Arrhythmias and anti-arrhythmic drugs

Question 1

Defects in impulse conduction

Answer 1

Re-entry
Conduction block
Accessory tracts

Question 2

Defects in impulse formation

Answer 2

Altered automacity

Triggered activity

Question 3

Altered automacity (physiological)

Answer 3

Modulation of SA node activity by the ANS

Question 4

Altered automacity (pathological)

Answer 4

Latent pacemaker takes over SA node function

• when SA node firing frequency is low
• if a latent pacemaker fires faster than the SA node rate

Question 5

Triggered activity (EAD)

Answer 5

Can occur in phase 2 or 3 of the ventricular AP

Prolonged AP

Question 6

Triggered activity (DAD)

Answer 6

Normal AP is followed by a depolarising event

Question 7

Re-entry

Answer 7

Conduction pathway of heart divides into 2 branches.
AP comes down the branches but collides with each other at a point so AP’s can’t progress further.

Uni-directional block: 
AP can't progress down normal route of 2 branches and can therefore only go down 1 branch. 
Slow backwards (retrograde) AP can occur at the non functioning branch so AP conducts slowly back up to the start of branch 1 again and the process repeats.

Question 8

First degree heart block (conduction block)

Answer 8

Prolonged PR interval

Treatment: none

Question 9

Mobitz type 1 second degree heart block (conduction block)

Answer 9

PR interval progressively increases until a QRS complex is missed resulting in a missed ventricular beat.

Question 10

Mobitz type 2 second degree heart block (conduction block)

Answer 10

PR interval is normal
Every ‘nth’ impulse fails to go through the AVN (i.e. no QRS complex)
Some P waves followed by QRS complexes, some not
QRS:Non-QRS - 2:1 or 3:1 ratio usually
Management: ventricular pacing

Question 11

Third degree -complete- heart block (conduction block)

Answer 11

Atria and ventricles work independently
P waves don’t link to QRS complexes
Management: ventricular pacing

Question 12

Accessory tract pathways

Answer 12

Usually - only point of electrical contact between A&V is AV node.

Bundle of Kent - extra tissue between A&V which bypasses the AVN. Impulse is conducted more quickly than that through AVN

Eg: WPW syndrome where delta wave excites ventricle before the normal QRS complex

Question 13

Anti-arrhythmic drugs: Class IA

Answer 13

Blocks voltage activated Na+ channels 
Moderate rate 
Cardiac AP - phase 0
Prolongs refractory period
eg: disopyramide
Rhythm control

Question 14

Anti-arrhythmic drugs: Class IB

Answer 14

Blocks voltage activated Na+ channels
Fast rate
Cardiac AP - phase 0
Prevents premature beats 
Rhythm control

Question 15

Anti-arrhythmic drugs: Class IC

Answer 15

Blocks voltage activated Na+ channels 
Slow rate 
Cardiac AP - phase 0
Depress conduction
eg: flecanide
Rhythm control

Question 16

Anti-arrhythmic drugs: Class II

Answer 16

Blocks B-adrenoceptors 
(eg: metoprolol) 
Cardiac AP - phase 4
Decreased rate of depolarisation at SA/AV node 
Rate control

Question 17

Anti-arrhythmic drugs: Class III

Answer 17

Blocks voltage activated K+ channels 
EG: amiodarone, sotalol 
Cardiac AP - phase 3
Prolongs AP duration 
Increased refractory period 
Rhythm control

Question 18

Anti-arrhythmic drugs: Class IV

Answer 18

Blocks voltage activated Ca2+ channels
EG: verapamil 
Cardiac AP - phase 2
Slows conduction in SA and AV nodes 
Decreases force of contraction
Rate control

Question 19

Adenosine

Answer 19

Useful to treat SVT but not VT
Very short acting
Administration route: IV bolus

Question 20

Arrhythmia investigations

Answer 20

ECG 
24 hr ECG 
Exercise ECG 
Echo
CXR 
Electrophysiological study - induce arrhythmia to study mechanism