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ANALGESIA

Preinsertion Paramedian Ultrasound Guidance for Epidural Anesthesia

Denis Tran, MEng^*, Allaudin A. Kamani, MD, Victoria A. Lessoway, RDMS, Carly Peterson, MD, King Wei Hor, MASc^*, and Robert N. Rohling, PhD^*

From the *Department of Electrical and Computer Engineering, University of British Columbia; Departments of Anesthesiology and Ultrasound, British Columbia Women’s Hospital and Health Centre; and Department of Mechanical Engineering, University of British Columbia, Vancouver, British Columbia, Canada.

Address correspondence and reprint requests to Robert N. Rohling, PhD, 2332 Main Mall, Vancouver, BC, Canada V6T1Z5. Address e-mail to rohling{at}ece.ubc.ca.

Abstract

BACKGROUND: Ultrasound is receiving growing interest for improving the guidance of needle insertion in epidural anesthesia. Defining a paramedian ultrasound scanning technique would be helpful for correctly identifying the vertebral level. Finding surrogate measures of the depth of the epidural space may also improve the ease of scanning.

METHODS: We examined 20 parturients with pre-epidural ultrasound in the paramedian plane, and the predicted depth was compared with the actual midline depth. The actual depth was also compared with subject biometrics, depth of transverse process, and thickness of lumbar fat.

RESULTS: The scanning technique allowed the depth of the epidural space to be measured in all subjects. The depth measured in ultrasound was strongly correlated to the actual depth (R² = 0.8 and 95% limits of agreement of –14.8 to 5.2 mm), unlike patient biometrics (R² < 0.25), the depth of the neighboring transverse processes (R² = 0.35 and 95% limits of agreement of –13.8 to 19.1 mm), or the thickness of overlying fat (R² = 0.66). The duration of the ultrasound scan was 10 min at the beginning of the trial and 3 min for the last subjects.

CONCLUSIONS: Paramedian ultrasound can be used to estimate the midline depth to the epidural space. The surrogate measures are not sufficiently correlated with the depth to the epidural space to recommend them as a replacement for the actual depth to the epidural space measurement.