HYPOBARIC AND NORMOBARIC HYPOXIA
There are two distinct approaches to elicit ambient hypoxia: hypobaric hypoxia and normobaric hypoxia. In hypobaric hypoxia, as described by the Dalton’s law of partial pressures, oxygen concentration of air remains constant at approximately 21% but hypoxia results from reduced ambient air pressure. In a large meta-analysis of high-altitude studies, a linear relationship between altitude and resulting arterial oxygen pressure could be identified for altitudes below 6000 meters. Notably, the analysis revealed a consistent decrease in arterial oxygen partial pressure by 1.6 kPa for every 1000 meters of altitude ascended [44].
In normobaric hypoxia, oxygen concentration is lowered through the addition of an inert gas, typically nitrogen, while ambient air pressure remains unchanged. Differences in the physiological response to hypobaric and normobaric hypoxia exist but have not yet been fully characterised [45, 46]. Effects of reduced pressure on the middle ear and other closed air-containing organs are evident. Important disadvantages of hypobaric hypoxia compared with normobaric hypoxia are that sophisticated and costly hypobaric chambers are required and that study participants and staff cannot easily move in and out of the hypoxia environment. Decompression to severe hypoxia may already go along with the risk of decompression illness. Despite these challenges, hypobaric chambers offer distinct advantages over alternative methods for simulating hypoxia. These chambers can be used to precisely control the level of hypoxia, which is important for studies that require consistent and repeatable conditions. By means of air pumps, pressure regulators, and control systems, the pressure inside the chamber can be adjusted relatively rapidly as needed. Furthermore, hypobaric chambers are often used for research or training programs that require large groups to be exposed to hypoxic conditions simultaneously. On the other hand, normobaric hypoxia is easier to implement, but requires more time to adjust the oxygen concentrations. The nitrogen required for normobaric hypoxia can be supplied onsite through concentrators, which operate with molecular sieves, or from a nitrogen tank. Room-in-room solutions for normobaric hypoxia are commercially available. For short term applications, hypoxic gas mixtures can also be supplied through a face mask, which offers the notable advantage of excluding the experimenters from the condition while allowing them in close proximity to the participants.