High altitude travel can lead to inflammation in the body and activation of innate immune cells. The investigators' prior research demonstrates that 1 to 3 days at 3800 m elevation leads to increased expression of genes in blood cells that code for proteins that signal cell damage (damage associated molecular patterns (DAMPs)), cell receptors involved in innate immune responses, as well as increases in monocyte and neutrophil cells which promote inflammation. This study will investigate the potential mechanisms underlying these effects using the drug Acetazolamide, a carbonic anhydrase inhibitor which is known to reduce symptoms of Acute Mountain Sickness.

The primary research goal is to investigate the mechanisms underlying the pro-inflammatory response we observe at high altitude. The investigators believe that hypoxemia is a primary driver of this inflammatory response. High altitude exposure causes both chronic sustained hypoxemia due to reduced atmospheric oxygen availability, as well as concomitant nocturnal intermittent hypoxia on top of reduced baseline arterial oxygen pressures. Each of these hypoxic stressors may be drivers of pro-inflammatory responses. To determine if this is the case, ACZ will be used to improve oxygenation (SpO2) and ameliorate sleep disordered breathing at high altitude, thereby reducing the impacts of these factors on the immune response to high altitude.

As a secondary outcome, the study will determine if the anti-inflammatory properties of ACZ may play a key role in modulating AMS symptoms independent of ventilatory stimulation. This may reveal novel mechanisms of action of ACZ, but it is not the primary goal of this study.

The investigators hypothesize that (1) hypoxemia (both daytime chronic sustained hypoxia and noctournal intermittent hypoxia) is an independent driver of innate immune activation and pro-inflammatory responses at high altitude, and that (2) ACZ improves AMS symptoms, in part, by reducing systemic hypoxia-induced inflammation and reducing the activation of innate immune cells (monocytes and neutrophils). This hypothesis will be tested with the following experimental aims: (Aim 1) Determine if ACZ treatment during high altitude exposure blunts plasma inflammatory cytokine expression and levels of circulating pro-inflammatory innate immune cell subsets; (Aim 2) Determine if ACZ treatment during high altitude exposure modulates immune cell function (innate inflammatory responses to bacterial and viral stimuli, cell migration, and bacterial killing efficacy).

To test these hypotheses, this study utilizes a placebo controlled double-blind study design. Participants are prospectively assigned to either placebo or Acetazolamide treatment groups. Acetazolamide is prescribed at 125 mg per dose, taken twice per day starting 2 days before ascent and continuing for 3 days at high altitude.

Prior to ascent, participants complete baseline testing at low altitude. Testing includes collection of basic physiological parameters including height, weight, blood pressure, ECG, lung function testing by spirometry, and collection of self-reported medical history.

To determine the impact of treatment and altitude on breathing, ventilatory chemoreflex testing will be conducted at baseline , prior to starting treatment administration, as well as on day 2 at high altitude. Testing will be performed using a modified rebreathing method as described by Duffin (2007). From these tests, baseline tidal volume, frequency, and minute ventilation, the ventilatory recruitment threshold, and the hypoxic and hypercapnic ventilatory responses will be quantified and compared.

To determine the impact of treatment and altitude on immune parameters, peripheral venous blood samples will be collected during fasting at baseline, each morning at high altitude, and after 7 and 30 days after return to baseline elevation. From blood samples, plasma will be used to quantify expression of circulating inflammatory biomarkers, peripheral immune cells will be collected for flow cytometry analysis of immune cell population distributions as well as functional culture assays.

Additional measures of acute mountain sickness will be measured at baseline and twice per day (morning and evening) at high altitude, as well as 7 and 30 days following return to baseline using the Lake Louise AMS questionnaire and the Environmental Symptoms Questionnaire. Sleep quality will be assessed with the Stanford Sleepiness Scale.

This study will include men and women who meet the following inclusion criteria:

• Between ages 18 and 65
• No history of cardiovascular or pulmonary disease (previously diagnosed or indicated by EKG, spirometry, and other medical history)
• No history of high altitude pulmonary or cerebral edema (HAPE or HACE)
• No history of obstructive or central sleep apnea
• No current use of anti-inflammatory medication, steroids, stimulants, or other agents that may interfere with ventilatory chemosensitivity
• No current pregnancy
• No travel above 8,000 feet elevation within 1 month of the first measures
• Non-smokers (including any type of vape use, marijuana usage)
• No current or recently treated systemic or serious local infection
• Must be fluent in the English language
• Participants must refrain from consuming alcohol for at least 1 week prior to the start of the study, throughout the duration of their participation, and for 2 days prior to their follow-up visits.
• Caffeine and other stimulants may not be consumed during the course of participation in the high-altitude portion of the study (including 12 hours before sea level baseline measures, then starting 2 days before ascent and ending upon return to sea level).
• Strenuous physical activity including hiking to higher elevations is not permitted during the high altitude portion of the study.

Individual participant-level data will not be shared. Summary results will be provided to the sponsoring company and may be published in peer-reviewed journals.

Individuals completing statistical analyses will be blinded to participant treatments. Treatment distributions are developed and maintained by a pharmacy team who prepare treatments for participants. Pharmacy team personnel provide a physician team member with the coded treatment orders for prescription and distribution to participants.