A nurse is caring for a client at 32 weeks gestation who is experiencing preterm labor. The client has been receiving magnesium sulfate for tocolysis for the past 12 hours. Which nursing intervention is the priority at this time?

Monitoring deep tendon reflexes and respiratory rate hourly is the priority to detect early signs of magnesium sulfate toxicity, such as CNS depression and respiratory depression. Other interventions are important but secondary to preventing life-threatening complications.

Clinical Context
A client at 32 weeks gestation is in preterm labor and has been receiving magnesium sulfate for 12 hours. Magnesium sulfate is a cornerstone therapy in obstetrics, used both for fetal neuroprotection in anticipated preterm birth and as a tocolytic agent ^[2]. However, it has a narrow therapeutic window, and its use requires vigilant monitoring for maternal toxicity ^[1].

Priority Nursing Intervention
The priority intervention is to monitor deep tendon reflexes (DTRs) and respiratory rate every hour. This assessment is critical for the early detection of magnesium sulfate toxicity, which can progress from loss of reflexes to respiratory depression and cardiac arrest.

In-Depth Rationale

Magnesium sulfate acts as a central nervous system depressant and a calcium channel blocker at the neuromuscular junction. As serum magnesium levels rise, the first warning sign of toxicity is the progressive loss of deep tendon reflexes (DTRs). This occurs because magnesium inhibits the release of acetylcholine at the motor endplate, reducing muscle cell excitability ^[1]. The patellar reflex is typically lost when serum magnesium levels reach approximately 8 to 10 mEq/L (4 to 5 mmol/L).

Respiratory depression follows the loss of DTRs and is a more severe sign of toxicity, occurring at serum levels of 10 to 12 mEq/L or higher. Magnesium’s mechanism of action involves antagonizing calcium-dependent processes, and this effect on the diaphragm and other respiratory muscles leads to hypoventilation and respiratory arrest ^[1]. Therefore, hourly assessment of DTRs and respiratory rate is the standard of care for detecting toxicity before it becomes life-threatening.

Analysis of Incorrect Options
- Option 2: Encourage the client to ambulate to promote circulation. This is contraindicated. The goal of tocolysis is to stop preterm labor, which requires uterine quiescence. Ambulation can increase uterine activity. Furthermore, magnesium sulfate can cause vasodilation and muscle weakness, increasing the client’s risk for falls .
- Option 3: Administer calcium gluconate as ordered for muscle cramps. Calcium gluconate is the antidote for magnesium sulfate toxicity and should be readily available at the bedside. However, it is administered only for signs of serious toxicity (e.g., absent DTRs, respiratory depression), not for routine muscle cramps. Administering it without indication would counteract the therapeutic tocolytic effect.
- Option 4: Increase fluid intake to prevent dehydration from the medication. While maintaining hydration is important, aggressive fluid administration is not a priority and can be dangerous. Magnesium sulfate is excreted renally, and fluid overload can precipitate pulmonary edema, a known risk in patients with preterm labor receiving tocolytics. Strict intake and output monitoring is required, not encouragement of increased oral intake.

The pharmacokinetics of magnesium sulfate, which is almost exclusively eliminated by the kidneys, underscores the need for this careful monitoring. Any degree of renal insufficiency can rapidly lead to toxic serum concentrations ^[1]. The foundational safety practice for nurses managing a magnesium sulfate infusion is the hourly assessment of DTRs, respiratory rate, and urine output to ensure the drug remains within its therapeutic window and to immediately identify the onset of toxicity.