Antidysrhythmic drugs

Question 1

List the 4 stages of myocardial depolarisation.

Answer 1

0- Fast depolarisation, Na+ channels open, allowing an inward current
1- Na+ channels begin to close
2- Opening of Ca2+ channels, allows sustained contraction
3- Outward K+ movement
4- Slow depolarisation, due to Ca+ and Na+.

Question 2

Which aspect of the ECG demonstrates ventricular conduction?

Answer 2

QT

Question 3

What is used to classify dysrhythmias?

Answer 3

Site and rate

Question 4

Compare types of atrial/supraventricular tachydysrhythmias.

Answer 4

Atrial fibrillation - persistent, irregular
Supraventricular tachycardia - rapid, regular (AV node or higher)
Paroxymal supraventricular tachycardia - intermittent

Question 5

List drugs used to control ventricular rate.

Answer 5

Digoxin, Ca2+ channel blocker, beta-blocker

Question 6

What is used to control sinus rhythm.

Answer 6

Direct current cardioversion

Question 7

Compare types of ventricular dysrhythmias

Answer 7

Ventricular ectopics - premature contractions
Ventricular tachycardia - rapid, regular
Ventricular fibrillation - irregular

Question 8

What are the two major mechanisms of dysrhythmias?

Answer 8

Abnormal pulse generation and abnormal impulse conduction/repolarisation.

Question 9

List mechanisms of abnormal impulse generation.

Answer 9

Altered normal automaticity

Ectopic aytomaticity

Question 10

List mechanisms of abnormal impulse conduction/depolarisation.

Answer 10

Conduction block
Re-entry circuits
After-depolarisations

Question 11

What are the different types of conduction block?

Answer 11

1st AV block - prolonged PR interval
2nd AV block - impulses not conducted, no QRS
3rd AV block - no conduction

Question 12

What are re-entry circuits?

Answer 12

Retrograde conduction re-exiting tissue that has already passed through the refractory period

Question 13

What is the caused of QT prolongation in after-depolarisation?

Answer 13

Impaired K+ efflux.

Question 14

When are antidysrhythmic drugs used?

Answer 14

Bradydysrhythmics: acutely
Tachydysrhythmics: only for life-threatening dysrhythmias or for moderate/severe symptoms

Question 15

What drugs are used for bradycardia?

Answer 15

Acute - atropine, adrenaline, isoprenaline

Chronic - Stop bradycardic medication, pacemaker

Question 16

List the classes of antidysrhythmics.

Answer 16

Class I - Na+ channel blockers, block conduction, phases 0, 4
Class II - beta blockers
Class III - block cardiac K+ channels and other actions, phase 3
Class IV - Ca2_ channel blockers, particularly in nodal tissue

Question 17

Describe class I antiarrhythmics.

Answer 17

Local anaesthetic properties

Block fast Na+ current at phase 0, slow cardiac conduction, depress automaticity.

Question 18

When are class I antuarrythmics used?

Answer 18

Used for ventricular and supraventricular dysrhythmias, often I.V.
Can be pro-dysrhythmic

Question 19

What gives class I antiarhythmics some selectivity?

Answer 19

Use dependent block of Na+ channels

Question 20

Lis the subclasses of class I antiarrhythmics

Answer 20

Class I A - prolongs AP
Class I B - shortens AP
Class I C - does not change AP duration

Question 21

Describe class II antidysrhythmics.

Answer 21

Reduce effects of Adr, NA and SNS (i.e. catecholamine toxicity).
Decrease sinus node automaticity and affect atrio-ventricular communication.

Question 22

When are class II antidysrhythmics used?

Answer 22

AF (exercise, mental stress), post-MI

Question 23

Describe class III antidysrhythmics.

Answer 23

Prolong AP by blocking K+ efflux, prolong refractory period (QT interval)
Can be pro-dysrhythmic as increase ventricular conduction (QT interval)

Question 24

Give examples of class III antidysrhythmics.

Answer 24

Sotalol - beta blocker with class III activity
Amiodarone - serious ADRS - thyroid, lungs, liver, deposits in eyes and skin)

Question 25

Describe Class IV antidysrhythmics.

Answer 25

Block slow Ca2+ influx responsible for conduction in nodal tissues. Suppress automaticity.
ADRs: headache, flushing, hypotension, peripheral oedema.

Question 26

When is verapamil used?

Answer 26

oral for AF and to prevent recurrence of paroxysmal supraventricular tachycardia. Care when combining with beta-blockers.

Question 27

List the unclassified antidysrhythmic drugs.

Answer 27

Digoxin

Adenosine (I.V.)

Question 28

What is the mechanism of action of adenosine in dysrhythmia?

Answer 28

Promptly converts supraventricular tachycardia to sinus rhythm
As well as working on adenosine receptors, stimulates pathways pathways similar to ACh absent in ventricles, hyperpolarises conduction tissue.

Question 29

What is the effect of digoxin in dysrhythmia?

Answer 29

Blocks atrio-ventricular conduction, slows ventricular rate in AF (may cause AV block)
Central vagal stimulation

At high doses, causes depolarisation and increased abnormal automaticity and ventricular dysrhythmia