This is an observational study to understand the changes in alveolar dead space in medical critically ill patients with severe infection (severe sepsis) requiring mechanical ventilation and the possibility to predict multi-organ failure.

The measurement of alveolar dead space used to require sophisticated equipment and time. New ventilators have microprocessors that allow rapid mathematical calculation with minimal intervention.

The patient will be followed during their ICU stay up to two weeks while on mechanical ventilation.

Patient will be followed daily for the initial 48 hours and then once per week for 2 weeks while mechanically ventilated.

Inclusion Criteria:

• Adults with severe sepsis or septic shock on mechanical ventilation
• Enrolled in the initial six hours of ICU admission

Exclusion Criteria:

• Patients with withdrawal or hospice order.
• Patients with terminal, irreversible disease, expect to decease in 48 hours from ICU admission.
• Patients with COPD.
• Patients transferred from outside ICU with ongoing sepsis management for more than six hours.

Adult patients admitted to medical intensive care unit with severe sepsis or septic shock requiring mechanical ventilation.