Pharmacology of rhythm control

Question 1

Where does electrical conduction of the heart begin?

Answer 1

At the sinoatrial node (SAN) which is near the SVC junction

Question 2

Describe the journey of electrical impulses

Answer 2

SAN –> AVN –> Bundle of His –> LBB –> RBB –> Purkinje network

Question 3

Explain the different phases of action potentials in cardiomyocytes (PHASES 0-4)

Answer 3

• Resting TMP in cardiomyoctes (ventricular AP) is -90mV

PHASE 0: is rapid Na+ influx through fast Na+ channels (+55mV) this is depolarisation

PHASE 1: is when you have the transient K+ channel opening, hyperpolarising the cell to 0mV

PHASE 2: is the plateau phase, balancing of Ca2+ influx through LTCC and K+ efflux through delayed rectifier K+ channels

PHASE 3: is when the Ca2+ channels close and the delayed rectifier K+ channels remain open

PHASE 4: (resting phase) is the Na+ and Ca2+ channels closed and recover from inactivation, whilst the open K+ delayed rectifier channels keep the TMP at -90mV

Question 4

Which cardiac cell types display pacemaker behaviour?

Answer 4

SA node
AV node
Purkinje fibers
Ventricular cardiomyocytes

Question 5

What is the absolute refractory period?

Answer 5

When the cell is absolutely unexcitable to a new stimulus

Question 6

What is the effective refractory period?

Answer 6

The ARP + short segment of phase 3 during which a stimulus may cause the cell to depolarise minimally but will not result in a propagated action potential

Question 7

What is the relative refractory period?

Answer 7

A greater than normal stimulus will depolarise the cell and cause an AP

Question 8

During which period are cells particularly susceptible to arrhythmias?

Answer 8

During the supra-normal period, with an inappropriately timed stimulus
- This is why one must synchronise the electrical stimulus during cardioversion to prevent inducing ventricular fibrillation

Question 9

What is the supra-normal period?

Answer 9

A hyperexcitable period during which a weaker than normal stimulus will depolarise the cells and cause an AP

Question 10

What is bradycardia? (in bpm)

Answer 10

<60bpm

Question 11

What is tachycardia? (in bpm)

Answer 11

> 100bpm

Question 12

Different causes of bradycardia?

Answer 12

• Sinus bradycardia
• Sick sinus syndrome
• AV block (1st degree, 2nd degree and CHB)

Question 13

Different types of tachycardia?

Answer 13

1. Supraventricular
   - Sinus tachycardia
   - AVRT/AVNRT/Atrial tachycardia (Atrioventricular reentrant tachycardia and Atrioventricular nodal reentry tachycardia)
   - Atrial flutter/AF
2. Ventricular
   - VT
   - VF
   - Torsades de pointes

Question 14

What are bradyarrhythmias?

Answer 14

1. Failure of pacemaker cells to generate appropriate electrical impulses AKA disorders of automaticity
   - Sinus bradycardia
   - Sick sinus syndrome
2. Failure to propagate electrical impulses appropriately AKA heart block
   - AV block (1st/2nd/3rd degree)

Question 15

What is automaticity?

Answer 15

• The heart rate gradually increases and gradually decreases

- Automaticity is when you get abnormal acceleration of phase 4 activity which occurs in some locations within the heart

Question 16

What are examples of automaticity?

Answer 16

Sinus tachycardia

Multifocal atrial tachycardia

Question 17

What are some causes of automaticity?

Answer 17

Metabolic causes
Ischaemia
Hypoxaemia
Acid-base disorders
High sympathetic tone
Use of sympathomimetic agents

Question 18

What is the most common mechanism for tachyarrhythmias?

Answer 18

Re-entry

- This is the mechanism for AVRT, AVNRT and VT

Question 19

What is the most common mechanism for tachyarrhythmias?

Answer 19

Re-entry

- This is the mechanism for AVRT, AVNRT and VT

Question 20

What is proarrhythmia?

Answer 20

The provocation of a new arrhythmia or the aggravation of a pre-existing one during therapy with a drug at doses considered toxic

Question 21

What is digoxin used to treat?

Answer 21

Atrial fibrillation
Atrial flutter
Congestive cardiac failure

Question 22

Mechanism of digoxin?

Answer 22

Mechanism 1: AV Node Inhibition: Digoxin has vagomimetic/parasympathomimetic effects on the AV node. By stimulating the parasympathetic nervous system, it slows electrical conduction in the atrioventricular node, therefore, decreases the heart rate = negative chronotropy (<hr>contractility (inotropy) of the heart, which causes smooth muscle contraction and vasoconstriction

Question 23

Side effects of digoxin

Answer 23

1. Abnormal cardiac rhythms
2. GI effects: anorexia, nausea, vomiting, diarrhoea
3. Visual effects: xanthopsia, blurring
4. Gynecomastia
5. CNS effects: confusion, agitation, nightmares and psychoses

Question 24

What is amiodarone used for?

Answer 24

SVT and VT

Question 25

What is the most frequently used anti-arrhythmic drug used in hospital settings?

Answer 25

Amiodarone

Question 26

How is amiodarone administered and why?

Answer 26

Through a central line as it can cause skin necrosis

Question 27

Side effects of amiodarone

Answer 27

1. Abnormal cardiac rhythm (bradycardia, heart block, ventricular arrhythmia)
2. Corneal microdeposits
3. Hypo and hyperthyroidism
4. Photosensitivity reactions
5. Blue-grey skin discolouration
6. Abnormal LFTs
7. Lung fibrosis

Question 28

Side effects of beta-blockers: BALD FISH

Answer 28

B) Bronchoconstriction and bradycardia
A) Arrhythmias
L) Lethargy
D) Disturbance in glucose metabolism

F) Fatigue
I) Insomnia
S) Sexual dysfunction
H) Hypotension

Question 29

Indications for lidocaine?

Answer 29

VT

- Used when amiodarone is contraindicated or ineffective