Antia-Arrythmic Drugs

Anti-arrhythemic Drugs

• Normal heart rhythm is generated & driven by the spontaneous
firing of pacemakers cells of the SA node (post. wall of the RA)
• These cells have an intrinsic firing rate of 100-110 depolarization / sec
• The intrinsic rate is under the control of ANS, (+ or -)
• At rest, the SA rate, (HR in sinus rhythm), 60-80 b/m below intrinsic firing rate).
• This reduced resting rhythm is due to vagal tone
• Efferent vagus nerve fibers innervating the SA node normally have a high level of activity under resting conditions.
• These nerves (Ach binds to M2 rec. on the SA nodal cells to cause the decrease in firing rate.
• The SA node is also innervated by sympathetic fibers AN (NE), binds principally to B1-in SA nodal cells. When the activity of sympathetic efferent nerves is increased, the SA node firing rate increases.
• Normally, there is a reciprocal relationship between the parasym (vagal) & sym influences acting on the SA node
In normal sinus rhythm:
• the impulses generated by the SA node travel through the atria and converge at the AV node where the speed of conduction is reduced to give the atria sufficient time to contract & empty their contents into the ventricles prior to ventricular contraction.
• Impulses from the AV node travel into the ventricles via the Bundle of His, and then branch into the left and right bundle branches, the terminal Purkinje fibers, and finally are conducted to the ventricular myocytes.
• Because of this spread of electrical activity from the atria to the ventricles, every atrial depolarization & contraction is normally followed by ventricular depolarization & contraction.
• Normally : one-to-one correspondence between atrial & ventricular depolarization & contraction.