Tachycardia-dependent atrioventricular (AV) block is affected by the atrial heart rate.
Synonym: phase 3 block
It is caused by a combination of:
(1) prolonged refractory period in the His-Purkinje system
(2) repetitive concealed penetration of supraventricular impulses into the blocking zone
It may be preceded by episodes of type 2 second degree AV block.
Occurrence:
(1) after acute myocardial infarction
(2) without known structural heart disease
AV conduction is:
(1) worsened by atropine (due to acceleration of the atrial rate)
(2) improved by carotid massage (reduces sinus rate)
(3) beta blockers (reduces sinus rate; may causes AV nodal block suppressing the concealed penetration of the atrial impulses)
Increase in the atrial rate can result in:
(1) complete AV block with prolonged asystole
(2) syncope
