Antiarrhythmics - Class III (K+ channel blockers)

Question 1

The cardiac electrophysiology in a pacemaker cell has only 3 phases, which ultimately lead to an action potential (as seen in the image). What is occurring at stage 4?

1 - pacemaker potential phase
2 - depolarisation phase
3 - Leaky K+ phase
4 - Ca2+ leaking into cell phase

Answer 1

1 - pacemaker potential phase

• similar to resting membrane potential
• around -65 mV

Question 2

The cardiac electrophysiology in a pacemaker cell has only 3 phases, which ultimately lead to an action potential (as seen in the image).To move between phase 4 and 0, there is a special type of channel located on the pacemaker cells that allows Na+ to flow into the pacemaker cells, raising the action potential to -50 mV. What is the channel called?

1 - Na+ channel
2 - Na+/K+ ATPase channel
3 - hyperpolarization-activated cyclic nucleotide-gated channels (HCN)
4 - Na+/Ca2+ co-transport

Answer 2

3 - hyperpolarization-activated cyclic nucleotide-gated channels (HCN)

• important when cells hyperpolarise following an action potential, these channels help the pacemaker cell get close to action potential and fir again
• called the funny current

Question 3

The cardiac electrophysiology in a pacemaker cell has only 3 phases, which ultimately lead to an action potential (as seen in the image). The funny current (Na+ entering the pacemaker cells) then enters phase 0. What is this phase called?

1 - pacemaker potential phase
2 - depolarisation phase
3 - Leaky K+ phase
4 - Ca2+ leaking into cell phase

Answer 3

2 - depolarisation phase

• Na+ enters cell through HCN
• Ca2+ enters the cell through Ca2+ channels
• pacemaker cells membrane potential becomes + (around 10mV

Question 4

The cardiac electrophysiology in a pacemaker cell has only 3 phases, which ultimately lead to an action potential (as seen in the image). In phase 0 Na+ and Ca2+ enter pacemaker cells causing depolarisation (aprox 10 mV). The pacemaker cell then enters phase 3. What of the following then happens here?

1 - Ca2+ channels close
2 - Na+ channels remain open
3 - K+ channels open and K+ leaves the cell
4 - all of the above

Answer 4

4 - all of the above

• lots of K+ channels
• overall this causes the membrane potential to drop called repolarisation
• phase 4 begins again

Question 5

In a myocyte cell the cardiac electrophysiology has 5 phases (0-4), which ultimately lead to an action potential (as seen in the image). Phase 0 is the resting phase (-90mV). What then enters the myocyte through gap junctions that raises the resting membrane potential (-90mV) to the threshold potential (-70mV)?

1 - Na+
2 - K+
3 - Ca2+
4 - Mg+

Answer 5

3 - Ca2+

Question 6

In a myocyte cell the cardiac electrophysiology has 5 phases (0-4), which ultimately lead to an action potential (as seen in the image). Phase 0 occurs when the the membrane potential reaches -70mV. Which channels then open causing depolarisation, reaching around 20mV?

1 - Na+
2 - K+
3 - Ca2+
4 - Mg+

Answer 6

1 - Na+

Question 7

In a myocyte cell the cardiac electrophysiology has 5 phases (0-4), which ultimately lead to an action potential (as seen in the image). Following depolarisation, we reach phase 1, called initial repolarisation. Which 2 of the following occurs here?

1 - Na+ channels close
2 - K+ channels open and K+ leaves the cell
3 - Ca2+
4 - Mg+

Answer 7

1 - Na+ channels close
2 - K+ channels open and K+ leaves the cell

• causes a drop in membrane potential

Question 8

In a myocyte cell the cardiac electrophysiology has 5 phases (0-4), which ultimately lead to an action potential (as seen in the image). Following initial repolarisation (phase 1), to stop the myocyte going into full repolarisation, another channel opens and the membrane potential plateaus, called the plateau phase. Which channel opens to maintain this plateau phase (phase 2)?

1 - Na+ channels
2 - K+ channels
3 - Ca2+ channels
4 - Mg+ channels

Answer 8

3 - Ca2+

• Ca2+ flows into the cell
• membrane charge is maintained
• responsible for heart contraction

Question 9

In a myocyte cell the cardiac electrophysiology has 5 phases (0-4), which ultimately lead to an action potential (as seen in the image). Following the plateau phase (phase 2) which is when the heart contracts, which of the following occurs in phase 3, called repolarisation?

1 - Ca2+ channels close
2 - K+ channels remain open
3 - Ca2+ is pumped out of the cell
4 - all of the above

Answer 9

4 - all of the above

• myocyte returns to resting membrane potential of around -90mV

Question 10

When looking at an ECG, what does the P wave represent in relation to an an action potential?

1 - sum of depolarisation in all atrial myocytes
2 - sum of depolarisation in all ventricle myocytes
3 - sum of repolarisation in all atrial myocytes
4 - sum of repolarisation in all ventricle myocytes

Answer 10

1 - sum of depolarisation in all atrial myocytes

Question 11

When looking at an ECG, what does the QRS wave represent in relation to an an action potential?

1 - sum of depolarisation in all atrial myocytes
2 - sum of depolarisation in all ventricle myocytes
3 - sum of repolarisation in all atrial myocytes
4 - sum of repolarisation in all ventricle myocytes

Answer 11

2 - sum of depolarisation in all ventricle myocytes

• atrial myocytes also repolarise here as well, but this is masked by the QRS

Question 12

When looking at an ECG, what does the ST segment represent in relation to an an action potential?

1 - sum of depolarisation in all atrial myocytes
2 - sum of depolarisation in all ventricle myocytes
3 - sum of repolarisation in all atrial myocytes
4 - plateau phase

Answer 12

4 - plateau phase

• this is when the ventricle contract and pump blood

Question 13

When looking at an ECG, what does the T wave represent in relation to an an action potential?

1 - sum of depolarisation in all atrial myocytes
2 - sum of depolarisation in all ventricle myocytes
3 - sum of repolarisation in all ventricle myocytes
4 - plateau phase

Answer 13

3 - sum of repolarisation in all ventricle myocytes

Question 14

When we look at an action potential, which phase would a class I (Na+ channel blocker) be able to have an affect?

1 - phase 4
2 - phase 0
3 - phase 1
4 - phase 2
5 - phase 3

Answer 14

2 - phase 0

• depolarisation is due to an influx of Na+
• this is only effective in myocytes and not pacemaker cells

Question 15

When we look at an action potential, which phase would a class III (K+ channel blocker) be able to have an affect?

1 - phase 4
2 - phase 0
3 - phase 1
4 - phase 2
5 - phase 3

Answer 15

2 - phase 3

• repolarisation is due to an outflow of K+

Question 16

Which 2 of the following are class III anti-arrhythmic core medications?

1 - Amiodarone
2 - Atenolol
3 - Verapamil
4 - Dronedarone

Answer 16

1 - Amiodarone
4 - Dronedarone (last resort drug)

Question 17

Which arrhythmia is Amiodarone specifically effective against?

1 - atrial fibrillation (AF)
2 - supraventricular tachycardia
3 - bradycardia
4 - systemically works on all arrhythmias

Answer 17

4 - systemically works on all arrhythmias

Question 18

What is the mechanism of action of Amiodarone and Dronedarone, the class III anti-arrhythmic core medication?

1 - inhibit noradrenaline and adrenaline binding with B1 cardiac receptors
2 - Na+ channel blockers
3 - Ca2+ channel blockers
4 -K+ channel blockers

Answer 18

4 -K+ channel blockers

• delays repolarisation
• prolongs action potential duration and prolongs QT duration on ECG
• means heart does not respond to unwanted electrical activity

Question 19

Amiodarone and Dronedarone are the class III anti-arrhythmic core medication that we need to be aware of. These drugs are indicated for use in a variety of tachyarrhythmias including atrial flutter and fibrillation, supraventricular tachycardia, ventricular tachycardia and fibrillation. When should these drugs be used though?

1 - malignant ventricular arrhythmias
2 - common sustained tachyarrhythmias
3 - AF and VT
4 - maintain sinus rhythm
5 - all of the above

Answer 19

5 - all of the above

• generally used when other medications or treatments have failed

Question 20

Amiodarone is able to do all of the following to myocardial cells, EXCEPT:

1 - increase spontaneous depolarisation
2 - slow conduction velocity
3 - increase resistance to depolarisation
4 - slows AV node conduction
5 - stabilises atrial and ventricular myocytes
6 - blocks accessory pathways
7 - no significant negative inotropic effect

Answer 20

1 - increase spontaneous depolarisation

• they decrease spontaneous depolarisation

Question 21

Amiodarone is an effective drug, but is also a very toxic drug. It has a number of adverse side effects. Which of the following is NOT an adverse effect of amiodarone?

1 - pneumonitis + fibrosis
2 - tachycardia
3 - AV block
4 - hepatitis
5 - arrhythmias
6 - photosensitivity with blue/grey discolouration
7 - thyroid abnormalities (iodine)
8 - corneal microdeposits
9 - peripheral neuropathy, tremor

Answer 21

2 - tachycardia

• typically can cause bradycardia due to its affects on conduction
• some of the adverse events are not reversible

Question 22

Amiodarone has a very long half life (30 days) due to being lipid soluble and can take months to clear. Which of the conditions should amiodarone not be prescribed in?

1 - hypotension
2 - heart block
3 - thyroid disease
4 - all of the above

Answer 22

4 - all of the above

Question 23

Amiodarone is contraindicated in patients with thyroid disease. What does amiodarone do to cause thyroid problems?

1 - initiates a type III hypersensitivity reaction against the thyroid
2 - absorbed by follicular cells and inhibits thyroid peroxidase
3 - inhibits T3 and T4 release into the circulation
4 - all of the above

Answer 23

3 - inhibits T3 and T4 release into the circulation

Question 24

Amiodarone should not be used with which of the following drugs due to interactions?

1 - verapamil
2 - digoxin
3 - diltiazem
4 - all of the above

Answer 24

4 - all of the above

• amiodarone can increase levels of all of the above and can cause bradycardia, AV block and heart failure

Question 25

Can amiodarone be prescribed by any qualified doctor?

Answer 25

• generally no
• should consult a senior doctor
• only exception is during a cardiac arrest it can be given for VF or pulseless VT

Question 26

Which drink should be avoided when taking amiodarone?

1 - alcohol
2 - cranberry juice
3 - apple juice
4 - grapefruit juice

Answer 26

• 4 - grapefruit juice
• avoid grapefruit juice down regulates cytochrome P450, which metabolises amiodarone
• less CP450 means an increases in exposure to amiodarone

Question 27

What is the key to monitoring for efficacy and safety pre and following administration?

1 - heart rate and rhythm
2 - LFTs
3 - echocardiogram
4 - MRI

Answer 27

1 - heart rate and rhythm

• long term use needs renal, thyroid and liver profiles
• monitoring QT prolongation

Question 28

In addition to monitoring a patients ECG, which of the following are routinely monitored in patients taking amiodarone?

1 - Lungs = Annual CXR
2 - Liver = 6-monthly LFTs
3 - Thyroid = 6-monthly TFTs
4 - all of the above

Answer 28

4 - all of the above