The study group will consist of 20 healthy non-smokers, with BMIs between 18-30 kg·m-2, free of cardiopulmonary diseases.

Subjects will wear multiple pulse oximeter devices to measure arterial oxygen saturation (SpO2).

Two fingers of each hand will be be equipped with identical FDA approved devices from two different commercial manufacturers.

The third finger of each hand be equipped with a Mayo device developed by the SPPDG program for use by the US Military.

Two wrist worn PPG-equipped "watches" (Garmin Fenix and the Mayo Clinic Developed SPPDG Watch) will be worn on each arm. One will be placed in the normal watch position (dorsal forearm) where pigmentation will generally be the darkest; the other will be placed on the volar forearm where pigmentation is generally less.

Subjects will also have an indwelling catheter placed in the brachial artery to collect samples for blood gases.

Subjects will be fitted with a mouthpiece and/or facemask to systematically reduce the fraction of inspired oxygen (FiO2) they breathe, starting with room air (21% O2), and then progressive hypoxic mixtures of 18%, 15%, 12%, and 10% FiO2 in sequence for five minutes each.

Blood flow to the hand downstream from the brachial catheter will be increased by brachial artery infusions of adenosine administered at 6.25 mcg/100ml tissue per minute for 30 minutes into the brachial artery to increase blood flow by 5 to 10-fold.

Arterial oxygen saturation will be obtained via a blood sample at the end of each stage of the breath down protocols and is reported as a percentage.

Arterial oxygen saturation will be obtained via pulse oximeter reading at the end of each stage of the breath down protocols and is reported as a percentage.