Which of the following is the earliest sign of magnesium toxicity in a patient with eclampsia on treatment with magnesium sulphate?
**Core Concept**
Magnesium toxicity can occur in patients with eclampsia treated with magnesium sulphate, and early recognition is crucial to prevent severe complications. Magnesium sulphate works by inhibiting neuromuscular transmission and stabilizing neuronal membranes.
**Why the Correct Answer is Right**
The earliest sign of magnesium toxicity is a decrease in the depth and rate of respiration, often manifesting as a respiratory rate of less than 10-12 breaths per minute. This occurs because magnesium competes with calcium ions for receptor sites on the neuromuscular junction, leading to muscle weakness and eventually respiratory depression. As the magnesium levels increase, the respiratory depression progresses, and the patient may exhibit a respiratory rate of less than 5-6 breaths per minute.
**Why Each Wrong Option is Incorrect**
**Option A:** Muscle weakness and flaccidity are indeed signs of magnesium toxicity but are not the earliest sign. These symptoms occur as the magnesium levels continue to rise and the neuromuscular transmission is further inhibited.
**Option B:** ECG changes are a sign of magnesium toxicity but are not the earliest sign. These changes include a prolonged QT interval and a widened QRS complex.
**Option C:** A decrease in blood pressure is a sign of magnesium toxicity but is not the earliest sign. Magnesium causes vasodilation, leading to a decrease in blood pressure.
**Clinical Pearl / High-Yield Fact**
In patients with eclampsia treated with magnesium sulphate, it is essential to monitor the respiratory rate closely, as a decrease in respiratory rate can be an early sign of magnesium toxicity.
**Correct Answer:** C. A decrease in the respiratory rate.