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.