The purpose of this study is to determine the ideal delivery of local anesthetics for postoperative analgesia when using continuous interscalene peripheral nerve catheters for shoulder surgery. New technology has emerged with electronic pumps that permit intermittent mandatory boluses of local anesthetic to be delivered in addition to continuous infusions. The pump would automatically deliver a bolus at a given interval, but would otherwise be idle for a period of time. This technology could improve the efficacy of local anesthetics, and thereby improve postoperative analgesia.~Continuous peripheral nerve catheters have been shown to improve pain control in several orthopedic surgical procedures when compared to opioids. However, it is not uncommon for patients to experience breakthrough pain in spite of a functional peripheral nerve block. Many times, this breakthrough pain can be managed with a bolus of local anesthetic administered by an anesthesia provider. Until recently, the vast majority of peripheral nerve catheter infusion pumps have only had the capability of providing a constant rate of local anesthetic administration. Newer technology allows for infusion pumps to deliver intermittent mandatory boluses of local anesthetic. This technique of local anesthetic administration has previously been evaluated with popliteal nerve blocks, but to our knowledge has never been evaluated for use with interscalene nerve catheters.~Currently, data would support the idea that a bolus of local anesthetic can improve analgesia with decreased side effects compared to a continuous infusion. However, there is little data evaluating the optimal volume of bolus, the optimal speed of delivery, or the optimal interval between local anesthetic boluses.
The purpose of this study is to determine the ideal delivery of local anesthetics for pain control when using continuous interscalene peripheral nerve catheters for shoulder surgery.~The study will be conducted in three stages. In the first stage the ideal bolus volume of ropivacaine will be determined. In the second stage the optimal rate of delivery of the bolus (infusion rate) will be determined. Finally, once both the ideal volume and infusion rate has been determined, the optimal interval between boluses of local anesthetic will be determined.