Hypobaric L5-S1 Study

Spinal anesthesia is the recommended type of anesthesia for total hip or knee arthroplasty, as it is associated with superior clinical outcomes compared to general anesthesia. However, many patients undergoing these surgical procedures have age-related spine disease, previous spine surgery, or other spinal deformities which make the injection of spinal anesthesia difficult.

One strategy is to administer the spinal anesthesia at the L5-S1 space: it is least affected by arthritic and degenerative changes, and as it is the largest intervertebral space, it has the lowest chance of causing spinal cord trauma.

However, administration of spinal anesthesia at the lowest lumbar intervertebral levels significantly decreases the odds of success of surgical anesthesia. This is due to the fact that the standard solution of 0.5% bupivacaine is isobaric with respect to the cerebrospinal fluid (CSF), leading to unpredictable distribution within the CSF and inconsistent extent of sensory block. It has been shown that an inordinately large dose of 25 mg (5 milliliter) of 0.5% bupivacaine is required to achieve the adequate extent of sensory loss, as opposed to more conventional doses of 12-15 mg (milligram). However, this leads to a very long duration of sensory and motor block, far in excess of that required for surgical completion. This delays patient discharge from the post-anesthetic recovery unit, mobilization, and recovery. This is at odds with the goals of modern total hip and knee arthroplasty, which emphasizes same-day mobilization, physiotherapy, and even same-day hospital discharge. Using lower doses of isobaric bupivacaine, on the other hand, increases the risk of inadequate or failed spinal anesthesia, requiring conversion to general anesthesia.

One strategy to address this dilemma, which has been employed with great success to date at the Toronto Western Hospital (TWH), is to use a solution of bupivacaine that is hypobaric relative to CSF. This promotes cranial distribution of bupivacaine within the CSF and blockade of the higher spinal nerve roots, thus ensuring adequate extent of sensory block, even when administering doses as low as 10 mg of bupivacaine. This hypobaric bupivacaine solution is prepared by mixing 2 mL(milliliter) of isobaric 0.5% bupivacaine with 1 mL of sterile water. The investigators have demonstrated the efficacy of this for successful surgical anesthesia of adequate, but not excessive, duration in a recently concluded observational study. Notably, two subjects in the cohort of 60 patients received spinal anesthesia at the L5-S1 level. Both patients had adequate sensory block height and duration for commencement and completion of surgery without need for anesthetic supplementation. The investigator(s) have performed spinal anesthesia at the L5-S1 level in many other patients over the last 3 years with the same dose of hypobaric bupivacaine and have not encountered failure to date.

The purpose of this observational study is to determine the success rate of spinal anesthesia with low-dose (10 mg) hypobaric bupivacaine injected into the L5-S1 space in patients undergoing total hip or knee arthroplasty surgery.

Inclusion criteria

• Patients undergoing elective unilateral total hip or knee arthroplasty under spinal anesthesia at TWH who are scheduled for inpatient overnight admission
• American Society of Anesthesiologists physical status class (ASA-PS) 1-3
• At least 20 years of age

Exclusion criteria

• Refusal to participate
• Inability to communicate due to language barrier or cognitive impairment
• Height > 200 cm
• Weight >120 kg
• Body mass index (BMI) >45 kg/m^2
• Contraindication or allergy to amide-type local anesthetic
• Contraindication to spinal anesthesia at the L5-S1 space (e.g., infection at the injection site, existing coagulopathy, severe spinal stenosis at higher vertebral levels)
• Previous spinal surgery in the lumbosacral spine
• Spinal anesthesia performed in the operating room rather than the block room
• Spinal anesthesia that includes administration of intrathecal opioids (e.g., morphine, fentanyl)
• Pre-existing sensory or motor impairment in the lower extremities
• Inability to identify the L5-S1 space with ultrasonography