15b – CV Arrhythmias Flashcards
Types of arrhythmias
- Tachycardia*
- Bradycardia
- Fibrillation*
- Conduction abnormalities
- *where most of our drugs target and treat
Tachycardia
- HR too high
- *normal in physical activity but problem if too far
Bradycardia
- HR too low
- *normal in sleep
- Could do a pacemaker in our animals, but tend not too
Fibrillation
- Uncoordinated, rapid excitation/contraction
Review of cardiac electrophysiology
- P wave: atrium depolarizes (SA node fastest)
- QRS complex: ventricular depolarizes
- T wave: repolarization of ventricle
Phases of electrophysiology: ventricle depolarization
Phase 0: rapid depolarization, large Na influx and tapers with time
Phase 1: Na Ca exchange
*Phase 2: PLATEAU phase: pausing in depolarized stated (VG Ca channels open=Ca INFLUX!)
Phase 3: K channels open (slower): K efflux begins=brings down membrane potential
Phase 4: resting membrane potential
Phase 4 in pacemakers
- Funny current: mix of Na and Ca channels that SLOWLY LEAK and cause a slow spontaneous depolarization until threshold=AP!
Supraventricular tachycardia (problem in pacemakers)
- *increased automaticity if
o Slope of phase 4 increased
o Lower threshold potential (TP is more NEGATIVE)
o Maximum diastolic potential (MDP) is more positive
Premature ventricular contraction
- *ventricular tachycardia
- Early or late afterdepolarization
o From abnormal Ca influx into cardiac cells DURING or immediately AFTER phase 3 of ventricular AP - *another QRS before the next P-wave could happen
o Absence of P waves!
o INDEPENDENT of atrial depolarization
Damage to Purkinje system (unidirectional conduction block)
- Ischemic or congenital damage to branches
- *no spread on damaged side so (no ‘forward excitation’)
o Gets excited from the non-damaged side - SELF-EXCITATION LOOP (‘backward excitation’)
o INDEPENDENT of pacemaker inputs - *most common cause of ventricular tachycardias
Anti-arrhythmic drugs
- Grouped based on where they act
- *major effect (ignore the other effects)
- SIDE EFFECT: fatal cardiac arrhythmias
Class I: effects
- *Na-channel inhibitor
- Decrease Na current=prolongs QRS phase
o Reduces HR and cardiomyocyte excitability - *do NOT want to block all of THEM
What are Class I anti-arrhythmic drugs used for?
- Ventricular arrhythmias
o Particularly tachycardias
o Some in supraventricular arrhythmias
Class Ia: targets and effects
- All cardiac cells
o Including ventricular myocytes - *decrease maximum of AP=longer depolarization=wider QRS
- Longer QT interval
o Repolarization is NOT effect
o Due to extra QRS time
Class Ib: targets and effects
- Damaged ventricular myocytes
o Usually problems with repolarization - Doesn’t effect much as there should be lots of HEALTHY cells still
- Only effecting phase 0 in damaged cells
- QRS is similar
Class Ic: targets and effects
- All cardiac cells, some preference for conduction cells
- Profound effect on phase 0=slows rate of depolarization
o Slows spread of excitation=WIDEN QRS
o Slightly longer QT interval
Class Ia: drug examples
- *Quinidine
- Procainamide
o not used much in dogs (hard for them to activate it)
Use Quinidine to
- slow down HR, but may suppress cardiac contractility
- good to use for supra and ventricular arrhythmias
- *VERY FLEXIBLE DRUG
- Beware of side effects
Class Ib: drug example
- Lidocaine
Class II: main effect
- *beta-1 adrenergic receptor antagonist
o Decrease cAMP=decreased contractility and HR - Negative chronotrope, dromotrope and inotrope
Class II: drug examples
- Metoprolol
- Atenolol
What are Class II anti-arrhythmic drugs used for?
- Supraventricular tachycardias
o Particularly where excessive sympathetic stimulation is the cause
Class III: main effect
- *K-channel inhibitor
o Prolonged refractory period (slow down repolarization) - DELAYS time until opening, but then all open and rapidly repolarization like normal
o Prolongs plateau phase
o Reduces HR - QRS is NOT affected
- Prolonged QT interval
- *relatively safe!
Class III: other effects (not a strong)
- Block beta1 adrenergic receptors
- Block Na channels
Class III: drug examples
- Amiodarone
- Bretylium
- Sotalol
What are Class III anti-arrhythmic drugs used for?
- Refractory ventricular tachycardias
o Especially those arising from re-entry
Amiodarone
- Complex with lots of iodine’s
o Can accumulate=’silver/purple cast’ to the skin - *takes a long time to reach steady state
Class IV: main effect
- Ca-channel inhibitor
- *more of an effect in pacemakers
o Decreases contractility and HR
o Vasodilation - *don’t want it to have an effect in cardiomyocytes
o it would DECREASE CONTRACTILITY - Ca channels involved in the funny current are inhibited=takes longer for it to generate enough depolarization
What are Class IV anti-arrhythmic drugs used for?
- Supraventricular tachycardias
- *good choice in hypertrophic cardiomyopathy
Class IV: drug examples and when do you NOT use them?
- Verapamil
- Diltiazem
- **DO NOT USE IN DILATED CARDIOMYOPATHY (could decrease contractility too much)
- Can use in hypertrophic cardiomyopathy
Class II and IV effects on SA node end up looking similar (pacemaker AP=no plateau phase)
- Phase 4 (funny current: spontaneous depolarization)
o *slower rate of depolarization (longer to reach threshold=SLOWS HR
o LONGER PR INTERVAL (p-wave may look similar) - May have a stronger repolarization
- *longer PP interval (more so the ‘TP interval’)
- **relatively SAFE
