The estimation of the partial pressure of carbon dioxide (PCO2) in the arterial blood is used to judge the adequacy of ventilation during spontaneous and controlled ventilation. Although the gold standard for monitoring PCO2 remains arterial blood gas sampling, this requires an invasive procedure and provides only an intermittent estimate of what is frequently a continuously changing value. The solution to this problem has been the development and validation of accurate noninvasive monitoring techniques which provide a continuous intraoperative estimate of PCO2.The most commonly used noninvasive technique to monitor PCO2 is measurement of the end tidal CO2 (PECO2) . However, sampling errors and patient -related issues such as ventilation-perfusion mismatch, patient positioning or decreases in pulmonary blood flow may influence the accuracy of PECO2 monitoring (1-3). Nasal capnography has been proved to be an accurate monitor during the post-operative period (4) but its ability to accurately detect hypoventilation associated with deep sedation has not been studied.

According to the American Society of Anesthesiologist standards for basic monitoring, continuous capnography is required for all patients undergoing general anesthesia but it is optional for MAC/sedation cases. The need for CO2 monitoring has been studied by other medical specialties that use procedural sedation, including gastroenterology (12) and emergency medicine (13, 14) and many specialties now recommend capnography as a standard monitor.

Patients receiving supplemental oxygen may experience significant persistent hypoventilation leading to progressive hypercarbia and acidosis which may go undetected for a significant time interval since the routinely monitored SpO2 may be maintained within normal range. A recent study has shown that despite the fact that end tidal CO2 is reliable in detecting apnea , increasing oxygen flow rates decrease the amplitude of measured CO2, probably via dilution, making the quantitative value less reliable as an assessment of adequacy of ventilation (15). Furthermore, during hypoventilation there is reduced alveolar ventilation and the end tidal CO2 is not a true reflection of arterial CO2.

Transcutaneous measurement of PCO2(PtcCO2) is a non-invasive method of measuring PCO2 that has been used much less frequently due to technical difficulties with earlier transcutaneous electrodes. Preliminary studies of the reliability of the current PtcCO2 electrodes (TOSCA, Linde Medical Sensors, and Basel, Switzerland) have shown good correlation of arterial and transcutaneous measurements in both adult volunteers and anesthetized subjects (5). PtcCO2 is measured with a sensor attached by a low pressure clip to an earlobe. The sensor probe heats the earlobe to 42 degrees Celsius to enhance blood flow. The current sensors have also been evaluated in anesthetized children (7, 8), anesthetized adults (9, 10) and critically ill neonates (11) and all these studies revealed a good correlation between PtCO2 and PaCO2.

A group of _40 patients scheduled to undergo Hysteroscopy surgery with deep sedation will be studied. Standard monitors will be utilized. Additionally a transcutaneous CO2 electrode will be attached to the ear lobe and a BIS monitor will be applied to the forehead.

The anesthetic management will be left to the discretion of the anesthesia provider (Anesthesia resident or CRNA, under the supervision of a faculty attending anesthesiologist).All patients will receive supplemental oxygen at 3L/ minute via nasal cannula, to maintain oxygen saturation greater than 90%.

Both the end tidal CO2 (ETCO2) and transcutaneous CO2 (TcCO2) (Linde Medical Sensors, Basel , Switzerland ) will be calibrated before use, according to the manufacturer's instruction.. The anesthesia provider will be blinded to the TcCO2 monitor but will have access to the ETCO2 values. The ETCO2 and TcCO2 values will be recorded simultaneously by an independent observer.

One arterial blood samples will be collected from each subject for measurement of PaCO2 at a deep level of sedation defined by a modified Ramsey score greater or equal to 5 (1= anxious/agitated/restless; 2= cooperative/oriented/tranquil; 3= drowsy/responds to commands only; 4= brisk response to shaking/loud sound; 5= sluggish response to shaking/loud sound; 6= no response) The ETCO2 and .TcCO2 will be recorded throughout the time of obtaining the blood sample every 15 seconds and the mean value will be used for comparison. Hypoventilation will be defined as a PaCO2 level greater than 45mmHg.

Sedation score (OASS) will be recorded on admission to recovery area. Time to meet discharge criteria will be recorded.

The experimental procedures: ABG draw, ETCO2 monitor placement on the ear, 2 Ramey scale assessments.

Inclusion Criteria:

• Age>18 years of age
• Sex: Non-pregnant, Female
• ASA PS: I, II
• Surgery: Elective Hysteroscopy surgery
• Consent: Obtained

Exclusion Criteria:

• Age < 18 years old
• Patients who refuses participation
• History of lung disease
• History of Obstructive sleep apnea