Session 9 - Drugs and the CVS Flashcards
What are the causes of arrythmias?
- Ectopic pacemaker activity
- Afterdepolarisations
- Re-entry loop
What are the causes of arrythmias?
- Ectopic pacemaker activity
- Afterdepolarisations
- Re-entry loop
What are the causes of arrythmias?
- Ectopic pacemaker activity
- Afterdepolarisations
- Re-entry loop
What might cause ectopic pacemaker activity
Damaged area of myocardium becomes depolarised and spontaneously active
Latent pacemaker region activated due to ischaemia
Explain why delayed after-depolarisations and early after-depolarisations occur
Delayed after- depolarisations are more likely to occur if there is high intracellular Ca+
Early after-depolarisations are more likely to occur if the AP are prolonged, the longer the AP the longer the QT. Can set up oscillations
What is meant by a re-entry loop. What can several small re-entry loops in the atria lead to
A block in the conduction of the myocardium prevents conduction in the correct direction and therefore excitation spreads in the wrong direction
Can lead to atrial fibrillation
What do Class 1 anti-arrythmic drugs do and give an example
Drugs that block voltage-sensitive sodium channels. Only when open or inactive state, therefore dissociates rapidly in time for next AP…. use-dependent block
e.g. Lidocaine
Why does lidocaine automatically block damaged myocardium
As the damaged myocardium has more inactive and open sodium channels (use-dependent)
What do class 2 anti-arrythmic drugs do and give an example
Beta adrenoreceptor antagonists (beta blockers)
Act at beta 1 receptors in the heart and block sympathetic action –> decrease in slope of pacemaker potential
Can slow the conduction at the AV node and therefore prevent supra ventricular tachycardias
e.g. propranolol, atenolol
What do class 3 anti-arrythimic drugs do and give an example
Block K+ channels therefore prolong the AP –> lengthening of the absolute refractory period
e.g. amiodarone
Why are class 3 anti-arrythmic drugs not used?
As they can become pro-arrythmic
What can amiodarone be used to treat
Tachycardia associated with Wolff-Parkinson-White syndrome (re-entry loop due to an extra conduction pathway)
What do class 4 anti-arrythmic drugs do and give an example
Block Ca2+ channels therefore decrease the slope of the pacemaker action potential, decrease AV nodal conduction and decrease the force of contraction
Also some peripheral and coronary vascodilation
e.g. verapamil
What drugs act in the periphery at Ca+ channels
Dihydropyridine Ca2+ channel blockers act on vascular smooth muscle –> vasodilation
What drugs act in the periphery at Ca+ channels
Dihydropyridine Ca2+ channel blockers act on vascular smooth muscle –> vasodilation
How does Adenosine work
Produced endogenously but can be given pharmacologically
Acts on A1 receptors at the AV node to enhance K+ conductance and therefore hyperpolarise the cell
What is heart failure?
Chronic failure of the heart to provide sufficient output to meet the body’s requirements
What are the features of heart failure
- Reduced force of contraction
- Reduced CO
- Reduced tissue perfusion
- Oedema
Explain why oedema forms in heart failure
As there is increased venous pressure and therefore there is increased pressure in the right ventricle so the hydrostatic pressure in the lungs is increased –> fluid leaves the capillaries
Name the different classes of drugs that can be used to treat heart failure
- Cardiac glycosides
- Beta agonists
- ACE-inhibitors
What is the mechanism of action of cardiac glycosides and give an example of one
Na+/K+ATPase blockers. This causes the NCX in the cell to become less effective as lost Na+ gradient as there is a rise in intracellular Na+ conc. Increase in intracellular Ca2+ therefore there is more stored in the SR –> increased force of contraction
Also increase vagal activity, slow AV conduction
e.g. Digoxin
What do beta agonists do
Increase the myocardial contractility by acting on Beta 1 receptors
Used in cardiogenic shock
e.g. dobutamine
Explain how ACE-inhibitors decrease the workload of the heart
Inhibit the action of angiotensin converting enzyme therefore prevent the formation of active angiotensin II from the inactive angiotensin I
Angiotensin II acts on the kidneys to increase Na+ and H20 reabsorption –> increase BP and blood volume
Angiotensin also is a vasoconstrictor
Therefore ACE-inhibitors reduce the after load and the preload
Name another type of drug that decreases blood volume
Diuretics
What is the primary action of organic nitrates
Venodilation –> decrease preload
What is the secondary action of organic nitrates
Dilation of collateral arteries
Give an example of an organic nitrate
Glyceryl trinitrate and isosorbide dinitrate
What conditions carry increase risk of thrombus formation
Atrial fibrillation
Acute MI
Mechanical prosthetic heart valves
Name 2 anticoagulants and their actions
Heparin - inhibits thrombin
Warfarin - antagonises vitamin K
Name an antiplatelet drug
Aspirin
Name 5 types of drugs that can be used to treat hypertension in theory
- Diuretics
- ACE-inhibitors
- Beta-blockers
- Ca2+ blockers on vascular smooth muscle
- Alpha 1 antagonists
