Lecture 10: Arrhythmias & Antiarrhythmic Therapy

What are the effects of arrhythmias?

• syncope

• weakness

• CHF decompensation

• reduced blood flow → reduced CO, reduced cerebral and coronary blood flow

• reduced renal perfusion

Describe the cardiac electrical system.

1. SA node: pacemaker in the RA → initiates electrical impulse

2. Atrial conduction pathways: impulse spreads through atrial muscle causing contraction

3. AV node: located at junction between atria and ventricles → delays impulse to allow ventricles to fill

4. Bundle of his: conducts impulse from AV node to ventricles

5. R&L bundle branches: carry impulses down the interventricular septum

6. Purkinje fibers: throughout ventricular walls → trigger ventricular contraction

What are the 4 classes of antiarrhythmic drugs according to Vaughan-Williams classification?

• Class I – Sodium (Na+) channel blockers

• Class II – Beta (β) blockers

• Class III –Potassium (K+) channel blockers

• Class IV – Calcium (Ca+2) channel blockers

What are the three classes of Na channel blockers (class I antiarrhythmics)?

Class 1A: Quinidine, Procainamide

• Blocks fast Na channels → Slows conduction moderately

• Works on normal and depolarized cells; used for atrial & ventricular arrhythmias

Class IB: Lidocaine, Mexiletine

• Block fast Na⁺ channels, especially in depolarized / ischemic cells

• Minimal effect on normal tissue, safer for ventricles, no effect on atrial tissue

Class IC: Flecainide, Encainide

• Strong Na⁺ block → marked conduction slowing, high efficacy but high risk if heart is diseased

• Used for supraventricular arrhythmias &life-threatening ventricular arrhythmias

What drug treats supraventricular and ventricular arrhythmias and is used to treat atrial fibrillation in horses?

quinidine

What are the possible side effects of quinidine?

• Proarrhythmic effect, Torsades de pointes

• Horses - Colic, hypersensitivity, urticaria, sweating

• Enhanced AV conduction – rapid heart rate

• GI signs

• Hypotension

What are the possible side effects of procainamide?

• Proarrhythmic

• GI signs

• Haircoat color change, Lupus-like changes

When is lidocaine used to treat arrhythmias?

• used for serious ventricular arrhythmias

• IV bolus followed by infusion (CRI) is best

• correct hypokalemia for optimal efficiency

When should a lidocaine dose be reduced?

with severe liver disease or CHF

What are the side effects of lidocaine?

• GI upset

• seizures

• cats and horses sensitive: neuro signs

What are the commonly used class II antiarrhythmics (beta-blockers)?

• Propranolol

• atenolol

• metoprolol

• carvedilol

• esmolol

What are the clinical uses for beta-blockers?

• Ventricular arrhythmias

• Supraventricular arrhythmias

• Congestive heart failure

• Systemic hypertension

• Hyperthyroidism

• Intoxication (ie. Chocolate, aminophylline)

What are the side effects of beta blockers?

• Negative chronotropism: Bradycardia, AV block

• Negative inotropism: CHF, weakness

• Hypotension

• Bronchospasm

• Neurologic

What are the commonly used class III antiarrhythmics and what do they do?

K Channel Blockers → Prolong action potential

Both can be used for ventricular and supraventricular arrhythmias

• Amiodarone (*also crosses over to all other 3 classes of antiarrhythmics)

• Sotalol (*also beta blocker)

What drug is commonly used in boxer cardiomyopathy?

sotalol

Amiodarone has a higher side effect profile than sotalol. What are the side effects?

• Hepatotoxicity

• GI upset, thyroid abnormalities

What are the commonly used class IV antiarrhythmics (Ca channel blockers)?

• Diltiazem

• Verapamil

• Amlodipine

What are the clinical uses of class IV antiarrhythmics (Ca channel blockers)?

• Supraventricular tachycardia

• Atrial fibrillation

• Systemic hypertension

• Hypertrophic cardiomyopathy

• Coronary vasodilation

What are the possible side effects of class IV antiarrhythmics (Ca channel blockers)?

• Negative chronotropism → Sinus bradycardia, AV block

• Negative inotropism → May precipitate CHF

• Excessive vasodilation → Hypotension

What is the mechanism of digoxin?

Positive inotrope (improves cardiac contractility)

• Blocks Na+ K+ ATPase pump

Negative chronotrope (vagomimetic effect)

• Slows heart rate

• Slows AV conduction

• Antiarrhythmic effect

What are the indications for digoxin?

• A fib or SVTs (slows ventricular response rate)

• DCM with reduced systolic function (improves contractility)

• Low output CHF when other inotropes not tolerated or efficacious

What cardiac drug has a very narrow therapeutic index and can quickly become toxic, causing anorexia, lethargy, depression, V/D, and cardiac arrhythmias, so it should be monitored closely?

digoxin

What is the treatment for this rhythm?

no treatment - normal sinus rhythm

What is this rhythm and what is the treatment?

sinus arrhythmia → incidental finding, no treatment

What is a sinus arrhythmia?

• Cyclic slowing and speeding of the sinus rate, most commonly associated with respiration

• Normal finding in dogs (rare to see in cats)

• Often accompanied by change in P wave configuration (wandering pacemaker) with taller, spiked P waves on inspiration and flatter on expiration

• May be associated with wandering pacemaker → Variable P wave height or appearance

What is the treatment for sinus tachycardia and sinus bradycardia?

sinus tachycardia: treat underlying cause - shock, hypotension, fever, CHF, pain, hyperthyroidism, anxiety

sinus bradycardia: treatment only needed based upon hemodynamic status - atropine, glycopyrrolate, epinephrine, pacing

What is this rhythm and how do you treat it?

atrial premature contractions = Early abnormal (ectopic) beats that arise from the atria

no treatment, but may warrant investigation as to cause

What causes atrial premature contractions?

• Structural heart diseases which cause enlargement and stretch of atria

• Congenital accessory conduction pathway

• Extra-cardiac illness: elyte abnormalities, anemia, thyroid dz, trauma

What is this rhythm and what is the treatment?

SVT

treatment if symptomatic: cardioversion, cardiac ablation, medical therapy

What rhythm caused by conditions that cause atrial enlargement, metabolic disease, autonomic tone variations, or congenital conduction system abnormalities can result in dyspnea, exercise intolerance, weakness, and syncope?

SVT

What drugs can be used for SVTs?

IV treatment for rapid response

• Diltiazem (Ca channel blocker) - IV for acute use, PO for chronic use

• Esmolol (beta blocker)

Oral treatment

• Calcium Channel blockers - Diltiazem

• Beta blockers

• K Channel blockers, Sotalol, Amiodarone

• Digoxin

What are the possible side effects of dilitazem?

• Negative chronotropism - Bradycardia, AV block

• Negative inotropism - May precipitate CHF

• Hypotension

• CV collapse

What are vagal maneuvers?

Parasympathetic innervation → at sinus node, atrial tissue & AV node

Carotid sinus compression:

• Compress carotid sinus against vertebral body

  • Sinus tachycardia – transient slowing of HR

  • Sustained SVT – Abrupt termination of the abnormal rhythm

• Ocular pressure

• Ice water – Dive reflex (not used in dogs or cats)

• Precordial thump – Not advised

What causes atrial fibrillation?

atrial dilation secondary to heart disease

What is this rhythm and what is the treatment?

atrial fibrillation

How is atrial fibrillation treated if there is heart disease involved?

• goal is to slow HR

• achieved by slowing AV conduction → digoxin, Ca channel blocker (diltiazem), beta-blocker if no CHF, less frequently amiodarone

How is atrial fibrillation treated if there is no heart disease involved?

• goal is converions to sinus rhythm

• drugs that sometimes work: quinidine, diltiazem, sotalol, amiodarone

• Cardioversion - ECG synchronized

What is this rhythm and what is the treatment?

VPCs

Treatment:

• None for isolated VPCs, may want to investigate cause

• Couplets or triplets with fast coupling rate or “R on T” phenomenon typically treated

• Holter monitor recommended to determine frequency and malignancy of the arrhythmia

What is this treatment and what is the treatment?

ventricular tachycardia

• treated with urgent anti-arrhythmic drugs (ex. lidocaine)

• key to identify underlying cause of rhythm

Does this rhythm need treatment?

Probably not – Slow rate (25 mm/sec strip, HR 130 , not scary)

What is accelerated idioventricular rhythm caused by?

Caused by enhanced automaticity of ventricular pacemaker cells vs reentry mechanisms like in V-tach:

• During reperfusion after injury

• Certain drugs/toxins

• Increased vagal tone

• Normal variant

• Electrolyte disturbances

• Structural heart disease

What is this rhythm and how is it treated?

v-fib

• medical emergency

• only prompt defibrillation and CPR can restore a viable rhythm and save life

What are the indications for defibrillation?

• Ventricular fibrillation

• Ventricular flutter

• Torsades de pointes

• Pulseless ventricular tachycardia

What is sick sinus sindrome?

• Signalment: middle-aged to older Mini Schnauzers, Westies, American Cocker Spaniels

• Cause: idiopathic fibrosis of the sinus node

• Periods of sinus arrest are often followed by a paroxysm of supraventricular tachycardia

• Can cause syncopy or sudden death

How is sick sinus syndrome treated?

• Treatment: Atropine response test to determine medical management (positive chronotrope) vs. pacemaker

• Medical management (if atropine responsive)

  • Propantheline, Isopropamide

  • Theophylline

  • Terbutaline (higher risk of side effects)

• Cardiac pacing

What can cause atrial standstill?

• can result from severe hyperkalemia (IV fluids - NaCl, calcium gluconate, sodium bicarb, dextrose and insulin)

• can also result from an atrial muscular dystrophy, hypothermia, terminal event

What is atrial standstill?

• Atrial activity is absent

• There are no P waves anywhere on the ECG strip

• If result of severe hyperkalemia → peaked T wave and bradycardia

What are the types of AV blocks?

Abnormal conduction from atria to ventricles

1st degree: slowed conduction

2nd degree: sometimes doesn’t conduct

Type 1: gradual prolongation before block (Wenckebach)

Type 2: consistent PR interval before block, High-grade – multiple consecutive non-conducted P waves

3rd degree: never conducts (complete heart block)

How are the different types of AV blocks treated?

1st degree: No treatment if asymptomatic

2nd degree:

Type 1: Atropine if symptomatic

Type 2: Usually needs pacemaker (+/- atropine short term if effective)

3rd degree: Pacemaker

What are the therapeutic options for bradycardias?

Medical therapy:

• Sympathomimetics: Theophylline, Terbutaline, Isoproterenol (Hypotension, ventricular tachycardia, ventricular fibrillation)

• Vagolytics (Parasympathetic antagonists): Atropine, Glycopyrrolate, Probanthine, isopropamide (Orally acting)

Temporary transvenous pacing lead

Permanent pacemaker implantation

What is the most commonly used type of cardiac pacing?

permanent cardiac pacing - transvenous