The maximum dose of lignocaine without adrenaline that can be admitted to a patient is
**Question:** The maximum dose of lignocaine without adrenaline that can be admitted to a patient is
A. 200 mg/kg
B. 5 mg/kg
C. 1.5 mg/kg
D. 1.5 mg/kg
**Correct Answer:** C. 1.5 mg/kg
**Core Concept:** Lidocaine (lignocaine) is a commonly used local anesthetic for peripheral nerve blocks, cardiac arrhythmias, and other medical conditions. The drug is administered in different formulations, including plain lidocaine, lidocaine with epinephrine (adrenaline), and lidocaine with epinephrine and vasopressin. When administering plain lidocaine without adrenaline, it is essential to consider the maximum safe dose to avoid potentially life-threatening complications like cardiac toxicity.
**Why the Correct Answer is Right:** The correct answer, 1.5 mg/kg, is derived from the fact that the maximum safe dose of plain lidocaine is limited by the risk of cardiac toxicity. Cardiac toxicity occurs when the plasma concentration of lidocaine exceeds 5 mg/L (500 µmol/L or 5 mM).
**Why Each Wrong Option is Incorrect:**
A. 200 mg/kg: This option is too high, reaching a plasma concentration that may lead to cardiac toxicity.
B. 5 mg/kg: Although this dose is lower than the cardiac toxicity threshold, it is still too high compared to the correct answer.
D. 1.5 mg/kg: This option is wrong because it does not account for the absence of adrenaline, which increases the plasma concentration of lidocaine by 25-50% due to the vasoconstrictor effect of adrenaline.
**Why the Correct Answer is Right (Clinical Relevance):**
Lidocaine without adrenaline is administered at a lower dose to prevent cardiac toxicity, which is a significant risk when using adrenaline-containing formulations. Vasoconstriction caused by adrenaline increases the concentration of lidocaine in the blood, thereby increasing the risk of cardiac toxicity.
**Why Each Wrong Option is Incorrect (Clinical Relevance):**
A. 200 mg/kg: Adrenaline increases the concentration of lidocaine in the blood, so using a higher dose without considering adrenaline increases the risk of cardiac toxicity.
B. 5 mg/kg: Adrenaline increases lidocaine concentration, making this option too high regardless of the absence of adrenaline.
D. 1.5 mg/kg: As mentioned earlier, the presence of adrenaline would increase the plasma concentration of lidocaine, rendering this dose too high for administering lidocaine without adrenaline.
**Clinical Pearls:**
- Always consider the influence of adrenaline on lidocaine concentration when administering plain lidocaine.
- Monitoring the plasma concentration of lidocaine helps prevent cardiac toxicity and ensures safe administration of the drug.