• Time from spinal injection of bupivicaine until recovery room discharge criteria met. [ Time Frame: Until recovery room discharge post surgery ] [ Designated as safety issue: No ]
  

• Need for pharmacologic rescue from inadequate spinal block during the procedure. [ Time Frame: Until end of surgery ] [ Designated as safety issue: Yes ]
  
• Time from entrance into recovery room until criteria for a recovery room discharge are met [ Time Frame: Until recovery room discharge criteria are met ] [ Designated as safety issue: No ]
  
• Time from entrance into recovery room until criteria for a femoral nerve block for post-operative pain control are met [ Time Frame: Until criteria are met for post operative nerve block in recovery room ] [ Designated as safety issue: No ]
  

With the doses of bupivacaine currently being used in spinal anesthetics at our institution, patients' sensory blocks are often much higher than needed to achieve adequate surgical anesthesia. This translates into a prolonged waiting period in the recovery room before the level regresses down to the pre-existing standard for discharge to the ward, as well as the pre-existing level before a post-operative nerve block for extended pain relief can be performed. This prolonged period in the recovery room leads to general delays in operating room usage and thus surgical cancellations. The excessive drug doses currently used also put patients at risk for increasing side effects including low blood pressure, slow heart rates, and nausea and vomiting.

By using a reduced dose of bupivacaine in a spinal anesthetic for a one sided knee replacement, we hope to significantly reduce the recovery room waiting times until the criteria for recovery room discharge or completion of a post-operative nerve block are met, while still providing adequate surgical anesthesia.

Patients will receive either 9 mg (intervention dose) or 13 mg (control dose) of bupivacaine through a spinal injection. Once the patients enter the recovery room after surgery the level of their sensory block will be tested as per usual protocol by the recovery room nurses every 15 minutes until a predefined level has been reached, and the two groups will be compared.

Inclusion Criteria:

• Patients undergoing elective one-sided total knee replacement for osteoarthritis at our institution where spinal anesthesia has been chosen by the patient and the anesthetist

Exclusion Criteria:

• Contraindications to spinal anesthesia (such as increase pressure in brain, bleeding disorder, serious valvular heart disease, serious infection in the blood or on your lower back, certain neurological disorders such as multiple sclerosis, desire for a general anesthetic)
• Allergies to local anesthetics, morphine or fentanyl
• Both knees being done at same surgery
• Revision of a previous knee replacement
• Knee replacement for reasons other than osteoarthritis (such as rheumatoid arthritis)
• Potential for difficult intubation in case of need for general anesthetic
• Patients under 150 cm or over 200 cm
• BMI greater than 40
• Lack of patient consent or patient refusal