Factors Contributing to the Pressure Wave Form Changes

Loss of resistance (LOR) is the most commonly used method to confirm the epidural space. The advantage of LOR is its simplicity; only saline or air filled syringe is required. LOR is felt through the sudden decrease of pressure and this pressure gradient is generated when the needle is within the passage of interspinous ligament, ligamentum flavum and epidural space. The presence of ligamentum flavum is crucial for the identification of epidural space by LOR. However, gaps in ligamentum flavum, paravertebral muscle and cyst in interspinous ligament can modify this passage and a false LOR is generated consequentially. The false positive rate of the lumbar and cervical area was reported to be 8.3~17% and 30~68%, respectively. If the false positive rate is high, repeated attempts of epidural steroid injection (ESI) are required, with additional discomfort or pain to the patient.

The high rate of false LOR has prompted the design of adjunctive modalities. Among these, epidural pressure waveform analysis (EPWA) using pressure transducer has been reported. If the epidural needle or catheter is positioned accurately in the epidural space, a pulsatile wave coinciding with arterial pulsations can be seen through the monitor.

Recent study suggested that significant abrupt pressure decrease occurs when cervical epidural injection was done via paramedian approach rather than midline.