As air is invisible, it is quite a difficult concept to imagine that we all do live in an ocean of pressurized air. Our very existence is dependent on this pressurized environment. Our body tissues are in a state of pressure equilibrium with our surrounding air pressure. Hence it is not really surprising that a sudden loss of this air pressure, will present a major challenge for our bodys pressure equilibrium systems.
Sea level air pressure is 760 mmHg, and only 20 minutes after taking off, cabin air pressure has been reduced to only 560 mmHg; a loss of 200 mmHg. It is the subsequent tissue depressurization of 200 mmHg that causes a fluid volume shift to occur, from the central circulation to the peripheral tissues. It is this peripheral fluid shift that causes the very common problem of peripheral oedema. In particular this tissue depressurization affects the most gravity-dependent ankles and feet of so many long-haul passengers in the hypobaric cabin environment.
Our blood vessels have a high component of hydrostatic pressure. Hence during cabin decompression they suffer a relatively smaller percentage decrease in pressure than our bodys other pressurized tissue compartments. This causes a movement of fluid out of the vessel capillaries s into the relatively more depressurized extracellular fluid compartment.
Photoplethysmographic measurement of the venous function in commercial airline passengers revealed total venous failure in our subjects, only 30 minutes after taking off. However in-flight Ultrasound testing demonstrated quite normal venous function. These results would indicate that the venous failure is occurring in the micro-venous vessels, rather than in the macro-venous veins of the calf muscle pump.
Is the current hypobaric cabin pressure ceiling of 2,400 m (or 560 mmHg) adequate, or should cabins be more highly pressurized like the smaller business jets?
The aim of this film is to make people more aware of the power of air pressure, and inspire to them to further explore this fascinating subject.
Pressure Medicine Institute, Melbourne, Australia
Duration : 0:10:0