A nurse is caring for a client who has hypomagnesemia and is receiving intravenous (IV) magnesium replacement. Which of the following findings indicates the client is experiencing magnesium toxicity?

Choice A rationale Nystagmus, or involuntary rapid eye movements, is actually a clinical sign associated with hypomagnesemia rather than magnesium toxicity. Low magnesium levels lead to neuromuscular hyperexcitability because magnesium normally acts as a calcium channel blocker; its absence allows for increased acetylcholine release at the neuromuscular junction. Normal serum magnesium levels range from 1.3 to 2.1 mEq/L. Since nystagmus reflects an irritable nervous system, it indicates that the magnesium deficiency has not yet been corrected or has become severe. Choice B rationale Kussmaul's respirations are deep, rapid, and labored breathing patterns typically seen in patients with metabolic acidosis, such as diabetic ketoacidosis. This respiratory pattern is a compensatory mechanism to blow off excess carbon dioxide and is not a characteristic finding of hypermagnesemia. In contrast, magnesium toxicity causes central nervous system depression, leading to respiratory depression or a decreased respiratory rate rather than the hyperventilation seen in Kussmaul's. This finding would point toward a different metabolic or acid-base emergency. Choice C rationale Lethargy is a hallmark sign of magnesium toxicity, reflecting the sedative effect of high magnesium levels on the central nervous system. As magnesium levels rise above the normal range of 1.3 to 2.1 mEq/L, the mineral acts as a potent depressant. This occurs because excess magnesium inhibits the release of neurotransmitters and reduces the sensitivity of the postsynaptic membrane. If levels continue to rise, this lethargy can progress to a loss of deep tendon reflexes, coma, and cardiac arrest. Choice D rationale While magnesium can affect smooth muscle motility, hypoactive bowel sounds are not the primary or definitive indicator of magnesium toxicity used in clinical monitoring. Hypermagnesemia typically causes systemic muscle weakness and vasodilation. A more classic gastrointestinal symptom of magnesium administration is diarrhea, as magnesium acts as an osmotic laxative. The nurse should prioritize assessing neurological status, deep tendon reflexes, and respiratory rate over bowel sounds when specifically monitoring for life-threatening magnesium toxicity during an active intravenous infusion.