Spinal anesthesia is one of the preferred anesthetic techniques for post-partum bilateral tubal ligation (PBTL). Bupivacaine is the most commonly used local anesthetic for neuraxial anesthesia for post-partum tubal ligation. Typically, hyperbaric bupivacaine would be injected into the spinal (intrathecal) space via a spinal needle; however, ongoing medication shortages have resulted in limited availability on a local and national level. One proposed alternative is isobaric bupivacaine; however, studies investigating its use for post-partum bilateral tubal ligation are limited.

The purpose of this prospective study is to determine the minimal effective dose (ED50 and ED95) of isobaric bupivacaine for adequate anesthesia during post-partum tubal ligation after vaginal delivery.

This prospective study proposes to enroll 40 post-partum women who are undergoing elective post-partum bilateral tubal ligation. The patient will be positioned sitting, and combined spinal anesthesia will be performed at L2-3 or L3-4 level. The presence of an epidural catheter enables the provider to supplement the spinal anesthetic if required. The starting dose of isobaric bupivacaine will be 10 mg based on prior similar studies1,4 and our institution's current practice. The dose of intrathecal isobaric bupivacaine was decided by using the up-down sequential method.

The motor block will be graded by a modified Bromage score: Score 1 = complete block, unable to move feet or knees; 2 = almost complete block, able to move feet only; 3 = partial block, just able to move knees; 4 = detectable weakness of hip flexion while supine, full flexion of knees; 5 = no detectable weakness of hip flexion while supine; and 6 = able to perform partial knee bend. The score of 1 or 2 within 10-15 minutes of injection will be evaluated as successful motor block.

The success of the sensory spinal block will be noted when bilateral T6 sensory level to pinprick is attained after 10 mins from the intrathecal drug administration, this is in accordance with previous studies1.

If spinal anesthesia is inadequate, the epidural catheter will be dosed up for supplementation. All observations, including block characteristics and the associated outcome (i.e., adequate or inadequate block), will be evaluated by a dedicated anesthesia provider.

Intraoperative adverse effects, such as hypotension, bradycardia, nausea, or vomiting, pruritus, or shivering were noted. Hypotension is defined as a decrease of more than 20% in basal systolic blood pressure within 30 minutes of intrathecal injection. The management of blood pressure including dose of pressor: phenylephrine and ephedrine will be at the discretion of the attending anesthesiologist. The use of pressor and anti-emetics will also be recorded.

The study will be take place in standard clinical context. The study does not deviate from our current practice, which includes the use of combined spinal anesthesia where the epidural catheter is available if supplemental local anesthetic is required.

Inclusion Criteria:

• Age 18 and older
• Patients undergoing elective post-partum bilateral tubal ligation after vaginal delivery
• Patients receiving spinal epidural anesthesia with combination of bupivacaine and fentanyl

Exclusion Criteria:

• American Society of Anesthesiologists physical status score >3,
• Contraindications to neuraxial analgesia
• Patients in whom a combined spinal epidural cannot be performed
• Body mass index >40 kg/m2
• Allergy or hypersensitivity to local anesthetics and fentanyl
• Severe liver, kidney or respiratory disease.
• Inability to understand the study protocol
• Refusal to provide written consent