Epidural insertion: how far should the epidural needle be inserted before testing for loss of resistance?

The aim of this study was to determine the depth to which an epidural needle can safely be inserted before testing for loss of resistance. This ensures that the epidural needle is advanced far enough to encounter ligament thus avoiding the confusion that can occur when testing for loss of resistance whilst still in subcutaneous tissues. A survey of the distance between the skin and the epidural space in 400 obstetric patients was performed. Three hundred and ten midline epidurals showed a highly significant correlation (P<0.001) between the patient's pregnant weight and the distance to the epidural space. Using these data confidence limits were calculated in order to quantify the distance that the epidural needle could safely be inserted before testing for loss of resistance. Dividing the patient's pregnant weight by 25 will give this distance in centimetres. Seventy six paramedian approaches were performed and a less good correlation was found between the depth of the space and the woman's weight. There was no correlation with height or foot size.