What is a Loss of Resistance (LOR) Syringe?
A Loss of Resistance (LOR) syringe is a specialized medical device used to identify the epidural space during epidural anesthesia by detecting the sudden loss of resistance that occurs when the needle passes through the ligamentum flavum into the epidural space.
Technical Description and Mechanism
- The LOR syringe is the most commonly preferred method for identifying the epidural space during epidural anesthesia placement 1
- The technique works by applying continuous pressure to the syringe plunger while advancing the Tuohy needle through tissue layers; when the needle tip enters the epidural space, there is a sudden and characteristic loss of resistance to injection 2, 3
- The syringe can be filled with either air or saline (liquid), each with distinct advantages and disadvantages 2
Types of LOR Syringes
Standard LOR Syringes
- Traditional glass or plastic syringes used with manual pressure application 2
- Require continuous operator pressure and tactile feedback to detect the loss of resistance 3
Advanced LOR Devices
- Spring-loaded syringes (e.g., Episure): Contain a coaxial compression spring that supplies constant pressure automatically, providing a more objective endpoint for epidural space identification 3
- Dual-chambered syringes (e.g., EpIdent): Feature two separate chambers—one for air and one for liquid—combining the advantages of both media while avoiding their individual disadvantages 2
- Pressure-monitoring devices (e.g., CompuFlo®): Measure tissue pressure in real time at the needle orifice, displaying pressure changes that help differentiate false from true loss of resistance 4
Clinical Significance and Safety
- Correct identification is critical: Incorrect identification of the epidural space using the LOR technique can lead to catheter misplacement and epidural failure 5
- False loss of resistance: Can occur when the needle passes through other tissue planes, potentially leading to inadequate or failed epidural blocks 4
- The LOR technique has been shown to provide a 2 mm greater posterior epidural space depth measurement compared to the hanging drop technique, suggesting a diminished risk of dural damage 6
- When using pressure-monitoring LOR devices, the drop in pressure associated with true epidural space identification (73%) is significantly greater than that recorded after false loss of resistance (33%) 4
Common Pitfalls
- Inexperienced providers have significantly higher rates of epidural failure and complications when using LOR technique 5
- Night shift placement (19:00-08:00) carries 6.33 times higher risk of inadvertent dural puncture, likely due to provider fatigue 5
- Negative aspiration through the catheter does not entirely preclude misplacement, as catheter aspiration is not completely reliable 5