L14: Understanding arrhythmias and Action of drugs on CVS Flashcards
What are the different types of abnormalities you can have to the heart rhythm?
Bradycardia Atrial fibrillation Atrial flutter Tachycardia--> Ventricular and supraventricular Ventricular fibrillation
What are some of the causes of tachycardia?
Ectopic pacemaker activity
Afterdepolarisations–> abnormal depolarisation following AP (delayed and early)
Atrial flutter/ atrial fibrillation
Re-entry loop–> conduction delay, accessory pathway
How does ectopic pacemaker activity arise?
Damaged area of myocardium –> depolarised and spontaneously active, or latent pacemaker region activated due to ischaemia (dominates over SA node)
What is delayed after-depolarisations (triggered activity)?
Occur after repolarisation phase
More likely to happen if intracellular Ca2+ increased (activate Cl- channels or Na+/Ca2+ exchanger–> brief inward current–> threshold reached–> AP generated)
What is the difference between delayed after-depolarisation and early after-depolarisations?
Delay–> after repolarisation
Early–> During repolarisation
What are early after-depolarisations?
Interupts normal repolarisation
Occurs if prolonged AP
Leads to oscillations
What is the re-enterant mechanism for generating cardiac arrhythmias?
Occurs when point of divergence with incomplete conduction damage
Unidirectional block
Excitation (impulse) takes the long route around
Can spread through the damaged area in opposite direction (unidirectional block) leading to the formation of a circuit–> areas re-excited quickly
Damage to atria, excessive stretch–> Multiple re-entrant circuits in atria–> atrial fibrillation
How may re-entry occur at the AV node?
AV node has fast and slow pathway in AV node → common pathway at end
- Slow pathway repolarisation faster than the fast pathway
Premature beat- slow pathway activated → reaches common pathway by which time fast has recovered from refractory period
Fast pathway activated in opposite direction → Sets up circuit
Depolarisation goes round the circuit
Many depolarisations down the bundle of His etc… → ventricles activated much faster rate than normal
What are supraventricular tachycardias?
Arise above the ventricles in the atria
Atrial fibrillation
AV node re-entry
What causes ventricular pre-excitation?
Accessory pathway between atria and ventricles
Creates a re-entry route
Depolarisation from atria to ventricles without passing through AV node
Wolff-Parkinson-White syndrome
What are some of the causes of brachycardia?
Sinus brachycardia
Conduction block
What causes sinus brachycardia?
Physiological–> fit, healthy athlete
Pathological
Sick sinus syndrome (intrinsic SA node dysfunction)
Extrinsic factors–> drugs–> beta blockers, some Ca2+ channel blockers
What is a conduction block?
Problems at AV node or bundle of His
Slows conduction at AV node due to extrinsic factors –> beta blockers, some Ca2+ channel blockers
What are the different classes of drugs that affect the rate and rhythm of the heart?
1- Block voltage sensitive Na+ channels
2- Antagonist of β-adrenoreceptors
3- Block K+ channels (not used–> pro-arrhythmic)
4- Block Ca2+ channels
How do Na+ channel blockers (Class one) work?
Prevent Na+ influx and depolarisation
e.g. Lidocaine –> Local anaesthetic
Use dependent block
Blocks channel in open or inactivated state
Little effect in normal cardiac tissue as it dissociates rapidly
Help prevent ventricle tachycardia
What is lidocaine used for?
Local anaesthetic
Propholyaxis–> prevent ventricle tachycardia after MI
Damaged areas of myocardium might depolarise and fire auomatically
More Na+ channels open during depolarisation–> lidocaine blocks then preventing depolarisation
Not commonly used–> other drugs avaliable
How do β-adrenoreceptor antagonist (class two) work?
Block sympathetic action –> β1 adrenoreceptors in the heart
Decrease slope of pacemaker potential in SA node and slows conduction at AV node
Propanolol, atenolol
What are β-blockers used to treat?
Supraventricular tachycardia–> slow conduction in AV node
–> slow ventricular rate in AF
MI–> causes increased sympathetic activity–> arrhythmias–> β-blockers stop
–> also reduced O2 demand preventing ischaemia