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.