The ratio of the doses of drugs required for analgesia by epidural administration compared to IV doses for some opioids. The lower the ratio (i.e. the smaller the epidural requirement) the less likely is any major systemic drug effect contribution to analgesia and also, the less likely are systemic adverse effects.

Delayed respiratory depression is due to CSF carriage of the drug to the brain stem and is usually not related to systemic concentration.

(Modified after Chrubachik et al. Postoperative epidural opioids, Springer, Berlin, 1993)