Hypobaric L5-S1 Study

Patients going through total hip or knee replacement (arthroplasty) have to be put under spinal anesthesia to prevent them from feeling any pain during surgery. Spinal anesthesia means having a small needle inserted between the bones of the lower lumbar spine (vertebrae). The choice of space used is decided by the anesthesiologist in charge of patient care. The space between these bones is often narrowed by arthritis or other age-related degenerative spine disease in patients having total hip or knee replacement surgery. The lowest L5-S1 (Lumbar 5- Sacral 1) space is the largest one and often least affected by arthritis, which makes it the easiest option. Despite this, it is not currently the most common space used for spinal anesthesia, because it is hard to find by palpation (feeling the bones through the skin), and because there is concern that local anesthetic will not always spread up high enough from this lowest space, which is needed for a successful spinal anesthesia. How far it spreads depends on the amount (dose) and density of local anesthetic given. There are two types of local anesthetic: one called isobaric, which has the same density as CSF (Cerebro-Spinal Fluid), and another called hypobaric, which has lower density than CSF. Isobaric local anesthetic spreads evenly throughout the CSF irrespective of patient positioning. However, a larger dose is required to achieve adequate anesthesia for surgery. Using larger doses, leads to a very long duration of sensory and motor block, far in excess of that required for surgical completion which leads to a slower recovery from anesthesia. Hypobaric local anesthetic, on the other hand, floats in CSF like oil on water. This allows anesthesiologists to use a smaller dose while still being sure that it will spread high enough to achieve adequate anesthesia for surgery. The smaller doses in turn allow for faster recovery of normal movement and feeling in the legs of patients after surgery. It has been observed that all of these dilemma can be solved by: 1. Using ultrasound imaging to find and mark the space, and 2. Injecting hypobaric local anesthetic. The goal of this observational study is to determine the success rate of spinal anesthesia with a lower dose of hypobaric bupivacaine injected into the L5-S1 space, when done on total hip or knee arthroplasty patients at Toronto Western Hospital.