Causes + tx of cardiac arrhythmias

How can arrhythmias be classified?

• Brady/Tachy

• Paraxysmal (episodic)/ sustained

• Tachy can be supraventricular/ Ventricular

  

How can arrhythmias be monitored?

Standard ECG (2-3 minutes)

Holter monitor (24h-7days)

Implantable device (years)

What is the general cardiac cause of weakness and syncope?

• Compromised cardiac output

What can cause a compromised cardiac output leading to weakness and syncope?

• Congenital heart disease

• DCM

• HCM

• Cardiac tamponade

• Neoplasias

• Arryhthmias

  • tachy —> too fast not enough time for ventricles to fill for adequate SV

  • brady —> too slow not enough O₂

What are the two types of action potentials in the heart? What is their main difference?

• Pacemaker (nodal) cells action potentials

  • Primary due to influx of Ca

• Cardiac myocytes

  • Primarily due to influx of Na

Recall the vaughan williams classification of antiarrhythmic drugs?

Class I- Na channel blocker (1° myocardial cells) —> slow upstroke of depolarisation

Class II- Beta blocker (1° nodal cells) —> slow depolarisation

Class III- K channel blocker (1° myocardial) —> lengthen AP duration

Class IV- Ca channel blocker (1° nodal cells) —> slow depolarisation

Misc: Digoxin —> slow conduction across AVN

'Never Buy Kennel Club Dogs'

What is this ecg showing?

Supraventricular tachycardia

• from above ventricles

• Fast conduction because only above ventricle affected so narrow complexes

• regular rhythm (except AFib)

Narrow QRS

What is arrhythmias are classed as SVTs?

• sinus tachy

• focal atrial tachy

• multi-focal atrial tachy

• orthodromic atrioventricular reciprocating tachy

• AFib

• atrial flutter

• focal junctional tachy

• junctional ectopic tachy

• atrioventricular node reentry tachy

When could a SVT look wide not narrow?

If bundle branches are damaged?

What is the most common pathological arrhythmia in animals?

AFib

What is AFib associated with?

• Most commonly atrial stretch from underlying heart disease (MMVD, DCM)

  • Can occur in abscence of heart disease if mass of atria is large in larger animals (lone AFib)

Explain what happens during AFib?

‘Fibrillating’ atria with chaotic depolarisation

• No coordinated atrial depolarisation so absence of p wave

  • May see fibrillating baseline

• Timing of depolarisation reaching AVN is irregular (ventricular rate irreg)

  • Irregular R-R interval

irregularly irregular

What is this ecg showing?

AFib

How is AFib treated?

• Manage underlying condition

• don’t try to convert to sinus rhythm (likely AFib with reoccur)

• AIM control the ventricular response rate to AFib (decr HR) —> <140bpm, but ideally <125bpm

• Drugs that slow conduction through AV node

  • Class 4s- Ca channel blocker —> Diltiazem

  • Digoxin

Better heart rate control with combination Digoxin + Diltiazem

What are beta blocker typically not use to tx AFib?

would worsen already poor systolic function

(NEVER use in uncontrolled CHF)

What effect does diltiazem have?

• slow conduction across the AVN

• Negative inotrope (clinically insignificant)

What effect does digoxin have?

• Negative chronotrope (slows heart rate)

• ‘Vagomimetic’ —> enhances vagal tone and slows conduction across the AV node

• Weak +ve inotrope

  • Blocks Na+/K+ ATPase → Ca2+ accumulation (inprove contract.)

What should you be careful of when using Digoxin?

• Digoxin toxicity

  • Start with cautious dose and check plasma levels

  • GI signs

What is Orthodromic atrioventricular re-entrant tachycardia (OAVRT)?

Wolf-Parkinson White syndrome

• Abnormal connection between atria and ventricles (band of muscle)

• Conduction from the atria to the ventricles can occur across the pathway

• ACCESSORY PATHWAY MEDIATED SVT

How does OAVRT present on ECG?

• Short P-R in sinus rhythm

  • slurred QRS upstroke = delta wave (ventricular pre excitation)

• Rapid narrow QRS complex tachycardia

What is the typical presentation of OAVRT?

• Labrador retrievers + GR

• Typically young (1-2 yr/o)

• Syncope and exercise intolerance

• Very fast HR

• Can present in heart failure bc tachycardia induce cardiomyopathy

How can OAVRT be treated?

Radiocatheter ablation (burn accessory pathway)

How is SVT treated in an emergency situation?

• Vagal manoeuvre

  • Ocular compression

  • Carotid massage

  • Gag reflex

• IV esmolol (beta blocker)

• IV diltiazem (IV verapamil if diltiazem unavailable)

• Oral diltiazem if IV preparation not available

• Oral sotalol can also be effective

In first opinion try oral options whilst evaluating- probably will work

What does a ventricular arrhythmia look like on an ecg?

Typically, wide QRS complexes

• not conducted along the specialised conduction fibres therefore slow conduction

ventricular ectopics = VECs + VPCs

What is the difference between ventricular escape complex and ventricular premature complexes?

Look the same on ECG

• Ventricular escape complexes occur after a long pause (‘escape’ death)

• Ventricular premature complexes (VPCs) occur before you’d expect them (premature)

What does this ECG show?

ventricular premature complex (triplet + couplet)

What are the different classifications of monomorphic VTACH?

• Non-sustained <30s

• Sustained >30s

• Incessant >12h

(wide QRS all look the same)

How does a VTACH present on ecg?

• wide QRS

• R-R interval constant (regular rhythm on ausc)

• ± P wave (not associated with the QRS or hidden within QRS)

What are the two most likely mechanisms of monomorphic VTACH?

1. Re-entry - a small circuit of electricity racing around and around an area of unhappy myocardium

2. Triggered activity – an unhappy island of ventricular myocardial cells discharging inappropriately at a fast rate

How does polymorphic VTACH differ from monomorphic?

More malignant cause – more than one region of unhappy ventricular myocardium

What signs of malignancy can be seen on ECG along side polymorphic ventricular tachy?

• Faster rates (the faster the worse)

• R on T phenomenon – two or more consecutive complexes so close together as to start their R wave on the preceding complex T wave

What can ventricular tachy trigger?

VFib

(tx of ventricular tachy aim to prevent VFib)

What does this ECG show?

VFib

What is the difference between accelerated idioventricular rhythm and ventricular tachy?

• Same morphology on ECG

• Only difference is rate

  • >180-200= VTACH

  • <180 = AIVR

• AIVR typically not haemodynamically compromised

  • Typically don't require treatment

  • Most often extra cardiac cause (eg: severe GI disease, splenic lesion, pancreatitis)

What does this ECG show?

accelerated idioventricular rhythm

(wide QRS, HR<180, BP normal)

List some differentials of causes of ventricular arrhythmias

• Underlying cardiac disease – CHF, cardiomyopathies, myocarditis, myocardial hypoxia, ischaemia etc.

• Catecholamines – stress, pain

• Acidosis

• Hypokalaemia

• Abdominal disease – GDV, splenic lesions (not just haemangiosarcoma), sepsis, pancreatitis etc.

• Thoracic trauma (myocardial trauma)

• Drug induced - eg: digoxin toxicity

How do you decide whether to treat ventricular arrhythmias?

Only treat if necessary so:

• Treat if haemodynamically significant (eg: hypotensive, collapsed etc.)

• Treat if very fast / close coupling intervals / ‘R on T’

• Treat if multifocal

What do you do before treating ventricular arrhythmias?

Ensure no electrolyte or acid-base disturbances

How do you treat ventricular arrhythmias in an emergency case?

• Lidocaine (Class IB)

  • Na⁺ channel blocker —> act on myocardial cells

  • bolus then cardiac resynchronization therapy

• Esmolol (Class II- not used much)

  • short acting beta blocker

• Amiodarone CRI (risk of anaphylaxis)

  • class 3 —> K channel blocker

Less common tx

• Magnesium chloride

• If completely refractory, electrical DC cardioversion (requires GA and a defibrillator)

What treatment do you use for a non emergency ventricular arrhythmia?

• Sotalol - Class 3

• Mexiletine - Class 1B

• Amiodarone- Class 3 (similar to digoxin)

  • Risks of hepatotoxicity and thyroid dysfunction (long term)

• Atenolol - Class 2

  • risk if underlying heart dz

List some of the causes of bradyarrythmias

• Sinus bradycardia (normal)

• Sinus arrest (no P wave)

• Sinoventricular rhythm (hyperkalaemia – think urinary obstruction in cats)

• AV Block

What tends to be the underlying cause of atrial standstill

• Hyperkalaemia (K+)

  • Urinary obstruction

  • renal failure (oliguric / anuric patients)

  • Hypoadrenocorticism (Addison’s disease)

How does atrial standstill occur and how does it present on ecg?

• Sinoatrial node is driving the rhythm as normal BUT there is no atrial depolarisation

  • Atria more sensitive to K+ than ventricles

• Absent P waves, spikey T waves, mild prolongation of the QRS complex

(atrial myocardium are more sensitive to high K+ than ventricular myocardium)

What is this ECG showing?

atrial standstill

What is this showing?

Atrial standstill with ventricular escape complexes

(no P wave, wide QRS + slow HR)

What are some of the causes of sinus arrest?

• Sick sinus syndrome (’Brady – Tachy syndrome – think West Highland White Terriers)

• Sinus node dysfunction

• Drugs (eg: amiodarone, digoxin, sotalol)

• Electrolyte changes

• Atrial myocarditis

(a sinus pause lasting for more than 3 times the normal P-P interval)

What does this ECG show?

sinus arrest

What are the classes of AV block?

First degree

• P: QRS normal but prolonged P-R interval

  • P for every QRS but elongated gap between P + QRS

Second degree

• P:QRS normal but some non conducted P wave

  • Mobitz 1- no pattern and tends to physiological

  • Mobitz 2 - pattern and more pathological

Third degree

• No association between P waves and QRS complexes, often fast sinus rate (freq p waves, wide QRS (ventricular escapes), HR often 20-40 in dogs

What does this ECG show?

Ist degree AV block

• P:QRS = 1:1 (just like normal sinus rhythm)

• Prolonged P-R interval (>130ms in a dog)

What are some of the causes and treatment of first degree AVB?

• High vagal tone

• Drug effect

• Breed morphology related (brachy)

• Tx- typically incidental with no treatment required

What do these ECGs show?

2nd degree AV block —> non-conducted P waves

top = Mobitz 1 (Wenkebach phenomenon)

bottom = Mobitz 2

How do the 2 types of second degree AV block present on ECG?

Mobitz 1

• time for the signal to pass through the heart gets longer and longer with each beat until a QRS is completely dropped (P-QRS get progressively longer until QRS dropped)

Mobitz 2

• not a prolongation of the P-R interval before the complete AV block/P-wave with missing QRS complex

• often a pattern

  • 2:1 = 2 P waves for every 1 QRS

  • 4:3 = 4 P waves for every 3 QRS

What does this ECG show?

third degree AVB

• No association between P waves and QRS complexes (electricity cannot get through the AV node)

• fast sinus rate —> frequent P wave

• QRS wide (VECs)

• HR abt 20-30bpm (dog) 100-130 bpm (cats)

How do you treat third degree AVB in dogs vs cats?

• Dogs- emergency, refer for pacemaker ASAP

• Cats- cats HR often 100-130 better at compensating so often incidental finding

How do you treat bradyarrhythmia?

• Treat underlying disorder

• Test response to anticholinergics (atropine) if vagally mediated

  • If HR increases suggests underlying disease causing high vagal tone

• If life threatining refer for pacemaker if poss or trial a B agonist (terbutaline)

• Oral meds

List some possible oral meds used in the treatment of bradyarryhtmias

• B agonist- terbutaline

• Anti-cholinergics – propantheline

• Beta sympathomimetics – terbutaline

• Xanthin derivatives – theophylline

• Pimobendan – more blood per heartbeat

Explain what happens in atropine response test

• Before= Sinus arrhythmia, sinus arrest, and ventricular escapes

• 30 minutes post atropine = If sinus tachycardia – proves bradycardia was vagally mediated

What are the two approaches to implant a pacemaker?

• Transvenous- via right jugular pacing lead passed through the right atrium, across the tricuspid valve into the right ventricle

• Epicardial- transabdominal pacing lead attached to epicardium (usually in cats bc size)

(gold standard tx for 3rd degree AVB or atrial standstill)