
What are the clinical signs of atrial fibrillation in horses?
No clinical signs at rest
Poor performance
When would you want to auscultate a horse to hear atrial fibrillation?
Auscultate a horse at rest to hear atrial fibillation. You are much more likely to miss atrial fibrillation if you auscultate just after the horse has exercised
What are the signs of atrial fibrillation on auscultation?
Irregulalrly irregular rhythm
No detectable S4 heart sound
How can atrial fibrillation cause poor performance in horses?
Atrial contraction contributes to cardiac output as atrial contraction pushes the remaining blood into the ventricles that didn’t enter during ventricular filling. With the loss of coordinated atrial electrical activity, the atria fail to contract resulting in inconsistent ventricular filling, resulting in a decreased stroke volume and cardiac output. Because ventricular contraction - while irregular - is normal in atrial fibrillation, cardiac output at rest is not markedly affected, however at exercise, the atrial contribution becomes more important as the demands on cardiac output approach maximum
What is cardioversion?
Cardioversion is a procedure used to convert atrial fibrillation into sinus rhythm
What are the contraindications for cardioversion?
Concurrent heart chamber enlargement
Chronic atrial fibrillation
If the horse is coping well with the atrial fibrillation
If the horse has had previous, unsuccessful cardioversion
Which antiarrhythmic drug is most commonly used for cardioversion?
Quinidine
How does quinidine achieve cardioversion?
Sequential doses of oral quindine are administered (usually 5-6 doses). Quinidine increases the refractory period of cells, slowing down the atrial electrical impulses. This allows the ventricles to respond to more of the atrial electrical impulses until eventually the electrical activity reaches sinus rhythm
What are the side affects of mild quinidine toxicity?
Mild depression
Muzzle oedema
Diarrhoea
What are some of the severe side affects of quinidine toxicity?
Ventricular tachyarrhythmias
Hypotension
Colic
Laminitis
Why is quinidine contraindicated in horses with concurrently enlarged heart chambers?
Quinidine can cause catastophic hypotension and ventricular tachydysrhythmias in horses with pathologically enlarged heart chambers
How can quinidine cause hypotension?
Quinidine is an α adrenoreceptor antagonist, resulting in vasodilation and hypotension
How can quinidine cause ventricular tachyarrhythmias?
Quinidine can cause ventricular tachyarrhythmias if the ventricles respond to every atrial electrical impulse which can result in ventricular tachycardia which can progress to ventricular fibrillation if the heart beats so fast that the oxygen demand exceeds oxygen supply
What can you do to monitor for signs of quinidine toxicity?
Monitor for changes in the ECG
How do you manage quinidine toxicity?
Stop quinidine administration
Administer mineral oil nasogastrically
Administer IV sodium bicarbonate
Antiarrhythmic drugs
Why would you want to administer nasogastric mineral oil to a horse with quinidine toxicity?
To reduce further absorption of the quinidine
Why would you want to administer sodium bicarbonate to a horse with quinidine toxicity?
Sodium bicarbonate will bind to free quinidine to reduce quinidine bioavailability
Which first line antiarrhythmic drug would you want to administer to patients with quindine toxicity and why?
Lidocaine infusion to manage the ventricular tacchyarrhythmias
When would a lidocaine infusion be indicated for treating ventricular tacchyarrhythmias?
A lidocaine infusion is indicated if the heart rate is above 100bpm
Which second line antiarrhythmic drug can be used in patients with quinidine toxicity?
Intravenous (IV) digoxin
Which antiarrhythmic should be administered if a horse undergoing quinidine cardioversion develops Torsades de Pointes?
Magnesium sulphate (MgSO4) bolus injections
What can be used to treat atrial fibrillation in horses with pathological heart chamber enlargement?
Transvenous electrical cardioversion
Which drug can be used following cardioversion to reduce the likelihood of reconversion to atrial fibrilliation?
Sotalol (beta-blocker)
What is the approximate cost of transvenous electrical cardioversion?
Approximately £3,000 to £5,000
What is the prognosis for atrial fibrillation?
The prognosis of atrial fibrillation is dependent on several factors. For horses that are used for high intensity exercise, the prognosis will be worse compared to leisure horses. If there is an underlying pathological cause, this will also worsen the prognosis. If the atrial fibrillation has been going on for a long time, this will also worsen the prognosis
What are the characteristic signs of ventricular tachycardia on an equine ECG?
- Tachycardia
- Wide QRS complexes with an abnormal morphology unrelated to any P waves (the P waves are often hidden in the complexes)
What are some of the possible underlying causes of ventricular tachycardia in horses?
Electrolyte imbalances
Myocardial inflammation
Myocardial hypoxia
Secondary to endotoxaemia
What investigative tests should you do for a horse presenting with ventricular tachycardia?
Serum electrolyte levels
Look for markers of cardiac inflammation/damage
What can you use to treat ventricular tachycardia?
Correct any electrolyte imbalances
Corticosteroids if there is evidence of an inflammatory aetiology
Prolonged rest
Lidocaine infusions
