The Effect of Local Anaesthetic Volume on Nerve Block Duration and Nerve Block Duration Variability

The trial is divided into two similar phases. Each phase will focus on a specific nerve. Phase 1 will focus on the common peroneal nerve and Phase 2 will focus on the sciatic nerve.

After baseline measurements, the investigators will insert a peripheral intravenous catheter. Then, the investigators will insert a suture-method peripheral nerve catheter (Certa CatheterTM). Procedures will be done in a sterile manner as a standard of practice. The investigators will use an ultrasound (US)-guided short-axis, needle in-plane technique. For the US-scan, the investigators will use a linear or a curvilinear transducer.

During interventions, volunteers will be monitored with continuous pulse oximetry. Placement of the peripheral nerve catheter will be facilitated by US using small injections of mepivacaine (carbocaine 2 %) in the skin and surrounding tissues and only isotonic saline (5 mL) in the perineural space to prevent blocking of the nerve before ropivacaine injection. The catheter orifice will be placed by pulling either end of the catheter and guided by the built-in echogenic markings seen on US. Before injection of ropivacaine, the investigators will do careful aspirations through the catheter in order to prevent intravasal injection.

Each ropivacaine injection will be administered via an infusion pump. This will ensure a constant infusion rate set to 10 mL per minute. During infusion, the volunteers will be monitored with continuous pulse oximetry.

Before instigation of the ropivacaine infusion, the investigators will use US to verify full absorption of the isotonic saline in the perineural space.

For each nerve, the subject will be randomly allocated to and receive one of five possible ropivacaine volumes. All volunteers and outcome assessors will be blinded to the ropivacaine administrations and infusion procedures. Outcome assessors will not be in the room, when the medication is prepared nor given. Preparation will take place behind a curtain and therefore also blinded to the volunteers.

Inclusion criteria

• Male or female 18-64 years
• American Society of Anesthesiologists Classification ≤ II

Exclusion criteria

• Body Mass Index ≤ 18 or ≥ 30
• Former surgery to the lower extremities
• Peripheral nerve disease
• Allergy to ropivacaine
• Pregnancy or breastfeeding
• Enrolment in other investigational drug studies or recent clinical trials that may interfere with this study
• Habitual use of any kind of analgesic treatment
• Anatomic abnormalities preventing successful US-guided peripheral nerve catheter insertion