This strange device is an indirect calorimeter made by CosMed Pulmonary Function Equipment which measures oxygen uptake and carbon dioxide production. It's used to measure the caloric needs of people to assure that they are getting adequate nutrition while ill and also to help guide people in healthy weight loss. It's not as uncomfortable as it looks but does take some getting used to. According to the machine I need about 2100 kcals/day which isn't too bad.
I don't care what this says I really, really needed those bignets at Cafe du Monde this morning even if they did constitute 150% of my daily caloric needs