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.



(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

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