A 52-year-old man is scheduled for lower limb orthopedic surgery under epidural anesthesia. After needle insertion at L3–L4 interspace and advancement, the anesthesiologist encounters resistance and is uncertain whether the epidural space has been entered. Which investigation is the most appropriate to confirm epidural space placement?

## Confirmation of Epidural Space Entry **Key Point:** Loss of resistance (LOR) is the gold standard and most practical bedside technique for confirming epidural space entry during needle advancement. ### Mechanism of LOR The epidural space is bounded by the ligamentum flavum posteriorly. As the needle penetrates the ligamentum flavum and enters the epidural space, the resistance suddenly decreases. This sudden change in resistance is detected by the anesthesiologist's hand as a loss of resistance when injecting air or saline through the needle. ### Why LOR is the Investigation of Choice | Feature | LOR | Fluoroscopy | Ultrasound | CT | |---------|-----|-------------|-----------|----| | **Real-time feedback** | Yes (tactile) | Yes (visual) | Yes (visual) | No | | **Bedside applicability** | Excellent | Requires equipment | Increasingly used | Not bedside | | **Cost** | Minimal | Moderate | Moderate | High | | **Radiation exposure** | None | Yes | None | Yes | | **Gold standard status** | Yes | Alternative | Emerging | Not standard | | **Immediate confirmation** | Yes | Yes | Yes | Delayed | **High-Yield:** LOR with air or saline is the universally taught, most practical, and most commonly used method to confirm epidural space entry. It requires no additional equipment beyond the syringe and needle already in use. **Clinical Pearl:** The anesthesiologist typically uses a 10 mL syringe filled with air or saline and applies steady pressure while advancing the needle. The sudden loss of resistance indicates entry into the epidural space where there is negative pressure relative to the ligamentum flavum. ### Technique Variants - **LOR to air:** Faster, more tactile feedback; risk of air embolism if injected into blood vessel (rare) - **LOR to saline:** Safer; slightly less tactile feedback; preferred in many centers **Warning:** Absence of LOR does not always mean the epidural space has not been entered—in some patients, especially those with obesity or degenerative changes, resistance may be minimal. Clinical judgment and additional signs (e.g., catheter advancement, test dose response) are essential. [cite:Gupta Textbook of Anesthesia Ch 28]