The epidural space is deeper in elderly and obese patients in the Japanese population

Epidural anaesthesia is an efficient method of providing both regional anaesthesia and post-operative pain relief. Detection of the epidural space is critical, but it is difficult to predict the depth of the epidural space. Published results are inconsistent. We retrospectively investigated the differences in the depth of the epidural space depending on the puncture site, approach type and physical findings of patients.

All surgical patients from 1994 to 2005 were included in the study and 4964 cases were analysed. The recorded distance from the needle tip to the skin surface was defined as the depth of the epidural. The differences in the depth were compared according to the puncture site, level and approach. We then searched for the factors determining the depth using multivariate regression analysis.

The depth in upper thoracic sites (T1/2-T9/10) was 5.0 (1.0) cm [mean (SD), n= 465] via the midline approach and 5.2 (1.0) cm (n= 1226) via the paramedian approach. For lumbar sites, the depth was 4.1 (0.9) cm (n= 1835) via the midline approach and 4.6 (1.0) cm (n= 298) via the paramedian approach. In the multivariate regression analysis, patient age, body weight and more cephalad puncture were significantly and positively correlated with the depth at thoracic sites.

The epidural space was deeper at upper thoracic sites than lower thoracic or lumbar sites. The depth with the paramedian approach was greater than with the midline approach at both sites. Patient age and weight were positive factors for depth.