Anti-arrhythmias

Question 1

Phase 0

Answer 1

• Rapid depolarization
• Fast Na+ influx

Question 2

Phase 1

Answer 2

• Partial repolarization
• K+ ion efflux
• Na channels inactivated

Question 3

Phase 2

Answer 3

• Plateau achieved
• Slow Ca2+ influx
• K+ continues efflux

Question 4

Phase 3

Answer 4

• Repolarization
• Ca2+ channels close
• K+ efflux continues
• Membrane voltage becomes negative

Question 5

Phase 4

Answer 5

-Pacemaker potential
- K+ channels close
- Some K+ influx
- Resting membrane voltage maintained until depolarization signaled to proceed to phase to phase 0

Question 6

Involvement of SNS

Answer 6

• Beta 1 receptor
• Positive inotropy and chronotropy
• Reduced cardiac efficiency

Question 7

Involvement of PNS

Answer 7

• M2 receptor
• negative inotropy and chronotropy
• inhibition of AV conduction
• reduce automacity

Question 8

Precipitating factor After-depolarization

Answer 8

• triggered depolarization interferes with action potential
• May be early P2&3 or delayed P4
• precipitate sustained firing

Question 9

Precipitating factors re-entry

Answer 9

• Propagating impulse fails to stop
• Cardiac excitement continues
• Heart is continuously stimulated> tachycardia

Question 10

Precipitating factors ectopic pacemaker activity

Answer 10

• Other cardiac tissues adopt pacemaker activity
• normal cells spontaneously depolarize> tachycardia

Question 11

Precipitating factors heart block

Answer 11

• Fibrosis/ ischaemic damage to conduction system
• Atria and ventricle beats independently of each other
• bradycardia

Question 12

Types of heart blocks

Answer 12

• 1st degree: slow conduction, all impulses reach ventricle
• 2nd degree: some impulses blocked> severe bradycardia
• 3rd degree: all impulses> bradycardia

Question 13

MOA of class I drugs

Answer 13

• Blocks voltage gated Na+ channels> reduces amplitude and slope of phase 0

Question 14

Class I drugs

Answer 14

• disopyramide (Int diss)
• Lidocaine (Fast diss): marked first-pass met
• Flecainide (Slow diss)

Question 15

Side Effects of Class I drugs

Answer 15

• Anticholinergic SE
• Drowsiness, Dizziness
• Bradycardia

Question 16

MOA of Class II drugs

Answer 16

Blocks beta adrenergic stimulation> reduces AV conduction> increases refractory period
- act on Phase 2&4

Question 17

Class II drugs

Answer 17

Propanolol atenolol

Question 18

MOA of Class III drugs

Answer 18

-Blocks voltage gated K+ channels> prolonging cardiac action potential
- Blocking repolarizing K+ channels> Increases refractory period
- Act on phase 3

Question 19

Class III drug and its side effects

Answer 19

• Amiodarone t1/2 142Days
• photosensitivity, pulmonary fibrosis, hypo-hyperthyroidism

Question 20

MOA of Class IV drugs

Answer 20

Block voltage gates Ca channel> slow conduction in SA&AV nodes> shortens plateau> reduce myocardial infarction
- Reduced after-depolarization and ectopic beats
- Acts on phase 2

Question 21

Class IV drugs

Answer 21

• Verapamil, diltiazem

Question 22

MOA of adenosine

Answer 22

• not classified by Vaughan Williams classification system
• Activates adenosine A1 receptors> hyperpolarizes pacemaker cells> slows conduction velocity> negative chronotropy effects

Question 23

Adenosine considerations

Answer 23

• 10sec T1/2
• metabolized in 2 ways:
• Deaminated by adenosine deaminase to inosine
• Adenosine kinase may recycle and re-phosphorylate it to adenosine monophosphate

Question 24

Adenosine side effects

Answer 24

Chest pains
Shortness of breath
Dizziness
Nausea