A multifactorial approach to the factors influencing determination of paravertebral depth

Predicting the depth of needle insertion for paravertebral nerve blocks is currently based on patients' anthropometric characteristics, and the block level. Although preliminary studies report relationships between paravertebral depth and the body mass index (BMI), the number of observations, to date, are inadequate to formulate reliable conclusions. This study was designed to determine which factors may predict the distance between the skin and paravertebral space, for patients undergoing paravertebral nerve block procedures.

After review of peripheral nerve block procedure records, 559 surgical patients who underwent a total of 1,318 thoracic paravertebral nerve blocks for postoperative pain control, were included in the study. The patients' gender, age, weight, height, BMI, thoracic level of the performed block, and depth of the needle insertion were analyzed, using stepwise multiple regression models.

With the exception of T5, a linear relationship was established between paravertebral depth and thoracic level, with depth increasing at inferior thoracic levels. The greatest depth was observed in younger patients with higher BMIs. Depth was more variable, and more strongly correlated with age and weight, at levels T4 throughT8, compared to levels T9 through T12. Age had no influence on depth between T9 and T12. In patients who underwent multiple blocks, awareness of depth of one paravertebral space allowed more accurate prediction of the depth of the other paravertebral spaces.

Age, weight and the thoracic level influence the depth of the paravertebral space between spinal levels T4 and T8, while only the thoracic level and body weight are implicated between T9 and T12.