What is the expected side effect of rapid IV administration of phenytoin?

Rapid intravenous administration of phenytoin can lead to cardiac dysrhythmias, particularly due to the drug's effects on cardiac conduction. Phenytoin, an antiepileptic medication used to control seizures, can depress sinoatrial node activity and may affect atrioventricular conduction, which can result in arrhythmias. The risk increases when phenytoin is administered too quickly, as this creates a surge of the drug in the bloodstream, which can impact heart rhythm.

The other options, while they may be side effects of phenytoin, are not directly related to the rate of intravenous administration. Ataxia, peripheral neuropathy, and drowsiness can occur with long-term use or at therapeutic levels but are not acutely induced by rapid IV administration. Therefore, cardiac dysrhythmia is the side effect most closely linked to the speed of phenytoin infusion.