Incomplete cardiac vagal blockade with atropine in the anesthetized dog

In dogs in neurolept-anesthesia the successive administration of alpha- and beta-adrenergic blocking agents and atropine, which should cause the functional equivalent of surgical denervation of the heart, always results in a marked tachycardia. The same is observed in conscious dogs, but not during methoxyflurane anesthesia. Bilateral vagotomy and administration of hexamethonium abolish the tachycardia. These observations demonstrate the presence of a vagally mediated chronotropic effect which becomes manifest when the inhibitory vagal effect is eliminated through blockade of the muscarinic receptors with atropine.