The Effect of Nasal Continuous Positive Airway Pressure (N-CPAP) on Arterial pCO2 During Spinal Anesthesia

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