Question

Which parameters are routinely monitored during magnesium sulfate maintenance therapy to detect magnesium toxicity?

• A. Respiratory rate (should be more than 14 per minute)
• B. Deep tendon reflexes (knee jerk present)
• C. Urine output (more than 100 mL in 4 hours)
• D. Serum sodium level every hour

Hint:

Think of the three checks before each dose: breathing, knee jerk and urine.

Answer 1

The Correct Option is A, B, C

Step 1: Understand why monitoring is needed:
Magnesium sulfate is used to prevent and treat eclamptic seizures. It has a narrow safety margin, so the mother must be watched for toxicity.
Magnesium is cleared by the kidneys. If urine output falls, the blood level rises quickly.
The usual levels are as follows. The therapeutic range is 4 to 7 mEq/L. The knee jerk is lost at about 8 to 10 mEq/L. Respiratory depression appears at about 12 to 15 mEq/L. Cardiac arrest can occur above 25 mEq/L.

Step 2: Recall the three bedside checks before every dose:
Before each maintenance dose, the nurse or doctor checks three things: the respiratory rate, the deep tendon reflexes and the urine output.
The next dose is given only if all three are in the safe range. If any one is not, the dose is held.

Step 3: Check statement (1) Respiratory rate:
Magnesium blocks neuromuscular transmission, so the respiratory muscles weaken. A fall in respiratory rate is a sign of toxicity.
The rate must stay at a safe level (at least 12 per minute in the standard teaching, and many wards use 14 or more as a working limit). The option states more than 14 per minute, which is within the safe range. So (1) is correct.

Step 4: Check statement (2) Deep tendon reflexes:
Loss of the patellar (knee) reflex is the earliest sign of magnesium excess. It appears before respiratory depression.
A present knee jerk shows that the level is still below the toxic range. So (2) is correct.

Step 5: Check statement (3) Urine output:
Urine output of at least 30 mL per hour, or about 100 mL in 4 hours, shows that the kidneys are clearing magnesium.
If the output falls below this, magnesium builds up and the dose must be cut or stopped. So (3) is correct.

Step 6: Check statement (4) Serum sodium level every hour:
Serum sodium has no link to magnesium toxicity, and it is not checked every hour. If a level is needed, it is the serum magnesium level, and that is checked only in selected cases such as renal impairment.
So (4) is wrong.

Step 7: Remember the antidote:
If toxicity occurs, stop the infusion and give 10 mL of 10 percent calcium gluconate slowly by the intravenous route (over 10 minutes).

Final Answer:
Respiratory rate, deep tendon reflexes and urine output are the routine checks. The correct options are 1, 2 and 3.