Arrhythmias Flashcards
What is an arrhythmia?
Disturbance to heart rate or rhythm
eg tachycardia, bradycardia, atrial fibrillation
What are supraventricular tachycardia?
Above ventricles (atria are tachycardic)
Causes of tachycardia
Ectopic pacemaker
Afterdepolarisations
Atrial fibrillation
Re-entry loop
What is ectopic pacemaker activity?
Damaged area of myocardium becomes depolarised and spontaneously active
Latent pacemaker can become active due to ischaemia (dominate over SA node)
What are afterdepolarisations?
Abnormal depolarisations following action potential
2 ways re entry loop can occur
Conduction delay
accessory pathway
Causes of bradycardia
Sinus bradycardia
Conduction block
Sinus bradycardia
Sick sinus syndrome (SA node dysfunction)
Extrinsic factors eg beta blockers/Ca2+ channel blockers
Conduction block
Problem at AV node OR bundle of His
Slow conduction at AV node due to extrinsic factors (beta blockers/Ca2+ channel blockers)
When are early afterdepolarisations likely to occur?
After prolonged AP (longer QT depolarised phase)
What can early afterdepolarisations lead to?
Oscillations
When are delayed after depolarisations likely to occur?
If intracellular Ca2+ is high (Na+Ca2+ exchanger?)
What can delayed afterdepolarisations trigger?
Activity and oscillations (self perpetuating)
What generates arrhythmias (re entrant mechanism)
Incomplete conduction damage (one direction block)
Excitation takes long route to spread
Spreads the wrong way
= circus of excitation (loops round and round, dont cancel eachother out)
What does multiple re-entrant loops in the atria cause?
Atrial fibrillation
Atrial fibrillation re-entry loops
Several small re-entry loops in atria
= atrial fibrillation
(chaotic signals at AV node = irregularly irregular)
Loop reasons for arrhythmias
AV nodal re-entry
Ventricular pre-excitation
What is AV nodal re-entry?
Fast and slow pathways in AV node
= re-entry loop
What is ventricular pre-excitation?
Accessory pathway between atria and ventricles creates a re-entry loop (should only be AV node and bundle of His)
(eg Wolff Parkinson White syndrome)
4 drug classes that affect rhythm of heart
- Block voltage gated Na+ channels
- Antagonists of B adrenoreceptors
- Block K+ channels
- Block Ca2+ channels
Drugs that block Na+ channels work on…
Only work when Na+ channel is open/inactivated (NOT WHEN CLOSED)
Preferentially blocks damaged depolarised tissue (inactive)
Time for Na+ channel blockers work for
Disassociate RAPIDLY
Blocks during depolarisation, released before next AP
= not much effect on normal cardiac tissue
Example of V gated Na+ channel blockers
Lidocaine (local anaesthetic)
Effect of lidocaine
Slows upstroke
Shortens AP
Slows conduction velocity
(only really affects damaged cardiac tissue (more Na+ channels open there)
