Exclusion criteria: Subjects with underlying conditions that could affect ventilation, perfusion, or metabolism including intubated subjects, subjects with clinical signs of cardiopulmonary instability, major trauma, thoracic trauma, shock, sepsis, psychiatric disorders and ASA class 3, 4, and 5. Also those unable to provide informed consent, nursing home residents, age less than 18 years, non English speaking, pregnant women, subjects under police custody, or physician discretion.