Session 9 Flashcards
What can drugs alter?
The rate and rhythm of the heart
The force of myocardial contraction
Peripheral resistance and blood flow - acting on peripheral resistance vessels to alter after-load and blood pressure.
Blood volume
What conditions are Cardiovascular drugs used to treat?
Arrhythmias (abnormal rhythm)
Heart failure
Angina
Hypertension
Risk of thrombus formation
NB: some drugs can act at more than one site and are used to treat more than one condition
What abnormalities of heart or rhythm may there be?
Bradycardia
Atrial flutter
Atrial fibrillation
Tachycardia
Ventricular fibrillation
Explain what is Atrial Flutter and Atrial Fibrillation
Atrial flutter: the impulse travels along a pathway within the RA, moving in an organised circular motion or circuit, causing the atria to beat faster than the ventricles - the heart beats fast but in a regular pattern.
Atrial fibrillation: many random electrical impulses firing off in the atria - atrial depolarisation is uncontrolled. The atria then fibrillate - only partially contract but very rapidly. Only some of these impulses pass through to the ventricles in a haphazard way. Therefore the ventricles contract in an irregular way and with varying force.
When could be the cause of Atrial Flutter or Atrial Fibrillation
May occur following conditions which put extra stretch and pressure on the atria (e.g. Mitral valve stenosis) and therefore damage the atria.
Damage to the atria may cause several short re-entrant loops to develop or may be as a result of an ectopic focal point of excitation.
Such points of excitation are frequently in the large veins entering the atria.
The actual heart rate depends on the frequency of impulse passing through the AV node.
A major concern with atrial fibrillation is the risk of thrombus formation.
Where could Tachycardia originate?
Ventricular tachycardia originates at ventricles (the bundle of His, Purkinje fibres or ventricular myoctes)
Supraventricular tachycardias originate at the atria or AV node.
Ventricular tachycardia may occur following MI as a result of ectopic pacemaker activity from damaged myocardium.
This could lead to ventricular fibrillation.
What is Ventricular Fibrillation?
Ventricles contract randomly causing a complete failure of ventricular function
- cardiac output cannot be maintained
- immediately life threatening.
List the causes of arrhythmia
Arrhythmias arise due to a disturbance of impulse generation, impulse conduction or both:
Ectopic pacemaker activity
After depolarisations
Re-entry loop
How can ectopic pacemaker activity cause an arrhythmia?
Damaged area of myocardium becomes depolarised and spontaneous activity is being generated (due to leakage of K+ ions).
Or
Latent pacemaker region is activated due to ischaemia - which dominates over SA node.
How can afterdepolarisations cause an arrhythmia?
Abnormal depolarisations following the action potential (triggered activity –> premature action potentials).
Early after- depolarisations can lead to oscillations - more likely if AP is prolonged (Q-T period is long)
Delayed after/depolarisations can trigger premature APs if it reaches threshold - more likely to happen if intracellular Ca2+ is high.
How can re-entry loop(s) cause an arrhythmia?
Conduction delay (e.g. Due to damaged area of myocardium)
Or accessory pathway (anatomical; there is a conduction problem at one of more points).
Incomplete conduction damage (unidirectional block) - excitation can take a long route to spread the wrong way through the damaged area, setting up a circuit of excitation.
What are the 4 basic classes of anti-arrhythmic drugs?
I. Drugs that block voltage-sensitive N+ channels
II. Antagonists of B-adrenoceptors
III. Drugs that block K+ channels
IV. Drugs that block Ca2+ channels
Note: anti-arrhythmic drugs can be pro-arrhythmic drugs as well - can aggravate or cause Arrhythmias under certain conditions so must be used with great care.
Describe the Mechanism of Action of Class I: Drugs that block Voltage-Dependent Na+ Channels
E.g. Local anaesthetic lidocaine (Class Ib)
Only blocks voltage-gated Na+ channels in open or inactive state (not in closed state).
Dissociates rapidly in time for next AP - doesn’t stop normal AP but might stop extra activity - stops another AP from being generated prematurely.
Use-dependent block.
Lidocaine is sometimes used following MI if a patient shows signs of ventricular tachycardia - given intravenously.
This is because damaged areas of myocardium may be depolarised and fire automatically which could lead to ventricular fibrillation.
More Na+ channels are open in depolarised tissue so lidocaine blocks these Na+ channels (use-dependent)- prevents automatic firing of depolarised ventricular tissue - the ventricular action potential can still take place but the local anaesthetic would block the Na+ channels once open.
Not used prophylactically following MI - overall benefit is not convincing.
Describe the Mechanism of Action of Class II: beta-adrenoceptor antagonists
Examples: Propranolol, atenolol (beta blockers).
Block sympathetic action - act at B1-adrenoceptors in the heart.
Decrease slope of pacemaker potential in sinoatrial node - slows down heart rate.
B-blockers are used following an MI; MI causes increased sympathetic activity and B-blockers prevent ventricular Arrhythmias which may be partly due to increased sympathetic activity.
B-blockers also reduce O2 demand - reduce myocardial ischaemia and this is beneficial following MI.
B-blockers slow conduction in AV node (slow down ventricular depolarisation) and can prevent supra ventricular tachycardias.
Describe the Mechanism of Action of Class III: Drugs that block K+ Channels
They prolong the action potential mainly by blocking voltage-gated K+ channels.
This lengthens the absolute refractory period.
In theory, the drugs should prevent another AP occurring too soon but in reality can be pro-arrhythmic therefore not generously used.
One EXCEPTION is Amiodarone but this drug has other actions in addition to blocking K+ channels. Used to treat tachycardia associated with Wolff-Parkinson-White syndrome (re-entry loop due to an extra conduction pathway). Amiodarone is an important anti-arrhythmic agent which acts on Na+, K+ and Ca2+ channels as well as B-adrenoceptors.
Describe the action of Cardiac Glycosides
Ca2+ is extruded via the Na+-Ca2+ Exchanger, driven by Na+ moving down the concentration gradient.
Cardiac glycosides block Na+/K+-ATPase so [Na+]in increases.
The rise in [Na+]in leads to decrease in activity (inhibition) of Na+-Ca2+ exchanger.
This causes increase in [Ca2+]in which causes more Ca2+ to be stored in SR.
This has a positive inotropic effect - increased force of contraction.
How do organic nitrates produce NO?
Reaction of organic nitrates with thiols (-SH groups) in vascular smooth muscle causes NO2- to be released.
NO2- is reduced to NO (nitric oxide).
NO is a powerful vasodilator, produced by endothelial cells lining the blood vessels.
NO activates guanylate cyclase which increases cGMP which lowers intracellular [Ca2+] which causes relaxation of vascular smooth muscle.
This dilatation of the collateral coronary arteries improves blood supply to the heart but it is only the SECONDARY effect. Organic nitrates do not alleviate angina by dilating arterioles.
What is the Primary Action of Organic Nitrates?
Main action: Venodilation
Venodilation lowers preload (central venous pressure) therefore reducing workload of the heart.
Heart fills less therefore force of contraction is reduced (Starling’s law) which lowers O2 demand.
