Among the commonly used IV induction agents (propofol, thiopentone, etomidate, and ketamine), which is the most common cause of pain on injection in clinical practice?

Explanation

## Pain on Injection Among IV Induction Agents ### Incidence and Mechanism **Key Point:** Propofol causes pain on injection in 30–80% of awake patients, making it the most common offending agent among standard IV induction drugs. Propofol's lipophilic formulation (20 mg/mL in soybean oil emulsion) activates nociceptors in the venous endothelium and perivascular tissue. The pain is often described as burning or stinging and occurs during the injection phase. ### Comparison of IV Induction Agents | Agent | Pain on Injection | Incidence | Mechanism | |-------|-------------------|-----------|----------| | **Propofol** | **Yes (most common)** | **30–80%** | **Nociceptor activation; lipophilic vehicle** | | Thiopentone | Rare | <5% | Alkaline pH; tissue irritation if extravasation | | Etomidate | Rare | <5% | Minimal vascular irritation | | Ketamine | Rare | <5% | Minimal local tissue effect | ### Strategies to Reduce Propofol-Induced Pain 1. **Pre-treatment with IV lidocaine** (40 mg) — most effective 2. **Use of larger veins** (antecubital fossa > hand veins) 3. **Slower injection rate** 4. **Dilution** of propofol (though not standard practice) 5. **Opioid pre-medication** (remifentanil, fentanyl) — reduces pain perception **High-Yield:** On NEET PG, when asked about side effects of IV induction agents, propofol's pain on injection is a classic high-yield fact. The question often pairs this with the remedy: IV lidocaine pretreatment. **Clinical Pearl:** In pediatric anesthesia, propofol pain is less common because children are already sedated before IV placement (inhalational induction first). In adults, it is a real clinical problem that requires proactive management. ### Why Other Agents Do Not Cause Pain - **Thiopentone:** Alkaline pH (pH 10–11) can cause tissue damage only if extravasated; does not cause pain during normal IV injection. - **Etomidate:** Minimal local irritation; dissolved in propylene glycol (not lipophilic oil). - **Ketamine:** Non-irritating to veins; no significant pain on injection. [cite:Gupta et al. Textbook of Anesthesia 5e]