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protective block

An incompletely understood mechanism whereby a pacemaker is protected from being discharged by the impulse from another centre; the mechanism, usually conceived as an encircling zone of unidirectionally refractory tissue permitting egress of impulses from the centre but preventing access to the centre, is seen in operation in ventricular parasystole where the parasystolic centre is protected from discharge by the sinus pacemaker and so is able to maintain its intrinsic rhythm undisturbed.

Synonym: entrance block, protection.