ECG Part 2 - Treating Cardiac Arrhythmias

Question 1

what are the two basic mechanisms that cause cardiac arrhythmias?

Answer 1

1. impulse formation disturbance - ectopic pacemaker
2. conduction disturbance - reentry phenomenon

Question 2

what is an ectopic pacemaker?

Answer 2

excitable group of cells anywhere in the heart that causes spontaneous depolarization of cardiac muscle independent of the sinoatrial node

can be within the sinoatrial node, atrioventricular node, AV conduction pathway, or atria/ventricles

Question 3

where do ectopic pacemakers usually occur?

Answer 3

typically occur in regions of cardiac muscle damage, ischemia, or excessive stretch (atrial enlargement causing atrial fibrillation)

Question 4

what causes re-entry phenomenon?

Answer 4

occurs when an electrical impulse is propagated around a region of damaged myocardium resulting in the passage of an action potential around this area - circus movement

causes a self-sustaining loop of action potential propagation resulting in a tachyarrhythmia

Question 5

what two properties are required for re-entry to occur?

Answer 5

1. slowed conduction
2. shortened refractory period

Question 6

what is the sequence of events with a cardiac arrhythmia?

Answer 6

1. normal action potential arrives at the damaged region
2. unidirectional block means that the action potential can only enter & exit cells in this area in one direction
3. slowed conduction means that the surrounding myocardium may be past the refractory period when the action potential emerges
4. action potential is conducted to cardiac muscle before the next impulse from the sinoatrial/atrioventricular node
5. action potential arrives back at the damaged region & is again propagated slowly in one direction & emerges after the refractory period of the rest of the myocardium
6. action potential conducted to the rest of the myocardium before the impulse from the sinoatrial/atrioventricular node
7. cycles continues as a re-entry pathway causing contraction at a different rate that is faster than normal pacemaker depolarization

Question 7

what is the general mechanism of action & use of anti-arrhythmic agents?

Answer 7

alter the ion channel conduction or inhibit beta-adrenergic cardiac stimulation

treat tachyarrhythmias & prevention of formation/conduction of ectopic action potentials

Question 8

what is the class I of anti-arrhythmic drugs? what are some examples?

Answer 8

membrane stabilizing - selectively blocks some fast sodium channels & slows conduction velocity

quinidine, procainamide, lidocaine, & mexiletine

Question 9

what is quinidine used for?

Answer 9

class I anti-arrhythmic that also has anti-cholinergic effects on the atria

used to treat supraventricular & ventricular arrhythmias

used in horses to treat atrial fibrillation

Question 10

what is lidocaine used for?

Answer 10

class 1 anti-arrhythmic that is the treatment of choice for ventricular arrhythmias in ICU as a bolus or CRI (rapid onset of action & short half life)

Question 11

what is mexiletine?

Answer 11

oral lidocaine analog that can be combined with sotalol

Question 12

what are class II anti-arrhythmics? what are some examples?

Answer 12

beta-adrenergic receptor blockers, either non-selective or type 1 selective

negative inotropes/chronotropes - used for supraventricular & ventricular arrhythmias

atenolol, propanolol, carvedilol

Question 13

how is atenolol used?

Answer 13

selective beta 1 blocker that is most commonly used in vet med - use caution in patients with reduced systolic function, do not stop abruptly, titrate!!!

Question 14

how is propanolol used?

Answer 14

non-selective beta blocker

can cause bronchoconstriction/bronchospasm

Question 15

what are the class III anti-arrhythmics? what are some examples?

Answer 15

potassium blockade prolongs the action potential & refractory period

sotalol (also a beta-adrenergic blocker) & amiodarone

Question 16

what is the most common drug used in dogs to treat ventricular tachyarrhythmias?

Answer 16

sotalol

Question 17

how is amiodarone used?

Answer 17

multiple effects - class I, II, & III

used for supraventricular & ventricular arrhythmias

significant side effects limit the use

Question 18

what are the class IV anti-arrhythmics? what are some examples?

Answer 18

slow calcium channel blockade that affects cardiac & vascular smooth muscle

diltiazem & amlodipine

Question 19

how is diltiazem used?

Answer 19

primary activity is in the cardiac cells - slows the automatic depolarization of the sinoatrial node & conduction through the atrioventricular node

also a negative inotrope & prolongs the refractory period

treatment used for atrial fibrillation

Question 20

how is amlodipine used?

Answer 20

primary activity occurs in the vascular smooth muscle - no significant cardiac effects

used to treat hypertension