Chapter 48 - ventricular tachyarrhythmias

Question 1

Firing rate of Purkinje fibres?

Answer 1

30-40 bpm in dogs

60-130 bpm in cats

Question 2

List three mechanisms of ventricular tachyarrhythmias

Answer 2

reentry
enhanced automaticity
triggered activity

Question 3

expected rates of idioventricular rhythm, AIVR and VT?

Answer 3

idioventricular = 30-40 (dogs), 60-130 (cats)
AIVR = between the two
VT = >150-180 (dogs), >220 (cats)

Question 4

difference between sustained & non sustained VT? significance?

Answer 4

> 30s, <30s

1st usually clinically insignificant

Question 5

define incessant VT and VT storm

Answer 5

recurrent episodes of sustained VT w/in 24h

Question 6

ECG features of VT

Answer 6

wide bizarre QRS complexes, >0.06s in dogs, >0.04s in cats, followed by wide T wave in opposite direction

Question 7

3 most reliable features of VT (cf SVT w/ aberrant ventricular conduction)

Answer 7

AV dissociation
fusion beats
capture beats

Question 8

why is AV dissociation not a perfectly sensitive feature?

Answer 8

if there is apparent association w/ independent atrial activity or retrograde ventricular conduction to the atrium –> signs of association don’t rule out VT

Question 9

why do fusion beats and capture beats occur?

Answer 9

two pacemakers competing (overdrive suppression leads to inhibition of slower foci by faster foci)

Question 10

if unable to differentiate between VT and SVT, what should you use to treat?

Answer 10

treat for VT (lidocaine) as safer - calcium channel blockers, B blockers etc for SVT will be ineffective for VT and worsen hypotension dt vasodilation/negative inotropic effects

Question 11

four electrolyte imbalances that can contribute to VT

Answer 11

hypokalemia (increases phase 4 depolarisation & prolongs action potential, risk for digoxin tox)
hypomagnesemia (needed for Na-K-ATP pump to maintain K+ conc)
hypocalcemia
hypercalcemia

Question 12

6 drugs that can prolong QT segment, predisposing to VT

Answer 12

procainamide, sotalol, domperidone, cisapride, chlorpromazine, erythromycin

Question 13

5 noncardiac causes of VT

Answer 13

hypoxia
electrolyte disturbances
acid-base disturbances
sympathetic stimulation
drugs

Question 14

10 cardiac causes of VT

Answer 14

cardiac tumors (+/- tamponade)
myocarditis
endocarditis
ischemia
DCM (esp Dobies)
ARVC
inherited ventricular arrhythmia of GSDs
severe subaortic stenosis
pulmonic stenosis
cats: HCM, concentric hypertrophy 2ndary to hypertension, hyperthyroidism

Question 15

expected progression of DCM in dobies

Answer 15

occult stage w/ echo signs of LV dysfxn (30% risk of sudden death) can last 2-4y
overt w/ CHF (30-50% risk of sudden death)

Question 16

most common origin of ventricular octopus in Dobies with DCM?

Answer 16

left ventricle - leads to right bundle branch block morphology

Question 17

prevalence of arrhythmias in DCM?

Answer 17

21% all breeds
16% Newfies
92% Dobies

Question 18

Indicators to treat VT?

Answer 18

symptomatic
>180-200 (hemodynamic compromise)
polymorphic
R-on-T alone may not be enough by itself

Question 19

Why not just treat anyway?

Answer 19

No evidence it will prevent sudden death

May precipitate it (all antiarrhythmics are proarrhythmic)

Question 20

First line treatment for VT?

Answer 20

Lidocaine (class IB)
Dogs: 2mg/kg q10-15min up to 4, CRI 25-80mcg/kg/min (beta blockers preferred in cats)
Can follow w/ oral mexiletine (IB) + atenolol (II)

Question 21

If lidocaine doesn’t work?

Answer 21

Procainamide (class IA)
10-15mg/kg IV over 1-2min, 25-50mcg/kg/min CRI, can follow with orals

Question 22

When to use B blockers? name 2

Answer 22

if sympathetically driven VT esp pheochromocytoma, thyrotoxic disease in cats - careful with ventricular dysfunction, can cause cardiovascular collapse (negative inotropy)
Esmolol, propranolol

Question 23

class III drug of choice?

Answer 23

sotalol 1-3mg/kg q12h PO

Question 24

common side effect of amiodarone? class?

Answer 24

anaphylaxis-like reactions (urticaria, facial edema)
Class III (also Ia, II, IV)

Question 25

The role of magnesium therapy in VT remains to be defined. T/F?

Answer 25

T

Question 26

Other potential treatments?

Answer 26

sedation/anaesthesia (if high sympathetic output)
rapid pacing (overdrive suppression GSDs w/ inherited ventricular arrhythmias)
synchronized electrical cardioversion/defib

Question 27

suggested minimum ECG monitoring period after starting treatment for VT?

Answer 27

min 24h

Question 28

ECG features of VT

Answer 28

1) wide QRS complex
2) AV dissociation
3) fusion beats
4) capture beats

Question 29

Most common non-cardiac causes of VT

Answer 29

1) hypoxemia,
2) electrolyte imbalance (hypokalemia, hypomagnesiumemia, hypocalcemia, hypercalcemia)
3) acid-base disorders
4) drugs (increase adrenergic tone)

Question 30

Most common cardiac disease associated with VT

Answer 30

1) ARVC in Boxers

2) DCM in Dorbie

Question 31

Other cardiac disease associated with VT (aside from ARVC and DCM)

Answer 31

cardiac tumor, myocarditis, endocarditis, ischemia

cat: idiopathic HCM, concentric hypertrophy secondary to hyperT4 or systemic hypertension

Question 32

T/F: anti-arrhythmic drugs do NOT prevent sudden death

Answer 32

True

Question 33

T/F: When the origin of a wide QRS tachycardiac cannot be determined (SVT vs VT), must be managed as if it were VT

Answer 33

True, because drugs used to stop SVT or to slow the ventricular response rate to rapid atrial impulses (i.e., calcium channel blockers and β-blockers) do not interrupt VT and worsen hypotension with their vasodilatory or negative inotropic effects.

Question 34

Rate of Purkinje fiber’s working as pacemaker?

Answer 34

Dog (30-40 bpm), Cat (60-130 bpm)

Question 35

Three most reliable diagnostics criteria of VT

Answer 35

1) AV dissociation 2) capture beat 3) fusion beat

Question 36

T/F: In ARVC, ventricular ectopies typically have a left bundle branch block morphology, indicating their right-sided origin

Answer 36

True

Question 37

Causes of slower non-sustained VT and AIVR

Answer 37

GDV, HBC, metabolic imbalances

Question 38

ECG characteristics of VT indicators of risk for sudden death and influence decision of treatment

Answer 38

1) polymorphic VT, continuously changing QRS
2) sustained VT with rates greater than 180-200 bpm
3) R on T- increased risk for VT and sudden death from V-fib

Question 39

Maximum dose of lidocaine recommendation?

Answer 39

8 mg/kg/hr

Question 40

What can be used for VTs that do not respond to lidocaine.

Answer 40

Pracanamide

Question 41

Drug for long-term management of VT, main anti-arrhythmic drug?

Answer 41

Sotalol, (especially in ARVC Boxer), many dogs respond successfully within a few hours of oral administration

Question 42

What is the common adverse reaction of Sotalol?

Answer 42

Anaphylaxis-like reaction (urticaria, facial edema)