This is a single site, prospective, non-blinded, non-randomized, non-interventional study designed to evaluate the effect of changes in patient position on the measurement of respiratory heat loss (enthalpy). This study would be carried out on patients whose operation requires the use of the steep Trendelenburg position independent of this study.

Inclusion Criteria:

• age greater than 18 years
• scheduled for an elective surgical procedure requiring general anesthesia, positive pressure ventilation using an endotracheal tube and the steep Trendelenburg position

Exclusion Criteria:

• Adults unable to give primary consent
• age less than 18 years
• pregnancy
• prisoners
• infusion of inotropic drugs
• positive end expiratory pressure (PEEP) requirements >8 cm H2O Women, minorities or non-English speaking subjects will not be excluded.

Elective surgery requiring steep Trendelenberg position.