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ANTIARRHYTHMICS
Dr. RAGHU PRASADA M SMBBS,MDASSISTANT PROFESSOR DEPT. OF PHARMACOLOGYSSIMS & RC.
Normal Sinus Rhythm
Heart rhythm is determined by SA node = Cardiac Pacemaker
Sinus rhythm Specialised pacemaker
cells spontaneously generate APs
APs spread through the conducting pathways
Normal sinus rate 60-100 beats/min
Cardiac Action Potential
Divided into five phases (0,1,2,3,4)• Phase 0 – rapid depolarization • Phase 1 – early repolarization• Phase 2 – plateau phase• Phase 3 – rapid repolarization • Phase 4 – resting phase, diastolic depolarization
0 1 2 3 4
• Effective refractory period• absolute refractory period• relative refractory period
1
0
2
3
4
ARP RRP
Cardiac Action Potential
What is an Arrhythmia ?
Irregular rhythm
Abnormal Rate
Conduction abnormality
What causes an arrhythmia?
Changes in automaticity of the PM Ectopic foci causing abnormal APs Reentry tachycardias Block of conduction pathways Abnormal conduction pathways (WPW) Electrolyte disturbances and DRUGS Hypoxic/Ischaemic tissue can undergo spontaneous
depolarisation and become an ectopic pacemaker
ECG showing
wave segments
Contraction of atria
Contraction of ventricles
Repolarization of ventricles
Vaughan-Williams Classification
Class Mechanism Example
I Na channel blockersMembrane Stabilisers
Lignocaine
II Beta Blockers Metoprolol
III K channel blockers Amiodarone
IV Ca channel blockers Verapamil
Other Digoxin. Adenosine.MgSO4. Atropine
Class I
IA IB IC
They ↓ automaticity in non-nodal tissues (atria, ventricles, and purkinje
fibers)
They act on open Na+ channels or
inactivated only
“use dependence”
Have moderate K+ channel blockade
IA – Quinidine, Procainamide, Disopyramide Slowing the rate of rise in phase 0 They prolong action potential & ERP ↓ the slope of Phase 4 spontaneous depolarization ↑ QRS & QT interval Slow rate of dissociation with open Na+ channels
Vmax
APD
Antimalarial, antipyretic, skeletal muscle relaxant & atropine like action.
A/E ▪ quinidine syncope from
ventricular tachycardia▪ Diarrhoea▪ “Cinchonism” – tinnitus,
vertigo, headache, nausea & blurred vision.
200-400 mg orally tds
C/I
AV block
QT prolongatio
n- Torsades de pointes Digoxin,
enzyme inducer
Myasthenia gravis
IA – QUINIDINE
IB – Lidocaine, Mexiletine, Phenytoin & Tocainide They shorten Phase 3 repolarization ↓ the duration of the cardiac action
potential Prolong phase 4 They show rapid association &
dissociation with inactiated Na+ channels
Vmax
APD
IB – Lidocaine
Used IV because of extensive 1st pass metabolism No vagolytic effects Least cartiotoxic CNS side effects LD – 150-200mg for 15mins MD – 1-4mg/min Used for VT Propranolol ↑ its toxicity
IC – Flecainide, Encainide, Propafenone & moricizine
markedly slow Phase 0 depolarization slow conduction in the myocardial tissue minor effects on the duration of action potential and
ERP reduce automaticity by increasing threshold
potential rather than decreasing slope of Phase 4 depolarization.
Vmax
«APD
Class II drugs – Propranolol, Metoprolol, Esmolol, Acebutolol
Depress phase 4 depolarization
depress automaticity prolong AV conduction
↑ ERPProlong PR interval HR
contractility
Class II drugs
Propranolol Esmolol
Resistant v arrhythmia SVT
10 – 80 mg TDS LD 500mg / kg / min for 1 min
1 – 3 mg in 50ml 5%D – 1 min MD 50mg / kg / min for 4 min
Contraindication
Asthma
Sinus Bradycardia
AV block
Severe CHF
Class III drugs - Amiodarone , Dronedarone, Vernakalant, Ibutilide, Bretylium, Dofetilide
K+ channel blockers AP / ERP without
affecting Phase 0 / 4 Prolong QT & PR APD
Amiodarone
Iodine – containing
Block K+ Na+ , Ca++
& β
HR & AV nodal
conduction
QT prolongatio
n
Uses =VF, VT & AF
Arrhythmic
death in post MI
LD-150mg slow IV
MD-1mg/min for
6hrs
A/E – heart block, pulmonary, hepatitis, dermatitis, corneal
deposits & thyroidism
Interaction – digoxin, diltizem & quinidine
class III
Dronedarone-Without iodine, short t1/2, AF Oral 400mg twice daily
Vernakalant-Na+ & K+, atrial ERP, A/D faster, AF
Azimilide-Block both rapid & slow k+ channel
Tedisamil
Class IV – Verapamil, Diltiazem
Mechanism-block L-type calcium channels.• Rate of phase 4 in SA / AV node• Slow conduction – prolong ERP• Phase 0 upstroke
Verapamil
Stronger action on heart than smooth muscle Used in supraventricular arrhythmia 80-120mg three times a day A/E – ankle oedema, constipation C/I – AV block, LVF, hypotention & WPW It digoxin toxicity
Uses
Sympathetically mediated
arrhythmia
Sinus tachycardia
AES
Supraventricular arrhythmia – AF /
PSVT
Ventricular arrhythmia – QT
VPC WPW
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