CV pharmacology 1 Classes of drugs

Question 1

Name the Vaughan-Williams classification of which drugs

Answer 1

• Antidysrhythmics
Class 1: I- drugs that block Na+ channels
Class 2: II- βeta blockers (block β 1 adrenoreceptor)
Class 3: III- drugs that prolong AP by blocking some K+ channels
Class 4: IV- drugs that block Ca2+ channels

Question 2

Aim of Class 1 and location

Answer 2

AIM: to bind to and BLOCK FAST Na+ channels, causing SLOWER depolarisation
LOCATION: cardiomyocytes

Question 3

What effect will class 1 drugs have on nodal tissue?

Answer 3

NO effect on nodal tissue as are present on cardiomyocytes

Question 4

How are class 1 drugs divided?

Answer 4

1. Divided into 3 sub classes. All of which block Na channels
2. 1a. 1b and 1c

Question 5

What do all the Class 1 drugs exhibit that affects which Na channels they block?
What does this mean?

Answer 5

1. All exhibit: use dependent Na channel blockade
2. Means they’re more likely to act on active (open or refractory) Na channels than inactive (resting)
3. This means the more active the channels are then the better these drugs bind and so work

Question 6

What affect do Class 1 drugs have on tachyarrhythmias and why?

Answer 6

1. Due to the “Use dependent Na Channel Blockade” Class 1 drugs reduce heart rate in tachyarrhythmias while not significantly affecting normal heart rates.
2. BECAUSE
3. In tachyarrhythmias, Na+ channels are open for reeeally long periods of time, which is NOT the case in normal heart rate.
4. Class 1 drugs only really act on OPEN Na+ channels = better at doing its job!

Question 7

What are class 1 drugs dependent on?

Answer 7

normal extracellular [K] for function and ECF.

Question 8

What affect does hypokalaemia and hyperkalaemia have on Class 1 drug function?

Answer 8

o Hypokalaemia (low K+) reduces drug function while hyperkalaemia increases it.

Question 9

Talk about Class 1 division 1a and give an example

Answer 9

e. g. QUINIDINE
1. Old drugs
2. Bind to Na channels and:
3. Moderate Na+ channel blockade so has a Intermediate rate of dissociation from Na Channels
4. Increase effective refractory period

Question 10

Talk about Class 1 division 1b and give an example

Answer 10

e.g. LIDOCAINE!!
1. Bind during phase 0 of AP then begins to dissociate in time for the next AP. So has a weak Na+ channel blockade.
Decreased effective refractory period
2. Means it prevents/ reduce chance of premature beats because channels are still blocked at this time.
3. Tachyarrhythmia lot of premature beats! So this helps prevent these!

Question 11

Talk about Class 1 1c and give example

Answer 11

e. g. FLECAINIDE
1. won’t talk as much
2. Binds and dissociates more slowly than 1b and 1a - strong Na+ channel blockade.
3. Reaches a steady state and reduces conduction through the His Purkinje System

Question 12

How do the class 1 drugs affect the effective refractory period?

Answer 12

1. Effects “effective refractory period” (ERP) by action on K+ channels
2. 1b Increases
3. 1b Decreases

Question 13

Class 1a antidysrhythmics in practise

Answer 13

QUINIDINE

Question 14

When do we use class 1 a Quinidine in practise.

Answer 14

1. It is an old drug and quite brutal
2. so only really used in Atrial Fibrilation in horses (and cattle) with structurally normal hearts.
3. because AF can occur spontaneously without dmaage to atria as have such large hearts.
4. Therefore can convert back to normal rhythm as NO underlying cardiomyopathy.

Question 15

What is atrial fibrilaition?

Answer 15

1. Atrial fibrillation happens when abnormal electrical impulses suddenly start firing in the atria.

Atria contract randomly and sometimes so fast that the heart muscle cannot relax properly between contractions, reducing heart’s efficiency and performance.

These impulses override the heart’s natural pacemaker, which can no longer control the rhythm of the heart. This causes you to have a highly irregular pulse rate

Question 16

How do we administer class 1 a antidysrhythmics Quinidine in practise?

Answer 16

1. Oral or Parenteral

2. NOT IV!!!

Question 17

Aim class 1 a quinidine

Answer 17

AIM: not necessarily to slow the heart down but to CONVERT it back to normal rhythm

Question 18

Side effects of class 1a Quinidine

Answer 18

1. Parenteral administration can be associated with considerable haemodynamic side effects
2. Mildly vagolytic (anticholinergic) which can inc HR and conduction
3. Negative inotrophy and vasodilation - can lead to Congestive HF
4. Various rhythm disturbances as the blockade persists
5. GI signs in horses not great.
6. Nervousness and depression :(

Question 19

How does class 1a QUINDINE have an adverse effect resulting in it acting as a negative inotrope and GI signs

Answer 19

1. blocking Na channels, reduced contractility

2. GI - due to vagolytic action which can cause cholic, be severe and fatal

Question 20

What would we prefer to use other than Quinidine to correct atrial fibrilation in hroses?

Answer 20

• If money and insurance, preferable to go for a heart ablation (laser scar or destroy heart tissues allowing incorrect electrical signals to cause an abnormal heart rhythm, to stop it fibrilating)

Question 21

Why not use Quinidine in small animals with atrial fibrilation?

Answer 21

1. Aim here not to slow down but to convert back to normal rhythm
2. Usually in horses due to huge heart, AF occurs spontaneously but STRUCTURALLY the heart is normal!
3. So we are converting back to normal rhythm with no underlying cardiomyopathy.
4. Whereas in SA and dogs, atrial fibrilation is mostly associated with disease of the heart muscle. So we have to help them to cope rather than cure it.