## **Core Concept**
Lignocaine (lidocaine) is a local anesthetic and antiarrhythmic drug that acts by blocking **sodium channels**. Its effects on the heart can be observed through electrocardiogram (ECG) changes, particularly at toxic levels. The therapeutic and toxic effects of lignocaine are closely related to its plasma concentrations.

## **Why the Correct Answer is Right**
The correct answer, **6 μg/mL**, is significant because at plasma concentrations above 6 μg/mL, lignocaine can start to exhibit toxic effects on the central nervous system and cardiovascular system, including ECG changes. Lignocaine's effect on the heart is primarily seen at higher concentrations where it can affect the **ventricular myocardium**, leading to widening of the QRS complex on the ECG. This is because at therapeutic levels (usually 1.5-5 μg/mL), lignocaine is effective against ventricular arrhythmias without significantly affecting the ECG. However, as levels rise above 6 μg/mL, the risk of toxicity increases, manifesting as ECG alterations.

## **Why Each Wrong Option is Incorrect**
- **Option A (1.5 μg/mL):** This is within the therapeutic range for lignocaine, where it is effective for treating ventricular arrhythmias without causing significant ECG changes.
- **Option B (3 μg/mL):** Still within the therapeutic range, where efficacy is optimal with minimal risk of toxicity or ECG changes.
- **Option D (12 μg/mL):** While this is definitely a toxic level, the question asks when ECG changes can **first** be observed, implying a lower threshold.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that lignocaine toxicity primarily affects the **central nervous system** at lower concentrations (e.g., >5 μg/mL) and the **cardiovascular system** at higher concentrations (>6-8 μg/mL), with ECG changes being a critical indicator of cardiac toxicity.

## **Correct Answer:** . **6 μg/mL**