Dexmedetomidine has been often used for procedural sedation. It has also has been shown to have a pain sparing effect. Therefore the investigators propose that if Dexmedetomidine is used for sedation in total knee replacements done under spinal anesthetic, the patients will have less pain up to 24 hours after the procedure.

Randomization will occur prior to patient being brought into operating room (by pharmacy or by the anesthesia research coordinator). A third anesthetist not involved in the project will mix up the syringe based on instructions in an opaque envelope, and provide the syringe to the attending anesthetist.

The current standard of care entails patients receiving a midazolam bolus for sedation during the operation. The investigators propose to substitute a syringe of dexmedetomidine or saline to be run as an infusion throughout the case for sedation. A midazolam bolus will be available at anesthetist discretion to achieve a moderate sedation score as defined by the American Society of Anesthesia.

Standardized Medication Protocol

Pre-op:

Tylenol and Naproxen

Spinal:

Marcaine .75%, 1.7cc (12.75mg) Fentanyl 10 micrograms

Sedation

1. st arm- Dexmedetomidine, drawn up into a 50cc syringe of saline, thereby making it 5ug/ml.
2. nd arm- Receives Normal Saline syringe

Plan for bolus of 0.5microgram/kg over 10 minutes, and then infusion of 0.5 microgram/kg/hr (Can be titrated from 0.2-0.7 microgram per kg per hr to achieve moderate sedation as defined by the ASA

Both groups will have a midazolam 0.5mg prn q 5minutes to be used as a rescue to achieve moderate sedation.

Post op- Initial setting of patient-controlled-analgesia pump to be set at morphine 1 mg, lockout of 8 minutes, to be increased to 1.5 mg with lockout of 6 minutes if pain not controlled. The investigators will increase the pump dose by 0.5 mg/dose/1 hr until pain controlled.

Risks The most common treatment-emergent adverse reactions for dexmedetomidine, occurring in 2% of patients in both ICU and procedural sedation studies, include- Hypotension, Bradycardia and Dry mouth. To minimize the risks to participants, an anesthesiologist will always be present in the room while the infusion is running, will monitor the patient, and will be allowed to treat as they see fit any of these risks.

Data Collection Data collection will be performed primarily by Ian Chan, anesthesiology resident.

Primary Outcome:

Total morphine consumption in the first 24 hours

Inclusion Criteria:

• Total Knee arthroplasty American Society of Anesthesiologist score of 1-3 Age 18-85 Elective total knee arthroplasty under spinal anesthetic.

Exclusion Criteria:

• Contraindication to Dexmedetomidine. (second or third degree heart block, renal or hepatic dysfunction) Contraindication to Spinal Anesthetic Pain being treated by opioids prior to operation Contraindication to premedication Previous total knee arthroplasty

24 hours