A MULTIVARIATE MODEL TO PREDICT THE DISTANCE FROM THE SKIN TO THE EPIDURAL SPACE IN AN OBSTETRIC POPULATION

Background and Objectives. Several attempts to relate epidural space d epth to individual patient parameters or details of technique have yie lded modest correlations. An attempt has been made to construct a mult ivariate model to predict the depth from the skin of the epidural spac e with use of several such factors. Methods. The depth of the epidural space from the skin was measured in 263 obstetric patients by using c alibrated needles. In 53 patients, the angle of insertion of the epidu ral needle was also measured, with protractors. For each patient, 14 d escriptors of patient habitus and anesthetic technique were recorded. Multiple regression analysis was used to construct models incorporatin g combinations of these variables to predict the epidural space depth; the model balancing the highest correlation coefficient and greatest statistical significance was selected. The validity of the model was t ested on the 53 patients in whom the angle of the needle had been meas ured but whose data have not been used to construct the model. Results . A model containing the seven variables present-weight, height, body mass index, age, parity, interspace, and patient position-yielded r =. 689, P < .0001. This model predicted the depth of the epidural space t o within 8 mm and predicted tile depth in the validation group well. C orrection for the angle of insertion of the needle did not improve the predictive power of the model. Conclusions. While this model is the b est predictor of epidural space depth yet published, it is probably no t sufficiently accurate to be clinically useful in confirming proper e pidural catheter placement. Further work in this area is probably not justified.