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.