The goal of this clinical trial is to compare the effects of single-level and bi-level erector spinae plane block in open inguinal hernia repair surgery. The main question it aims to answer are:

• Whether bi-level ESP block will improve pain control after open inguinal hernia repair surgery
• Whether bi-level ESP block will improve quality of recovery after open inguinal hernia repair surgery

Participants will receive erector spinae plane block, and will be randomised into 2 groups, single-level ESP block and bi-level ESP block, before open hernia repair surgery. They will be followed up after operation for assessment of pain control and quality of recovery.

Open inguinal hernia repair is increasingly performed as a day case surgery. As a result, satisfactory acute pain control is very important to reduce discomfort, as well as to facilitate early mobilisation and recovery. Regional anaesthesia, eg. erector spinae plane block (injection below para-spinal muscles) is increasingly implemented as part of multi-modal analgesia to establish better post-operative pain control and spare the use of opioids which can lead to unwanted side effects eg. sedation, nausea and vomiting. Acute post-surgical pain from open inguinal hernia repair consists of subcutaneous, deep somatic and visceral components. The erector spinae plane (ESP) block acts by local anaesthetic (LA) spread to ventral and dorsal rami of spinal nerves, producing somatic and visceral pain relief. Therefore, it can effectively relieve acute post-surgical pain resulted from open hernia repair. Bi-level ESP block has been utilised clinically to provide multi-dermatomal analgesia in pain management eg. flail chest; or anaesthetic management eg. open inguinal hernia repair, scoliosis surgery, mastectomy. However, to date, there have been no studies comparing the analgesic efficacy of bi-level and single-level erector spinae plane blocks. The investigators postulate that comparing with single-level ESP block, bi-level ESP block can facilitate LA spread into paraspinal areas more effectively, producing more reliable analgesia; and therefore would reduce post-operative pain scores and improve quality of recovery in patients undergoing open inguinal hernia repair.

To investigate the above clinical question, a parallel-group observer-blinded randomised clinical trial was designed. Patients will be randomised into 2 groups. One group of patients (2ESP) will receive ESP block at ipsilateral T12 and L1; while the other group of patients (1ESP) will receive ESP block at ipsilateral L1. L1 was chosen as an injection level, since it is at the midpoint of corresponding spinal nerve roots innervating the groin (T12-L3). For bi-level injection group, thoracic instead of another lower lumbar level was chosen to avoid motor blockade caused by excessive local anaesthetic spread into lumbar plexus, leading to delayed mobilisation which is undesirable for patients undergoing ambulatory surgery.

Inclusion Criteria:

• Patients above 18 years old
• ASA (American Society of Anesthesiologists) class I to III
• Unilateral open inguinal hernia repair

Exclusion Criteria:

• Patient refusal or unable to consent to study
• Contraindication of ESP block including patient refusal, injection site infection, coagulopathy, hypersensitivity to local anaesthetic
• Pre-existing chronic pain
• Alcohol and substance dependence
• Pre-existing psychiatric diseases