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Effect of diluent volume on analgesia produced by epidural fentanyl

Injection of opioids into the epidural space produces postoperative analgesia by acting on receptors in lamina 11, IV, and V of the substantia gelatinosa (1). Morphine, due to its low lipid solubility produces numerous side effects when injected into the epidural space; most notably delayed respiratory depression (2). Fentanyl, a highly lipid soluble narcotic agonist, has a more rapid onset of action and a significantly lower incidence of side effects than those of morphine. In a dose-response study in patients after cesarean delivery, 50 pg of epidural fentanyl was found to provide maximal analgesia with minimal incidence of side effects (3). The effect of varying the volume of injectate on the onset, duration, and incidence of side effects produced by 50 pg of fentanyl has not been reported. This standard dose of fentanyl diluted in larger volumes could increase the surface area of drug exposure and thus augment the number of opioid receptors affected. Conversely, diluting the opioid in greater volume may reduce the concentration gradient between the central nervous system and epidural space and thereby decrease both the rapidity of onset and duration of analgesia. Fentanyl is being given epidurally in several different dilutions. To determine the most effective diluent volume, a double-blind, randomized study was undertaken with varying volumes of normal saIine solution to dilute a single 50 pg dose of epidural fentanyl.