Upper dose limit of lignocaine in a 70 kg adult is
## **Core Concept**
The core concept being tested here is the pharmacokinetics and toxicology of **lignocaine (lidocaine)**, a commonly used local anesthetic. Understanding the maximum safe dose of lignocaine is crucial to prevent systemic toxicity.
## **Why the Correct Answer is Right**
The correct answer, **B. 4.5 mg/kg**, reflects the recommended maximum dose of lignocaine for local anesthesia. The standard upper dose limit for lignocaine without adrenaline (epinephrine) is **4.5 mg/kg**, not exceeding **300 mg** in total. For a 70 kg adult, the calculation would be (4.5 , text{mg/kg} times 70 , text{kg} = 315 , text{mg}), but the total dose should not exceed **300 mg** to ensure safety. This dose limit helps prevent **central nervous system** and **cardiac toxicity**.
## **Why Each Wrong Option is Incorrect**
- **Option A:** This dose seems too low for a maximum limit, as even **2 mg/kg** would only be **140 mg** for a 70 kg person, which underestimates the commonly recommended dose.
- **Option C:** **7 mg/kg** is higher than the generally recommended maximum dose for lignocaine without adrenaline, increasing the risk of toxicity.
- **Option D:** While **5 mg/kg** might seem reasonable, it exceeds the traditionally recommended upper limit of **4.5 mg/kg** for lignocaine.
## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the addition of **adrenaline (epinephrine)** to lignocaine can allow for a higher dose (**7 mg/kg**) because adrenaline causes vasoconstriction, delaying systemic absorption. However, the classic, safe upper limit without adrenaline is **4.5 mg/kg** or **300 mg total**, whichever is less.
## **Correct Answer: B. 4.5 mg/kg**