The standard atmospheric pressure of 760 mm Hg exerted at mean sea level may fluctuate by 15-20 mmHg, under normal weather conditions.
Lowering of barometric pressure is generally associated with lowered oxygen content of inspired air.
If the oxygen supply is maintained adequately, humans can tolerate lowering of atmospheric pressure for prolonged periods.
The volume of gases held in closed body cavities changes inversely with the external pressure.
Exposure to increased barometric pressure This may occur in deep-sea divers, tunnel workers and patients under hyperbaric therapy chambers.
The volume of gases held in the middle ear, paranasal sinuses, lungs, intestines, dental cavities, etc, diminishes under the high pressure and transudation occurs into these cavities.
Severe rise of pressure leads to distortion and damage to tissues.
In the middle ear, it may lead on to transudation and in severe cases even rupture of the tympanic membrane-otitic barotraumas.
Changes in the volume of air in the paranasal sinuses leads to headache, nasal discharge, or pistaxis.
Compressed air used for breathing at depths by sea divers, is generally safe but the increased partial pressure of nitrogen can cause nitrogen narcosis.
This is characterized by euphoria, error of judgement and intellectual impairment.
Oxygen toxicity may occur if pure oxygen is administered at a pressure of 2.
8 bar (equivalent to a depth of 18 m water) especially when the individual is engaged in vigorous activity.
Confusion, convulsions and damage to the lungs may result from oxygen toxicity.
Effects of sudden reduction in barometric pressure (dysbarism) Those who live in high altitudes, passengers in unpressurized aircraft, divers and workers in pressure chambers who are decompressed without proper supervision are susceptible to the ill effects of sudden reduction in ambient pressure.
Slow and graded decompression allows the gases contained in body cavities and dissolved in the blood and tissue fluids to be liberated gradually without producing untoward effects.
In persons suffering from upper respiratory infection, sinusitis, or Eustachian blockage, symptoms like ear pain, vertigo, giddiness, or deafness may occur.
Expansion of gas in the alimentary tract results in abdominal distension and dental pain results from increase in the volume of gases within dental cavities.
Aseptic necrosis of bone may occur in some cases.
As a result of rapid decompression more serious consequences occur due to formation of bubbles from liberation of nitrogen dissolved in the blood and tissue fluids.
Acute decompression sickness follows rapid reduction of environmental pressure sufficient to cause the formation of bubbles from dissolved gases in the tissues.
The bubbles act as emboli.
This disease is seen in divers and those working underwater in harbors, ports, naval installations, off-shore oil drilling, and mining industry.
In patients with airway obstruction, expansion of air in the lungs leads to pulmonary ruptures with resultant pneumothorax or surgical and mediastinal emphysema.
Factors which influence clinical manifestations are obesity and physical activity during the compression phase and individual susceptibility.
Obese subjects suffer more.
In addition to the mechanical effects due to the liberated gases, secondary phenomena also develop.
These are fat embolism, sludging of erythrocytes in the microcirculation, platelet activation and disseminated intravascular coagulation.
The gas bubbles themselves may be transient, but the effects they induce are prolonged.
Increased vascular permeability leads to fluid loss into tissues, hypovolemic shock, hemoconcentration and hyperviscosity states.
The classical form of decompression sickness occurs in Caisson disease.