Respiratory distress is a common problem in an intensive care unit. There are multiple mechanisms that are used to help patients who are in respiratory distress including mechanical ventilation, continuous positive airway pressure (CPAP), bilevel positive airway pressure (BiPAP), high flow oxygen, and oxygen supplementation through nasal cannula or a facemask.

The purpose of this study is to evaluate the mechanism by which Vapotherm, a high flow oxygen system, provides breathing support. Vapotherm provides high flow oxygen at different flow rates, meaning one can increase the amount of oxygen flow to help with breathing support. The investigators believe that this high flow oxygen system may provide similar breathing support that a continuous positive airway pressure machine (CPAP) machine does.

High flow nasal cannula oxygen therapy (HFNC) is a method of oxygen delivery now commonly used in persistently hypoxic patients refractory to conventional modes of oxygen supplementation (i.e. nasal cannula, facemask, non-rebreather facemask). Initially used in neonates, it is now increasingly popular in the adult population. While the investigators know how HFNC provides oxygen supplementation, the physiologic mechanism of correcting hypoxemia is still unclear. There are five mechanisms of hypoxemia, four which correct with oxygen supplementation - decreased fraction of inspired oxygen (FiO2), hyperventilation, ventilation-perfusion (V/Q) mismatch, and diffusion defect; and one that does not - shunt. The hypoxemia refractory to supplemental oxygen suggests the presence of physiologic shunt. The conventional non-invasive therapy to reduce shunt fraction requires raising end-expiratory lung volumes by raising end-expiratory airway pressures using the application of continuous positive airway pressure (CPAP) or bilevel positive airway pressure (BiPAP). The Vapotherm® (Vapotherm®, Stevensville, Maryland) product of high flow oxygen therapy (Precision Flow®) is one the investigators frequently use in intensive care units at the University of Maryland Medical Center. It does not create a leak proof seal in the nose as seen in CPAP and BiPAP. Positive pressure generation has been studied in another high flow oxygen system called Optiflow™ (Fisher & Paykel Healthcare, Ltd., Auckland, New Zealand). However this device differs from Vapotherm® in that its nasal bores are large and create a seal in each nares thereby affected both ventilation and the level of positive end expiratory pressure (PEEP) generation. Additionally, these studies measured positive expiratory lung pressures, not volume. Positive end expiratory alveolar pressure and increase expiratory lung volumes in adults have not yet been demonstrated using Precision Flow®.

HFNC is intriguing because studies and clinical data have shown it is a relatively non-invasive method of oxygen delivery that appears to correct hypoxemia better than other non-invasive methods. It is more comfortable than a CPAP machine and thus is better tolerated among patients, especially those who are critically ill and possibly altered. While it has been used in neonates for some time, its use with adults is new and needs more research.

The investigators hypothesis is that HFNC corrects persistent hypoxemia by producing increased end-expiratory lung volumes thus keeping alveoli open throughout the respiratory cycle which other oxygen supplements are unable to do. Using healthy volunteers the investigators will measure end expiratory lung volumes on HFNC and compare them to those obtained with CPAP at graded pressures.

Inclusion Criteria:

• Age between 18 and 75
• Able to follow and understand simple instructions to collect spirometry

Exclusion Criteria:

• Younger than 18y/o
• Older than 75 years old
• History of chronic obstructive pulmonary disease (COPD)
• History of asthma
• History of congestive heart failure
• Measured ratio of forced expiratory volume at 1 second/forced vital capacity (FEV1/FVC) <70 when undergoing spirometry

Normal healthy adult volunteers

6 hours