Participants will be dosed 250mg Acetazolamide (p.o.) three times per day for two days prior to and a single dose on the day of study.

Participants will be dosed 100mg Methazolamide (p.o.) twice daily separated by a placebo for two days prior to and a single dose on the day of study. The placebo dose is provided to match the dosing schedule between conditions.

Participants will take (p.o.) placebo pills three times per day for two days prior to and a single dose on the day of study.

To quantify the isocapnic hypoxic ventilatory response, the hypercapnic ventilatory response, and the hypercapnic hypoxic ventilatory response, ventilation will be measured throughout controlled changes in end-tidal gas levels. Each protocol will consist of 90s steps in end-tidal oxygen partial pressure from baseline through 65, 57, and 47 mmHg. For hypercapnic hypoxia, the end-tidal partial pressure for carbon dioxide will be increased from baseline to +6 mmHg for 7 minutes before reducing the end-tidal partial pressure of oxygen as above. The poikilocapnic hypoxic ventilatory response will be determined by measuring the change in ventilation from baseline throughout 60 minutes of poikilocapnic hypoxia (fraction of inspired oxygen = 0.12)

Pulmonary artery systolic pressure (PASP) will be derived using the modified Bernoulli equation and the regurgitant velocity across the tricuspid valve. Estimates of right atrial pressure will be evaluated based upon the collapsibility index of the inferior vena cave during a sniff test. The pulmonary artery pressure response will be measured during 60 minutes of exposure to poikilocapnic hypoxia (fraction of inspired oxygen = 0.12)

To quantify the isocapnic hypoxic cerebral blood velocity response, the hypercapnic cerebral blood velocity response, and the hypercapnic hypoxic cerebral blood velocity response, cerebral blood velocity in the middle and posterior cerebral arteries will be measured throughout controlled changes in end-tidal gas levels. Each protocol will consist of 90s steps in end-tidal oxygen partial pressure from baseline through 65, 57, and 47 mmHg. For hypercapnic hypoxia, the end-tidal carbon dioxide partial pressure will be increased from baseline to +6 mmHg for 7 minutes before reducing the end-tidal oxygen partial pressure as above. The poikilocapnic hypoxic ventilatory response will be determined by measuring the change in ventilation from baseline throughout 60 minutes of poikilocapnic hypoxia (fraction of inspired oxygen = 0.12)