This is a single site, prospective, non-blinded, non-randomized, non-interventional study designed to evaluate the effect of changes in alveolar minute ventilation on the measurement of respiratory heat loss (enthalpy).

This is a single site, prospective, non-blinded, non-randomized, noninterventional study designed to evaluate the effect of changes in alveolar minute ventilation on the measurement of respiratory heat loss (enthalpy). Each patient will serve as their own control. The study will enroll adult patients without cardiac or respiratory illness. It is designed to evaluate the effect of the ventilatory pattern (tidal volume, rate) on respiratory heat loss measured using the VQm™ monitor during standard intra-operative clinical care that requires positive pressure ventilation (PPV)

Inclusion Criteria:

• age greater than 18 years
• scheduled for an elective surgical procedure with an anticipated duration of more than 2 hours requiring general anesthesia and positive pressure ventilation using an endotracheal tube
• patient status ASA 1, 2 or 3
• no significant cardio-vascular or respiratory disease
• supine surgical position

Exclusion Criteria:

• Adults unable to give primary consent
• age less than 18 years
• pregnancy
• prisoners
• pre-existing cardiac or pulmonary disease
• infusion of inotropic drugs
• positive end expiratory pressure (PEEP) requirements >8 cm H2O
• increased intra-abdominal pressure (e.g. planned surgical pneumoperitoneum or ascitis)
• intra-operative patient position other than supine

adult patients without cardiac or respiratory illness

The study will enroll adult patients without cardiac or respiratory illness.

24 hours