PHYSIOLOGIC EFFECTS OF INCREASED GAS PRESSURE

Increased Barometric Pressure

Some effects of altered ambient pressure are summarized in Figure 70-2 .

An increase in environmental pressure is accompanied by significant adiabatic heat production, whereas decompression generates cooling. This effect of changes in pressure results in an increase in chamber temperature during compression and significant cooling and precipitation of water droplets during decompression. These phenomena may limit the rate of compression in manned chambers to maintain temperature within a comfortable range.

Additionally, pockets of trapped gas will either contract or expand on compression or decompression. These pockets include gas in the middle ear and paranasal sinuses, intestinal gas, pneumothorax, and gas pockets within monitoring and life support systems. The changes in gas volume occur according to Boyle's law:

PV = Constant (1)

such that a doubling of environmental pressure (P) will cause the volume (V) of a gas-filled cavity to decrease by half. This effect also underlies one of the major beneficial effects of hyperbaric treatment of pathologic gas, as in arterial gas embolism or decompression sickness (see later).

A comparison of units of pressure used clinically with those in common use in hyperbaric environments is shown in Table 70-3 .