The relationship among cardiac pacemakers. Overdrive suppression.

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Abstract
Cardiac pacemakers are suppressed temporarily when driven at a fast rate by a faster pacemaker or by applied stimuli. This overdrive suppression of atrial pacemakers involves the release of acetylcholine, although a residual factor, possibly K accumulation outside the cell membrane, may play a role. The suppression of the sinus node by drive applied to distant parts of the heart (such as the venticles) suggested to Lu, et al. the interesting concept that propagated action potentials themselves can liberate neuromediators.