10. Antiarrhythmics Flashcards
what must occur for the heart to contract efficiently?
- To function efficiently, heart needs to contract sequentially (atria, then ventricles) and in synchronicity
- Relaxation must occur between contractions
what do each part of the ECG rhythm show?
p wave - atrial contraction
qrs complex - ventricular contraction
t wave - repolarisation of ventricles
what conditions may Result in rate and/or timing of contraction of heart muscle that may be insufficient to maintain normal cardiac output (CO)?
Heart condition where disturbances in
– Pacemaker impulse formation
– Contraction impulse conduction
– Combination of the two
what is the resting membrane potential and what is it maintained by?
• A transmembrane electrical gradient (potential) is maintained, with the interior of the cell negative with respect to outside the cell
• Caused by unequal distribution of ions inside vs. outside cell
– Na+ higher outside than inside cell
– Ca+ much higher outside than inside cell
– K+ higher inside cell than outside
• Maintenance by ion selective channels, active pumps and exchangers
What is the fast and slow action potential?
Fast: in normal cardiac myocytes - atrium and purkinje fibres included
Slow: in SA and AV nodes
WHat are the different parts of the Fast action potential?
0: Na+ in (depolarisation)
1: K+ out (partial repolarisation)
2: Ca++ in (plateau)
3: K+ out (repolarisation)
4: resting potential
What are class 1 drugs and what are their general effects on the fast action potential?
Na channel blockers
- Marked slowing conduction in tissue (phase 0) - depolarisation not as rapid - stop rhythm moving across tissue
- Minor effects on action potential duration (APD)
What are class 2 drugs and what are their general effects on the fast action potnetial?
Beta blockers
- reduced calcium influx in phase 2 therefore diminish phase 4 depolarisation and automaticity (??)
What are class 3 drugs and what are their general effects on the fast action potnetial?
Potassium channel blockers
- stops efflux of potassium
- Increase action potential duration (APD) (increased refractory period)
What are class 4 drugs and what are their general effects on the fast action potnetial?
Calcium channel blockers
- Calcium channel blockers decrease inward Ca2+ currents resulting in a decrease of phase 4 spontaneous depolarization
- affect plateau phase of action potential
What are the different parts of the slow cardiac action potential?
0: Ca++ in (depolarisation)
3: K+ out (repolarisation)
4: funny current - slow spontaneous depolarisation (slow Na+ in)
What is the effect of CCBs on the slow action potential?
Decreases rate of depolarisation
slope of phase 0 = conduction velocity so therefore conduction velocity decreased
- increases refractory period
What drugs affect automaticity of cardiac action potentials?
steeper slope of depolarisation: - beta agonists shallower slope of depolarisation: - muscarinic agonists - adenosine
What are 2 mechanisms of arrhythmogenesis?
- abnormal impulse generation
- abnormal impulse conduction
What are the different types of abnormal impulse generation? (3)
Automatic rhythms:
- enhanced normal automaticity (increased AP from SA node)
- ectopic focus (site other than SA node)
Triggered rhythms:
- early/delayed after depolarisations
What are the different types of abnormal impulse conduction?
Conduction block: - 1st, 2nd, 3rd degree Reentry: - circus movement (reentrant tachycardia) - reflection
Give 3 examples conditions that cause reentry tachycardia.
- Wolf parkinson white syndrome
- AV nodal reentry tachycardia
- Area of ischaemia can also cause reentry
What is Wolf-Parkinson White?
Abnormal conduction between the atria and ventricles leading to tachycardia
describe what happens in reentry pathways?
two pathways for electric conduction down AVN - slow and fast - conduction usually goes down fast pathway but in some conditions that pathway can be blocked so go down slow pathway and then back up into atria via fast pathway - AVNR tachycardia
In case of abnormal generation, what is the purpose of drugs? Which drugs do this?
- Decrease of phase 4 slope (in pacemaker cells)
- raises threshold for depolarisation
Beta blockers and CCBs
In case of abnormal conduction, what is the purpose of drugs? Which drugs do this?
- ↓conduction velocity (remember phase 0) (Na+ blockers)
- ↑Effective refractory period (so the cell won’t be reexcited again) (K+ blockers)
summarise what arrhythmia’s occur due to?
– Automatic or triggered activity
– Re-entry due to scar, anatomy of AV node slow and fast pathway/ WPW
summarise aim of antiarrhythmic drugs?
– Reduce abnormal impulse generation
– Slow conduction through tissue
– Block AV node to terminate some rhythms
Give examples of class 1B drugs.
Lidocaine (IV only), mexiletine (oral only)