Pectoral nerve block (PECS) has been used for post-operative pain relief in patients undergoing breast surgery. It has been shown that adenosine has an effect on pain modulation. Magnesium sulphate has been used as an adjuvant to local anaesthetics in many nerve block techniques. We hypothesised that adenosine may affect the PECS block quality. We aimed to compare the effect of adding adenosine to the local anaesthetic and compare its effect to magnesium sulphate when used for PECS block.

The present study was conducted on 90 adult patients aged between 20 and 65 years old. Patients scheduled for modified radical mastectomy with axillary clearance surgery American Society of Anaesthetist (ASA) classes I, II, and III patients were included in the study.

Patients were randomly allocated into on of three groups according to the adjuvant used with the local anaesthetic, 30 patients in each group using a computerised program. All patients received PECS block. Group (C) received PECS block with 0.25% bupivacaine (control group), group (A) bupivacaine with added adenosine, and group (M) bupivacaine with added magnesium sulphate.

The local anaesthetic syringes were prepared by an independent anaesthetist. The researchers and the patients were blinded to the local anaesthetic adjuvant in the syringes. All patients received 30 ml local anaesthetic for PECS. The 30 ml local anaesthetic used for each patient contained bupivacaine hydrochloride 0.25%, bupivacaine hydrochloride 0.25% and 12 mg adenosine, and bupivacaine hydrochloride 0.25% and 500 mg magnesium sulphate for groups C, A, and M respectively.

Patients' demographic data were collected including age, BMI, ASA, and duration of surgery. In the post- anesthetic care unit (PACU) the visual analogue score (VAS) was assessed on arrival and then every 15 minutes. The duration and the quality of the block was recorded.

Inclusion Criteria:

• Patients scheduled for modified radical mastectomy with axillary clearance surgery ASA classes I, II, and III patients were included in the study

Exclusion Criteria:

• 1) contraindications for regional anesthesia such as coagulopathy, local infection and fungating breast cancer, 2) history of allergy to the medications used in the study, 3) patient with history of drug abuse, 4) previous breast surgery except for diagnostic biopsies, 5) history of treatment for a chronic pain condition, 6) psychiatric disorder.