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Year 1 Semester 2 Cardiovascular System > S10) Arrythmias & CVS Drugs > Flashcards

S10) Arrythmias & CVS Drugs Flashcards

1
Q

Outline, briefly, the possible action of drugs on the cardiovascular system

A

CVS drugs can alter:

  • The rate and rhythm of the heart
  • The force of myocardial contraction
  • Peripheral resistance and blood flow
  • Blood volume
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2
Q

What are arrhythmias/dysrhythmias?

A

Arrhythmias are abnormalities of heart rate or rhythm

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3
Q

Provide 5 examples of arrhythmias

A
  • Bradycardia
  • Tachycardia (ventricular/supraventricular)
  • Atrial flutter
  • Atrial fibrillation
  • Ventricular fibrillation
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4
Q

Describe four causes of arrhythmia which manifest as tachycardia

A
  • Ectopic pacemaker activity – latent pacemaker region activated due to ischaemia and dominates over SAN
  • After depolarisations – abnormal depolarisations following the action potential
  • Atrial flutter / atrial fibrillation
  • Re-entry loop – conduction delay due to accessory pathway
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5
Q

Describe two causes of arrhythmia which manifest as bradycardia

A
  • Sinus bradycardia

I. Intrinsic SAN dysfunction

II. Extrinsic factors e.g. drugs (beta blockers, CCBs)

  • Conduction block

I. Problems at AVN / bundle of His

II. Slow conduction at AVN due to extrinsic factors e.g. drugs

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6
Q

When are delayed after-depolarisations likely to happen?

A

Delayed after-polarisations are more likely to happen if intracellular Ca2+ high

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7
Q

When are early after-depolarisations likely to happen?

A
  • Early after-polarisations are more likely to happen if AP prolonged
  • Longer AP = longer QT
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8
Q

Describe the re-entrant mechanism for generating arrhythmias

A
  • Incomplete conduction damage (unidirectional block)
  • Excitation can take a long route to spread the wrong way through the damaged area, setting up a circus of excitation
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9
Q

How does atrial fibrillation occur?

A

Atrial fibrillation arises due to several small re-entry loops in the atria

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10
Q

Briefly, explain how AV nodal re-entry occurs

A

Fast and slow pathways in the AV node create a re-entry loop

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11
Q

Briefly, explain how ventricular pre-excitation occurs

A

An accessory pathway between atria and ventricles creates a re-entry loop such as in Wolff-Parkinson-White syndrome

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12
Q

There are 4 basic classes of anti-arrhythmic drugs affecting the rate and rhythm of the heart.

What are they?

A
  • Drugs that block voltage-sensitive Na+ channels
  • β-adrenoreceptors antagonists
  • Drugs that block K+ channels
  • Drugs that block Ca2+ channels
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13
Q

Describe the action of drugs which block voltage-dependant Na+ channels (Class I)

A
  • Only blocks voltage-gated Na+ channels in open/inactive state, thus preferentially blocks damaged depolarised tissue
  • Blocks during depolarisation but dissociates in time for next action potential
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14
Q

In four steps, describe the action of β-adrenoreceptor antagonists (class II)

A

⇒ Act at β1-adrenoreceptors in the heart

⇒ Block sympathetic action

⇒ Decrease slope of pacemaker potential in SAN

⇒ Slows conduction at AVN

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15
Q

Describe the effects of beta blockers

A

Beta blockers slow conduction in AV node:

  • Prevent supraventricular tachycardias (decrease in SNS activity)
  • Slows ventricular rate in patients with AF
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16
Q

Explain why beta blockers are beneficial following myocardial infarction

A
  • MI causes increased sympathetic activity
  • β-blockers reduce O2 demand, hence reducing myocardial ischaemia
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17
Q

Describe the action of drugs that block K+ channels (class III)

A
  • Block K+ channels
  • Prolong the action potential
  • Lengthens the absolute refractory period
  • Preventing another action potential occurring too soon (can be pro-arrhythmic)
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18
Q

Describe the actions of drugs that block Ca2+ channels (class IV)

A
  • Decreases slope of action potential at SAN
  • Decreases AV nodal conduction
  • Decreases force of contraction (negative inotropy)
  • Some coronary and peripheral vasodilation
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19
Q

What is heart failure?

A

Heart failure is the chronic failure of the heart to provide sufficient output to meet the body’s requirements

20
Q

Identify 4 features of heart failure

A
  • Reduced force of contraction or reduced filling
  • Reduced cardiac output
  • Reduced tissue perfusion
  • Oedema
21
Q

Which types of drugs are used in the treatment of heart failure?

A
  • Positive inotropes to increase cardiac output (not routinely used)
  • Drugs which reduce work load of the heart (afterload and preload)
22
Q

Which drugs increase myocardial contractility in the treatment of heart failure?

A
  • Cardiac glycosides
  • β adrenoreceptor agonists
23
Q

In 5 steps, explain how cardiac glycosides increase myocardial contractility

A

⇒ Ca2+ is extruded via the Na+-Ca2+ exchanger

⇒ Cardiac glycosides block Na+/K+ ATPase

⇒ Rise in [Na+]i decreases activity of Na+-Ca2+ exchanger

⇒ Causes increase in [Ca2+]i

⇒ Increased force of contraction

24
Q

Cardiac glycosides may be used in heart failure when there are arrythmias.

Describe the action of cardiac glycosides on heart rate

A
  • Action via CNS to increase vagal activity
  • Slows AV conduction
  • Slows the heart rate
25
Q

Describe two uses of β – adrenoreceptor agonists

A
  • Cardiogenic shock
  • Acute but reversible heart failure e.g. following cardiac surgery
26
Q

Which drugs reduce the work load of the heart?

A
  • ACE inhibitors
  • Diuretics
  • β – adrenoreceptor antagonists
27
Q

Describe the action of ACE-inhibitors

A

Prevent the conversion of angiotensin I to angiotensin II:

  • Reduce Na+ and H2O reabsorption
  • Vasodilation of systemic vasculature
28
Q

Explain why ACE inhibitors are so useful in the treatment of hypertension

A
  • Decrease vasomotor tone (blood pressure)
  • Reduce afterload of the heart
  • Decrease fluid retention (blood volume)
  • Reduce preload of the heart
29
Q

What is angina?

A
  • Angina is a condition arising due to insufficient O2 supply to the heart for a limited duration caused by artheromatous plaque formation in the coronary arteries
  • It results in heart tissue Ischaemia which presents as chest pain
30
Q

How does one treat angina?

A
  • Reduce the work load of the heart:

I. β blockers

II. CCBs

III. Organic nitrates

  • Improve the blood supply to the heart:

I. Organic nitrates

II. CCBs

31
Q

Describe the action of organic nitrates

A
  • Reaction of organic nitrates with thiols in vascular smooth muscle causes NO2- to be released
  • NO2- is reduced to nitrous oxide
  • NO is a powerful vasodilator
32
Q

How does nitrous oxide cause vasodilation?

A

⇒ NO activates guanylate cyclase

⇒ Increases cGMP

⇒ Lowers [Ca2+]i

⇒ Causes relaxation of vascular smooth muscle

33
Q

In four steps, describe the primary action of nitrous oxide

A

Acts on venous system (venodilation):

⇒ Reduced venous pressure

⇒ Reduced filling → reduced contractility (Starling’s Law)

⇒ Lowers O2 demand

⇒ Reduces work load of the heart

34
Q

What is the secondary action of nitrous oxide?

A

Acts on coronary collateral arteries to improve O2 delivery to the ischaemic myocardium

35
Q

Why do organic nitrates preferentially act on veins?

A

Less endogenous nitric oxide in veins:

  • Most effective on veins > arteries
  • Little effect on arterioles
36
Q

Illustrate how venodilation by organic nitrates is a major contribution to their overall action

A
37
Q

Illustrate how the dilation of collateral coronary arteries by organic nitrates is only a minor contribution to their overall action

A
38
Q

Provide an example of an organic nitrate

A

Glyceryl trinitrate (GTN spray – sublingual)

39
Q

Why do we need antithrombotic drugs?

A

To treat certain heart conditions which carry an increased risk of thrombus formation e.g. atrial fibrillation, AMI, mechanical prosthetic heart valves

40
Q

What are the two types of antithrombotic drugs?

A
  • Anticoagulants
  • Antiplatelet drugs
41
Q

Identify some anticoagulants and describe their actions

A
  • IV Heparin – inhibits thrombin (short term)
  • Oral Warfarin – antagonises action of vitamin K (long term)
  • Other: fractionated subcutaneous heparin, oral thrombin inhibitors
42
Q

What is the most common antiplatelet drug and when is it given?

A

Aspirin – following AMI / in high risk of MI

43
Q

What is hypertension?

A

Hypertension is the persistent elevation of blood pressure

44
Q

What are the possible targets of hypertensive drugs?

A
  • Lower blood volume
  • Lower cardiac output directly
  • Lower peripheral resistance
45
Q

Identify 5 hypertensive drugs and describe their respective actions

A
  • ACE-inhibitors – decrease Na+ and H2O retention and TPR
  • CCBs – vasodilation in vascular smooth muscle
  • Diuretics – decrease Na+ and H2O retention
  • β-blockers – decrease cardiac output (not routinely used)
  • α1 adrenoceptor antagonist – vasodilation (not routinely used)

Year 1 Semester 2 Cardiovascular System (17 decks)

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