Interscalene plexus block is a widely used technique to provide anesthesia and analgesia for surgery at the upper extremity (shoulder and upper arm); it is standard-of-care in many institutions worldwide. Local anesthetic is being injected around the nerves supplying the arm (Plexus brachialis) at a specific location in the arm (between the scalenus muscles, thus called "interscalene plexus block"). The optimal volume of injection with regard to efficacy, safety and avoidance of untoward effects has been subject to intense debate for a long time. In spite of evidence that small volumes (between 5 and 7 mls) are effective for adequate postoperative analgesia, larger volumes up to 40 mls are still frequently used in many practices. However, with the use of such large doses, adverse events are known to occur with increased frequency, including paralysis of the diaphragm or spread of local anesthetic to the spinal cord.

This study is intended to help evaluate the effects of small or larger injection of local anesthetic around the brachial plexus, and to correlate the distribution with clinical efficacy, block duration, and possible side effects. It is a randomized, controlled, observer-blinded trial; patients undergoing shoulder or upper arm surgery will be randomly allocated to receive either 5mls or 20mls of local anesthetic for their interscalene plexus block. Magnetic resonance imaging will be performed immediately afterwards, followed by a series of neurological exams during the hospital stay. Test of lung function (spirometry) and ultrasound of the diaphragm will be used to evaluate effects of the block on respiratory mechanics.

Background: Interscalene plexus blockade (ISB) is frequently used during surgery of the upper limb. The use of ultrasound has significantly reduced time of onset and total volume of local anesthetics in comparison to nerve localization by stimulators. (1-4) The introduction of this technique has been correlated with decreased volumes of effective local anesthetics from 40 milliliters and more down to 10 milliliters and less.(3-4) As the injection site of ISB is located in close proximity to the anatomical structures of the spinal cord, spread of local anesthetics could potentially cause adverse events, including contralateral blockade and total spinal anesthesia. Case reports on these complications have been published previously. (5-11)In a recently published study we showed that the phenomenon of epidural spread can be reproduced in cadavers and seems to be positively correlated to injection-volumes exceeding 10 milliliters.(12) Moreover, we observed a trend towards cranial spread of the contrast agent in all investigated specimens along the course of the nerval roots towards the spinal cord. In another cadaveric study by Orebauch et al., cranial epidural spread was also detectable following injection of dye into the nerval roots of the brachial plexus.(13) Magnetic resonance imaging has not yet been used for visualization of the spread of local anaesthetics in ISB.

Given that ultrasound provides excellent visualization of target-structures in regional anesthesia and therefore enables physicians to place injection needles with enormous accuracy, the question arises whether there is a critical volume for local anesthetics increasing complications such as epidural spread and in consequence contralateral blockade.

The aim of this study is to confirm the findings of our cadaveric study in vivo and to show that the frequency of epidural spread correlates with injection of increased volumes of local anesthetics.

Primary endpoint: Spread of local anesthetics to the epidural space

Secondary endpoints:

• Contralateral epidural spread.
• Spread of local anesthetics to the phrenic nerve.
• Bed side spirometry and ultrasound investigation of the diaphragm in the PACU.
• Oxygen saturation in the PACU
• Self-reported block duration.
• Self reported pain scores for the first 24 postoperative hours
• Time to first analgesic consumption on demand.
• Total analgesic consumption (ropivacaine PCA).

Inclusion Criteria:

• Age between 18 and 75 years
• Surgery of the shoulder (shoulder arthroscopy, open shoulder joint surgery, rotator cuff surgery, tendon transfer, shoulder arthroplasty, humerus fracture surgery)
• Patients willing to undergo magnetic resonance imaging prior to surgery
• Informed consent to participate in the study
• American Society of Anesthesiologists physical score I, II or III

Exclusion Criteria:

• Refusal to participate in the study
• Inability to understand the study protocol due to language barrier
• Serious cardiac or pulmonary disease such as decompensated heart failure, recent myocardial infarction (less than one month in the past), heart block greater than 2nd degree, obstructive sleep apnea and chronic obstructive lung disease greater than 2nd degree
• renal impairment with an calculated glomerular filtration rate below 60ml/min
• Hypersensitivity to ropivacaine or gadolinium or other contraindications against peripheral nerve blocks
• Chronic opioid usage greater than 15 mg oral morphine equivalents daily, the daily use of adjunctive pain medications (gabapentins, tricyclic antidepressants, serotonin-norepinephrine reuptake inhibitors)
• Schizophrenia or bipolar disorders, uncontrolled anxiety, claustrophobia
• Peripheral neuropathy
• Hepatic or renal impairment
• Ongoing illicit drug or alcohol abuse
• Metal implants or other contraindications for magnetic resonance imaging
• Coagulopathy
• Participation in additional clinical trials within 4 weeks before screening
• Hearing impairment
• Pregnancy