An altitude test evaluates how the human body reacts to reduced oxygen availability, a condition known as hypoxia. The test simulates atmospheric conditions experienced at high elevations, providing insight into an individual’s physiological tolerance and adaptive capacity. The primary principle is the controlled reduction of the partial pressure of inspired oxygen, mirroring the thinner air found in mountainous regions or aircraft cabins. By systematically exposing a person to a low-oxygen environment, scientists can safely observe and quantify the body’s responses. This technique is applied across multiple fields, including pre-flight medical screening, athletic training, and the diagnosis of altitude-related susceptibilities.
Simulated Altitude Testing Methods
The simulation of altitude is achieved through two distinct methods, manipulating air composition or pressure to induce hypoxia. Normobaric hypoxia involves reducing the fraction of oxygen in the air while maintaining normal atmospheric pressure. This is commonly accomplished using hypoxic generators that filter out oxygen or dilute ambient air with nitrogen gas. The reduced-oxygen air is then delivered via a face mask, or the entire environment, such as a tent or room, can be converted into a low-oxygen space.
The second method is hypobaric hypoxia, performed within a sealed structure called a hypobaric or altitude chamber. Powerful vacuum pumps within the chamber actively reduce the total atmospheric pressure, causing the air to thin out just as it does at high elevations. In this simulation, both the total air pressure and the partial pressure of oxygen are lowered simultaneously, unlike the normobaric method. Studies indicate that hypobaric conditions may elicit slightly different physiological responses, such as a higher ventilatory drive.
Monitoring equipment is essential for safety and data collection. A pulse oximeter continuously tracks the oxygen saturation of the blood (SpO\(_{2}\)), which is the percentage of hemoglobin carrying oxygen. Metabolic carts may also be integrated to measure oxygen consumption and carbon dioxide production, providing a detailed metabolic profile. Technicians gradually adjust the simulated elevation while closely monitoring the subject’s cardiovascular and respiratory status.
Clinical and Diagnostic Applications
The High Altitude Simulation Test (HAST) is a frequent clinical application used to assess a patient’s fitness for commercial air travel. Commercial aircraft cabins are typically pressurized to an altitude equivalent of 6,000 to 8,000 feet, where the inspired oxygen concentration drops from 21% to approximately 15%. Patients with cardiopulmonary conditions, such as severe anemia or Chronic Obstructive Pulmonary Disease (COPD), may experience significant drops in blood oxygen saturation (SpO\(_{2}\)) under these conditions. The HAST involves having the patient breathe a 15% oxygen mixture for about 20 minutes while their SpO\(_{2}\) is monitored.
HAST results help physicians determine if a patient requires supplemental oxygen during a flight, often prescribing a flow rate sufficient to keep the SpO\(_{2}\) above 88% to 90%. Altitude simulation also predicts an individual’s susceptibility to Acute Mountain Sickness (AMS). Measuring the drop in arterial oxygen saturation after short exposure at simulated high elevation can predict the risk of developing AMS with high accuracy. Individuals who experience a profound drop in SpO\(_{2}\) are considered less tolerant and may be advised to take preventative measures.
Sleep medicine utilizes altitude simulation to investigate how hypoxia affects sleep-disordered breathing. Studies show that exposing patients with moderate Obstructive Sleep Apnea (OSA) to a simulated altitude of 9,000 feet can convert their condition into severe Central Sleep Apnea (CSA). This occurs because reduced oxygen stimulates the respiratory drive, leading to excessive breathing (hyperventilation) that lowers carbon dioxide levels (hypocapnia). This imbalance triggers central apneas, demonstrating altitude’s impact on respiratory control during sleep.
Measuring Performance and Acclimatization
Altitude testing is a routine procedure in sports science to quantify an athlete’s aerobic capacity and track adaptation to a low-oxygen environment. Maximal oxygen uptake (VO\(_{2}\) max) testing is frequently conducted in a simulated altitude chamber using a graded exercise protocol. This test measures the maximum rate at which the body can consume oxygen during strenuous activity. By performing this test at different simulated altitudes, coaches determine the extent of performance impairment and monitor recovery during acclimatization.
The results optimize the “Live High, Train Low” (LHTL) protocol, a common training strategy for endurance athletes. LHTL involves living or sleeping at a simulated altitude, often between 6,500 and 9,800 feet, to stimulate physiological adaptations. High-intensity training sessions are performed at sea level. The test helps determine the optimal “altitude dose,” or the specific time and elevation required for the living component.
Acclimatization progress is tracked by monitoring several physiological markers over time. The most important is the increase in total hemoglobin mass, which signifies a greater oxygen-carrying capacity due to the production of new red blood cells. Researchers also track changes in hematocrit (the ratio of red blood cells to total blood volume) and resting heart rate at altitude, which decreases as acclimatization improves. An increase in minute ventilation (the volume of air breathed per minute) indicates successful ventilatory acclimatization.