Shoulder surgery is known to be extremely painful requiring high doses of opiate analgesics (morphine) to control pain. Morphine has numerous unwanted side effects including sedation, hallucinations and vomiting.

It is now standard procedure for patients undergoing shoulder surgery to have their pain controlled by means of an injection made around the nerves where they pass through the neck to supply the shoulder with sensation (similar to the numbing injections made by the dentist before a dental procedure). This injection is called a 'nerve block'.

Nerve blocks provide complete analgesia after shoulder surgery allowing clinicians to discharge patients home on the day of surgery, pain free and with no opiate side effects. However, a side effect of nerve blocks at this level is involvement of the phrenic nerve, which is anatomically close to the injection point. This may cause (temporary) paralysis of the diaphragm and in some cases, severe respiratory dysfunction.

Research shows that reducing either the volume or the concentration of the drug injected, can reduce the consequent respiratory dysfunction. However no study has compared both volume and concentration in parallel, to see which of these has the more significant effect in reducing respiratory dysfunction. Furthermore there has been no assessment of how these changes may affect the duration of analgesia received and patient coping after discharge.

The investigators propose to conduct a double blind randomised controlled trial at the Royal Surrey County Hospital, enrolling patients presenting for elective day case arthroscopic (key hole) surgery over a period of 6 months. Patients will receive one of four treatment allocations:

1. Low concentration-high volume of local anaesthetic
2. Low concentration-low volume of local anaesthetic
3. High concentration-high volume of local anaesthetic
4. High concentration-low volume of anaesthetic drug of local anaesthetic.

The aim of this study is to inform an optimum dosing regimen for patients in order to facilitate maximal pain relief and quality of recovery with minimum respiratory dysfunction.

Inclusion Criteria:

All patients undergoing day case shoulder surgery that would normally be managed with a nerve block for post-operative pain relief will be considered eligible for the study. Inclusion criteria are detailed below

1. Age > 18 and < 80 years
2. American Society of Anaesthetists (ASA) grades I - III. (ASA grade determines how 'fit' a patient is for surgery, with 1 being a normal healthy patient and 5 being a moribund patient that is not expected to survive. 'Fitter' patients are being chosen as it is felt that severe associated illness may influence our results).
3. A composite pain score of 0 when assessed in the post anaesthetic care unit (PACU) at 30mins post-operatively. This will be required in order for the patient to be deemed to have an 'effective' nerve block and be entered into the study. Patients reporting pain at this point will be removed from the study and offered either a rescue brachial plexus block or alternative analgesia. (Published research by Riazi et al and by McNaught et al, indicates that 5mls of local anaesthetic solution is adequate for establishing an effective ISBPB in this immediate post-operative period).

Exclusion Criteria:

1. Significant respiratory disease including Chronic Obstructive Pulmonary Disease or unstable asthma
2. Renal or hepatic impairment
3. Allergy to local anaesthetic
4. Opiod tolerance (more then 30mg of morphine or its equivalent per day)
5. Body mass index > 40
6. Body weight < 56kg (due to potentially toxic doses of local anaesthetic drug if using higher volume of 0.75% concentration at below this weight)