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Antidysrhythmic Drugs - Beta Blockers (Lecture 18)
ANTAGONISTS AT BETA1-ADRENOCEPTORS - PROPRANOLOL
Useful for both supraventricular and ventricular arrhythmias
Class 2 - cardiac β1 adrenoceptor antagonist
PRO-ARRHYTHMOGENIC ACTIONS OF SYMPATHETIC NERVOUS ACTIVITY IN
THE HEART & THEIR CONTROL BY BLOCKADE OF
Beta blockers act at multiple sites on the heart.
Activation of the sympathetic nervous system
(SNS) is one of the factors that can lead to the development of arrhythmias and beta blockers can be useful in controlling arrhythmias that have a sympathetic component.
The SNS increases the rate and force of contraction under "physiological" conditions and can also exacerbate or trigger tachycardias.
It also increases the automaticity (tendency to form ectopic pacemakers) of atrial and ventricular tissues. Beta blockers oppose these effects.
Beta blockers reduce the slope of the pacemaker potential (phase 4).
This slows the rate of discharge of the SA node.
They also slow conduction through the AV node.
This slows the sinus rhythm. ANTIDYSRHYTHMIC USES OF ANTAGONISTS AT BETA1-ADRENOCEPTORS:-
Beta blockers are particularly useful at supressing arrhythmias when there is a strong component of SNS involvement, for example arrhythmias induced by:
Excessive catecholamine release (e.g. adrenaline from a phaeochromocytoma
(adrenal medulla tumour))
Increased sensitivity to catecholamines (e.g. thyrotoxicosis (hyperthyroidism))
There are also useful in preventing arrhythmias following a heart attack (lots of pain and panic so usually strong SNS activation).
BETA-ADRENOCEPTOR ANTAGONISTS - UNWANTED EFFECTS: and so should not be used in patients with and can lead to…--
Precipitation of cardiac failure/heart block:
Patients with heart disease
Oxprenolol (partial agonist)
Mask of sympathetic sweating, tachycardia and tremor
Propranolol (not a problem with atenolol)
DRUG THERAPIES AFFECTING ION CHANNELS INVOLVED IN
CARDIAC ACTION POTENTIAL:-Cardiac rhythm (electrical activity)
Ion channel activity
Drugs used to treat dysrhythmias alter these parameters.
These drugs fall into classes I and
IV according to the VW system
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