Causes of and Treatments of Cardiac Arrhythmias Flashcards
How are arrythmias subdivided?
Into tachyarrhythmias and bradyarrhythmia’s
How are tachyarrhythmias further divided?
Into supraventricular (originating from the AV junction or above) and ventricular (originating from below the AV junction).
What must be carried out before treating arrythmias?
Record an ECG - need to know the rhythm disturbance
List some cardiac causes of Episodic Weakness / Syncope
- Congenital heart disease (esp. Aortic stenosis, rev. PDA)
- Dilated cardiomyopathy – poor stroke volume
- Hypertrophic cardiomyopathy (cats)
- Cardiac tamponade
- Cardiac neoplasia
- Cardiac arrhythmia: Tachyarrhythmia, Bradyarrhythmia
Compare the appearance of ventricular vs supraventricular tachyarrhythmias on an ECG
Supraventricular = Narrow QRS complexes
Ventricular =
- Wide and bizarre QRS complexes
- T-wave is in the opposite direction of the QRS complexes
Describe the 4 classes of antiarrhythmic drugs
Class 1 = Sodium channel blockers
Class 2 = Beta-blockers
Class 3 = Potassium channel blockers
Class 4 = Calcium channel blockers
How do sodium channel blockers work as anti-arrhythmic drugs, name an example?
Slow upstroke (phase 0) of action potential
Lidocaine
How do beta blockers work as anti-arrhythmic drugs, name an example?
Slows depolarisation
Atenolol
How do potassium channel blockers work as anti-arrhythmic drugs, name an example?
Delays repolarisation, lengthens action potential duration
Sotalol
How do calcium channel blockers work as anti-arrhythmic drugs, name an example?
Act on nodal tissue, SAN or AVN - slow conduction
Diltiazem or Verapamil
Describe the two types of supraventricular tachycardia
Can be paroxysmal (starts and stops abruptly) or
sustained
Compare the ECG of supraventricular tachycardia to atrial fibrillation
Atrial fibrillation is an irregular R-R interval
SVT has a regular R-R interval
Describe the emergency treatment options for a supraventricular tachycardia
- Vagal manoeuvre: physical enhancement of vagal tone at the AV node
- IV Esmolol (beta-blocker)
- IV Diltiazem (calcium channel blocker) - oral diltiazem if no IV preparation
- Oral sotalol also possible (potassium channel blocker)
What is an accessory pathway?
A piece of myocardium that bypasses the AV node, may be one mechanism underlying SVT
Abnormal connection between the atria and ventricles
Name an example of SVT associated with an accessory pathway
Wolff-Parkinson White syndrome
Labradors and Retrievers appear to be predisposed.
Describe the appearance of an ECG in a dog with an accessory pathway/Wolff-Parkinson White syndrome
- Conduction to ventricles across the pathway – short PR interval
- Slurred QRS upstroke called delta wave
What is atrial fibrilation?
A form of supraventricular tachycardia
Requires different treatment
When does atrial fibrillation occur?
Atrial fibrillation can only occur if there is a critical atrial mass (so it is more likely to occur in giant breed dogs, horses and humans).
It occurs in other breeds of dog or even cats if there is marked atrial stretch - this means there is severe structural heart disease
What is the goal of treating atrial fibrilation?
Not possible to convert AF back to sinus rhythm
- Goal is to control the ventricular response to atrial fibrillation (slow the heart rate), by slowing conduction across the AV node.
List 3 drugs that can be used to treat atrial fibrillation
Digoxin
Ca channel blockers - Diltiazem
Beta blockers - Atenolol
Why does treatment have to be monitored when using digoxin?
Potential for toxicity
If a patient is being treated with Digoxin but ventricular rate is too fast which drug can you add to treatment?
Diltiazem - Ca channel blocker
Better heart rate control with combination of Digoxin and Diltiazem than either drug alone.
Can you add beta-blockers to digoxin treatment for atrial fibrillation?
- Should never add beta blockers to uncontrolled congestive heart failure
- If dog has poor systolic function (e.g. DCM), it will usually fail to tolerate beta blocker
What are the effects of digoxin on the heart?
Positive chronotrope
Weak positive inotrope
Describe the appearance of multiform (polymorphic) ventricular tachycardia on an ECG
- Bizarre, wide QRS complexes with an oppositely directed T wave
- Initial focus, not fast (180 bpm)
- Multiform
- 2nd focus faster rate (shows R on T)
When is treatment for a ventricular tachycardia actually indicated?
- The arrhythmia is haemodynamically significant (check signs of cardiac output).
- It is very fast (> 200 bpm)
- It is multifocal (i.e. different morphology of the ventricular ectopics through the trace)
- Close coupling with preceding complex (R on T phenomenon).
Emergency treatment of ventricular tachycardia usually starts with which drug?
Lidocaine
- can be given as a bolus followed by a constant rate infusion
What should you do if lidocaine isn’t working as an emergency treatment for ventricular arrythmias?
Check and correct for any hypokalaemia, other electrolyte or acid base disturbances
List some possible underlying causes of ventricular ectopics
- Underlying cardiac disease; CHF; cardiomyopathies
- Catecholamines (e.g. stress, pain etc.)
- Acidosis
- Hypokalaemia
- Abdominal disease: GDV, splenic lesions (not just splenic haemangiosarcoma), sepsis, perforated gastric ulcer, pancreatitis etc.
- Thoracic trauma (myocardial contusions)
- Drug induced (e.g. digoxin toxicity)
Which drug is most commonly used as an oral antiarrhythmic?
Sotalol
List the different Bradyarrhythmias
Sinus bradycardia
Sinus arrest
Sinoventricular rhythm (hyperkalaemia)
1st degree AV Block
2nd degree AV Block
3rd degree AV Block
What is the cause of atrial standstill?
Hyperkalaemia
List some causes of hyperkalaemia
Addison’s disease
Urethral obstruction
Renal failure
Describe the appearance of an ECG with atrial standstill
Sino-ventricular rhythm (SAN drives the rhythm) (so variable R-R as with sinus arrhythmia)
- Absent P waves, spikey T waves, mild prolongation of the QRS complex (atrial myocardium most susceptible to increased K+).
How is atrial standstill treated?
Correct the electrolyte balance
What is the most common cause of AV block?
Disease of the AV node, such as fibrosis, or myocarditis
Describe the appearance of the ECG in 1st degree AV block
P-R interval prolonged above normal but P:QRS = 1:1
2nd degree AV block is divided into?
Morbitz Type 1
Morbitz Type 2
Describe Morbitz type 1 appearance on an ECG
There is gradual lengthening or variation in the PR interval before a non-conducted P wave
Describe Morbitz type 2 appearance on an ECG
The PR interval does not prolong and it remains constant before the AVN fails to conduct a P wave
- It can also occur in a repeating pattern e.g. only one out of 3 P waves are conducted to result in a QRS complex (3:1 conduction)
Describe the appearance of 3rd degree AV block on an ECG
- No relationship between P waves and QRS complexes.
- QRS complexes arise from the inherent pacemaker activity of the ventricles, so they are ventricular escapes, usually occurring at a rate of 30 – 60 bpm in dogs
- Atria and ventricles are functioning independently due to absence of conduction through the AVN
What is sick sinus syndrome?
A generalised conduction disease seen in Westies, cairns, miniature Schnauzers and certain other breeds
- Sinus node dysfunction
- Sometimes failure of any escape complex may result in long periods of sinus arrest.
- Periods of arrest over 8 -10 seconds can result in syncope
What is an atropine response test?
Used in diagnosing bradyarrhythmia’s
Excludes vagally mediated bradyarrhythmia’s
How are life-threatening bradyarrhythmia’s treated?
Try atropine/ glycopyrrolate IV in the first instance.
Can also try terbutaline (Beta-2 agonist) IV
What is the only method of treating bradyarrhythmias that is likely to significantly reduce clinical signs and improve exercise tolerance?
Pacemaker implantation
