The proposed study intends to investigate several aspects of the respiratory effects of intravenous sedation of patients undergoing spinal anesthesia for knee replacement surgery. The study will include assessment of PaCO2 during the intraoperative period. PaCO2 is expected to be elevated as a result of intravenous sedation and postural factors. Further, the study will investigate how application of Nasal Continuous Positive Airway Pressure (N-CPAP) may impact PaCO2.

Inclusion Criteria:

• American Society of Anesthesiology Class I, II, or III patients
• Age over 21 years and under 70 years
• Planned surgical procedure is total knee arthroplasty (TKA) to be performed at Cape Fear Hospital
• Body Mass Index less than 40
• The patient and their attending anesthesiologist select spinal anesthesia with intravenous sedation, intrathecal morphine, and femoral nerve block as the preferred anesthesia plan

Exclusion Criteria:

• Prior diagnosis of Obstructive Sleep Apnea based upon sleep study
• History of Stroke with residual neurologic deficit
• Prior diagnosis of Emphysema, Chronic Bronchitis, Chronic Obstructive Pulmonary Disease, or Asthma, or other chronic lung disease
• Recent history of significant nasal obstruction, epistaxis, or facial abnormality that may interfere with proper fit of a nasal CPAP mask
• Pregnancy
• Mental or other disability preventing a patient from personally giving informed consent
• Chronic narcotic or benzodiazepine treatment or dependency
• Allergy to Midazolam, Fentanyl, or Propofol
• Severe upper respiratory infection within the past three weeks