Which of the following agents does not elicit pain on injection?

Explanation

## Correct Answer: D. Ketamine Ketamine is the only IV anesthetic agent among the options that does NOT cause pain on injection. Unlike propofol, thiopentone, and etomidate—which all trigger venous pain through direct activation of nociceptors in the vessel wall—ketamine has minimal to no local irritant properties. The pain with other agents is mediated by activation of C-nociceptors and release of inflammatory mediators (bradykinin, substance P) in the endothelium. Ketamine's chemical structure and pH do not produce this irritant effect. In Indian clinical practice, when IV anesthesia is administered via peripheral veins (common in resource-limited settings), ketamine remains a preferred choice partly because it avoids the need for premedication with opioids or lidocaine to prevent injection pain. This is particularly valuable in emergency and trauma scenarios where rapid induction is needed without additional drug administration. Ketamine also preserves airway reflexes and cardiovascular stability, making it ideal for induction in hemodynamically unstable patients—a frequent scenario in Indian tertiary care and trauma centers. ## Why the other options are wrong **A. Etomidate** — Etomidate causes significant injection pain (reported in 30–80% of patients) due to direct activation of venous nociceptors. Pain is more common with peripheral IV administration and smaller veins. In Indian practice, etomidate is often used with lidocaine premedication or opioid co-induction to mitigate this. The pain is a major drawback despite its excellent hemodynamic stability. **B. Thiopentone** — Thiopentone causes injection pain in 5–10% of patients, though less frequently than propofol or etomidate. The pain is due to its alkaline pH and direct irritation of the vessel wall. Thiopentone is now rarely used in India due to regulatory restrictions and availability issues, but historically it was a common cause of injection pain when used for rapid sequence intubation. **C. Propofol** — Propofol is notorious for causing injection pain in 50–80% of conscious patients—the highest incidence among IV anesthetics. The lipid emulsion formulation irritates venous endothelium and activates local nociceptors. Indian anesthesiologists routinely use lidocaine premedication, opioid co-induction, or larger central veins to prevent propofol injection pain, especially in awake patients. ## High-Yield Facts - **Ketamine** does not cause injection pain because it lacks direct nociceptor-activating properties, unlike propofol (50–80%), etomidate (30–80%), and thiopentone (5–10%). - **Propofol injection pain** is mediated by activation of C-nociceptors via the lipid emulsion; prevented by lidocaine, opioids, or larger veins. - **Etomidate** causes pain due to direct endothelial irritation; excellent hemodynamic stability makes it useful in shock despite pain liability. - **Ketamine** preserves airway reflexes and cardiovascular stability—preferred for induction in hemodynamically unstable and trauma patients in Indian emergency settings. - **Thiopentone** is now rarely available in India; historically caused pain via alkaline pH and direct vessel irritation. ## Mnemonics **PECK: Painful Induction Agents** **P**ropofol (50–80% pain), **E**tomidate (30–80% pain), **C**ompound (thiopentone, 5–10% pain), **K**etamine (NO pain). Use this to remember which agents cause injection pain and which don't. **Memory Hook: 'Ketamine Keeps It Cool'** Ketamine is the only IV anesthetic that does NOT irritate veins on injection—it 'keeps it cool' (no inflammation, no pain). All others need premedication or tricks to prevent pain. ## NBE Trap NBE may pair this question with scenarios describing "injection pain prevention strategies" (lidocaine, opioids, larger veins) to lure students into selecting propofol or etomidate as the "correct" agent when the question explicitly asks which agent does NOT cause pain. The trap is confusing "how to prevent pain" with "which agent avoids pain inherently." ## Clinical Pearl In Indian emergency departments and trauma centers, ketamine is often the induction agent of choice for hemodynamically unstable patients—not only because it preserves airway reflexes and blood pressure, but also because it can be administered via peripheral veins without the need for additional pain-prevention measures, saving precious minutes in life-threatening scenarios. _Reference: KD Tripathi Essentials of Medical Pharmacology Ch. 10 (IV Anesthetics); Harrison Principles of Internal Medicine Ch. 373 (Anesthesia); Bailey & Love Short Practice of Surgery Ch. 11 (Anesthesia)_