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2 hormones that affect the rhythm of the heart
acetylcholine and noradrenaline
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in SA and AV node, pacemaker cells have a __ conductance which gradually decreases, resulting in depolarization --> an action potential
k+
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how do vagal fibers make the heart beat slower (what do they release, what does it do?)
release acetylcholine onto muscarinic receptors -- they increase the K+ conductance, so it takes longer to get the K+ low enough to initiate an action potential
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how do anti-arrhythmic drugs like atropine keep the heart steady?
block the vagal influence on the AV node
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sympathetic fibers release __ onto __ receptors in the pacemaker tissues and myocardium to decrease K+ conductance, making it easier to reach threshold --> faster HR
noradrenaline onto beta-1
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noradrenalin decreases K+ conductance to give a faster HR. It also increases contraction force. How does it do that?
by increaseing Ca++ conductance (positive inotropic effect)
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sinus bradycardia, an arrhythmia common after acute MI, is treated with __
atropine
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drugs effective in supraventricular arrhytmia (occuring in atrium or AV node) are called __, their action is __
- Class IV agents
- block Ca++ channels
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2 drugs for supraventricular arrhythmia
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verapamil
- for supraventricular arrhythmia - class IV
- powerful agaist AV node where conduction is totally dependent on Ca++ conductivity
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digitalis
- for supraventricular arrhythmia - class IV
- slows conduction, prolongs refractory period in AV node and bundle of His
- doesn't stop atrial fib but slows it and strengthens ventricular beat
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drugs used in ventricular arrhythmia -- their names? their class name?
class IB - they block voltage-dependent Na+ channels
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lidocaine - given how?
only given i.v.
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lidocaine - works by what mechanism?
blocks inactivated Na+ channels
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lidocaine - treats what? what class is it?
- for ventricular arrhythmia
- class IB
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mexiletine
- for ventricular arrhythmia - class IB
- similar to lidocaine, but can be given orally and has more side effects
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class IA drugs - 2 names, do what?
- disopiramide
- quanidine
- fight both types of arrhythmia by blocking open voltage dependent Na+ channels
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drugs for both supra and ventricular arrhythmias, and their class name
- disopiramide -- class IA
- quanidine -- class IA
- amiodarone -- class III
class IA -- block open voltage-dependent Na+ channels - class IB, which treat ventricular arrhythmia, only block those not already open
class III - slow heart repolarization and prolong the duration of action potentials
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disopiramide - admin how?
oral
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disopiramide - treats what?
both types of arrhythmias -- so it's a class IA agent
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disopiramide - side effects
HoTN, nausea, vomiting
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disopiramide
- fights both types of arrhythmias
- class IA agent
- oral administration
- used in combo with digitalis
- may cause HoTN, nausea, vomiting
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class III agents - treat what, basic description, name one
- both types of arrhythmia
- slow heart repol and prolong duration of APs
- amiodarone
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quanidine
- class IA - treats both types of arrhythmia
- used for arrhythmias resistant to other drugs
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amiodarone - basic description, side effects
class III, treats both kinds of arrhythmia by prolonging the effective refractory period, esp of Purkinje fibers and ventricular muscle cells
liver damage, neuropathy -- so it's a last choice med
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3 alternatives to drugs for treating arrhythmias
- pacemaker
- surgical ablation of the ectopic focus
- ablation of the ectopic focus via electrodes
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inotrope
- an agent that alters the force or energy of muscular contractions
- negatively inotropic agents weaken the force of muscular contractions
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pacemaker, used when?
- complete heart blocks
- tachyarrhythmias
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surgical ablation of ectopic focus is successful when?
supraventricular arrhythmias
which would otherwise be hit by class IV agents - verapamil or digitalis
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ablation of ectopic focus via electrodes
"on an intracardiac catheter (endocavity ablation)"
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