This study aims to compare two medications, dexmedetomidine and fentanyl, when used alongside a commonly used spinal anesthetic called bupivacaine in patients undergoing lower limb surgery. The main focus of the study is to see which combination causes fewer problems with urination after surgery - a condition known as postoperative urinary retention (POUR). POUR can lead to discomfort, delayed recovery, and a need for catheterization. Dexmedetomidine is a newer drug that may help reduce this problem compared to fentanyl, which is more commonly used but may increase the risk of urinary retention. This trial will help determine which medication combination provides better pain relief while reducing urinary side effects. The study is being conducted as a randomized, double-blind trial at a hospital in Nepal and will include 190 adult patients.

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Spinal anaesthesia is a common and effective method used to manage pain during lower limb surgeries. It allows patients to remain awake and comfortable while providing excellent pain control. To make spinal anaesthesia last longer and provide better pain relief, certain medications called adjuvants are added to the main anaesthetic drug, bupivacaine. Two commonly used adjuvants are fentanyl, an opioid, and dexmedetomidine, a medication that works on the body's alpha-2 receptors to provide sedation and pain relief.

However, a known side effect of spinal anaesthesia especially when combined with certain adjuvants is postoperative urinary retention (POUR). This condition occurs when a patient is unable to urinate normally after surgery. POUR can be uncomfortable, may delay recovery, and often requires insertion of a urinary catheter, which can increase the risk of infection and hospital stay.

This clinical trial, called the D-FAB POUR Trial, is designed to find out which of these two adjuvants dexmedetomidine or fentanyl is more likely to cause or prevent POUR when used with bupivacaine in patients undergoing elective lower limb surgery.

The study will involve 190 adult patients. Each participant will receive spinal anaesthesia with bupivacaine and be randomly assigned to also receive either dexmedetomidine or fentanyl. Neither the patients nor the doctors will know which medication is given in a process known as double-blinding to ensure fairness and eliminate bias.

The researchers will monitor the ability of patients to urinate at the third and sixth hours after surgery, using ultrasound to measure bladder volume. If the patient is unable to pass urine and the bladder is overly full, temporary or longer-term catheterisation may be needed. These outcomes will help determine how each drug affects bladder function.

In addition to urinary outcomes, the study will also look at side effects like nausea, vomiting, itching, changes in blood pressure or heart rate, sedation level, and how much extra pain relief the patient needs after surgery.

The goal of this research is to help doctors choose the best adjuvant to use in spinal anaesthesia, one that provides effective pain control without increasing the risk of urinary problems. The findings may lead to improved safety, comfort, and recovery for patients undergoing lower limb operations.

Inclusion Criteria:

• Adults aged 18 to 60 years
• Scheduled for elective lower limb surgery under spinal anesthesia
• Classified as American Society of Anesthesiologists (ASA) physical status II or III
• Minimum height of 150 cm (to ensure uniform drug volume)
• Able to understand the study procedures and provide written informed consent (in English or Nepali)

Exclusion Criteria:

• Contraindications to spinal anesthesia (e.g., coagulopathy, infection at injection site, hemodynamic instability)
• Known allergy or hypersensitivity to bupivacaine, fentanyl, or dexmedetomidine
• Chronic pain conditions or long-term opioid use
• Neurological, Cardiac, Renal, or uncontrolled diabetic conditions
• Current treatment for prostatic pathology or use of alpha-2 blockers
• Presence of a Foley's catheter preoperatively
• Estimated surgical duration: >2.5 hours
• Inability or unwillingness to provide informed consent

De-identified individual participant data (IPD) related to primary and secondary outcomes (including urinary retention, adverse events, and analgesic requirement) will be made available upon reasonable request from qualified researchers. Data will be shared after publication of the main study findings. Access will require a data use agreement and approval by the principal investigator and ethical committee.

6 months after publication of the primary study results in a peer-reviewed journal.

Who will have access:

Qualified researchers, academic collaborators, and clinicians involved in similar fields of study may request access to the de-identified individual participant data (IPD).

What will be accessible:

De-identified datasets related to primary and secondary outcomes - including incidence of postoperative urinary retention (POUR), hemodynamic data, sedation scores, rescue analgesic use, and adverse events. Supporting documents such as the study protocol, statistical analysis plan, and case report forms (CRFs) may also be shared upon request.

How to access:

Interested researchers must submit a formal written request to the principal investigator (Dr. Sujan Dhakal, szndkl44@gmail.com) explaining the purpose and scope of the intended data use. Requests will be reviewed by the study team and ethics oversight body. Upon approval, access will be granted through a secure data-sharing agreement, ensuring compliance with confidentiality and ethical standards.

In this double-blind study, both the participants and the care providers (anesthesiologists) will be blinded to group allocation. The study drugs will be prepared in identical syringes by an independent assistant who is not involved in patient care or outcome assessment. Outcome assessors responsible for measuring postoperative urinary retention and other clinical parameters will also be masked. However, the data analyst will not be blinded to group assignments during statistical analysis, as unblinded data will be required for appropriate group comparisons.