[An attempt at a mathematical prediction of the depth of insertion of the needle in peridural anesthesia].

The purpose of the investigation was to correlate the distance from the skin to the flaval ligament with the individual data of patients such as bodyweight, height, sex and age with the aim of predicting the depth of insertion of the needle. In a retrospective study we examined 448 patients (217 men and 231 women). In the anaesthetic records the depth of insertion of the needle was noted together with the patients' personal data. Evaluation of the material showed that there is no correlation of statistically significant relevance between the depth of insertion and the bodyweight and age of the patient, but there is a correlation between depth of insertion on the one hand and the variables bodyweight, overweight and the quotient body height/weight on the other. This correlation showed a somewhat higher significance in women than in men. The correlation can be expressed in a formula like y = K + fx, whereby for the relation between depth of insertion and body weight the equation for men is y = 1.87518 + 0.02761x. The equation for women is y = 2.23985 + 0.03559x. With these formulae a rough prediction for the depth of insertion can be made, with x representing the bodyweight and y the expected depth in cm. The measured values fluctuated between 3.5 and 7.4 cm in the 217 men, with the medium value at 4.08 cm. In the 231 women the measured values fluctuated between 3.3 and 6.9 cm with the medium value at 4.67 cm.