DECOMPRESSION SICKNESS, CAISSON'S DISEASE

mus effects of 02 (see below). However, in premature infants, vasoconstriction may affect retinal vessels, leading to relrolental fibroplasia and blindness. 3. High PI02 can produce pulmonary edema. 4. Within the cells an excess P02 in the milochondria, (where the P02 is of the order of only 1 mm Hg, in normal conditions) may damage some of the enzymes of Krebs cycle, causing the Krebs cycle to stop. This means stoppage of aerobic respiration and a paradoxical state, viz, development of anaerobic condition due to availability of excess oxygen, is reached. Unloward effects menlioned above, due to administration of excess oxygen may be called "oxygen poisoning" DECOMPRESSION SICKNESS, CAISSON'S DISEASE When a person is exposed to an environment of excessive barometric pressure, partial pressures of gases in his alveo- li will be abnormally high, which means the amount of gases in physical solution in his plasma will also be abnormally high. When this person suddenly reenters an atmosphere of exactly one atmospheric pressure, the plasma can no longer hold all the gases in solution and a part of them form bubbles. These bubbles eventually may block small arteries and produce symptoms due to local ischemia (lack of blood supply); they can damage by other means also. This phenomenon is called decompression sickness. Decompression sickness can be acule or chronic. Acute decompression sickness. 'Caisson's disease'. This most commonly occurs in professional divers, who go deep under the sea, where owing to the high pressure of waler, the total baromelric pressure becomes great. If this diver is suddenly broughl to the surface water (baromelric pressure exactly 760 mm Hg), bubble formation and arterial blocking as described above, may occur The gas, nitrogen deserves special attention. A small quantity of N2 is normally present in solution in the plasma al sea level pressure. In hyperbaric (where barometric pressure is high) condilion, more N2 goes into solulion. The lipids being better solvent for N2 (than water) more N2 can be found in the lipid tissues. Brain and other nervous tissues being very rich in lipid, now conlain excessive N2 and when the decompression occurs, bubbles form excessively, in Ihe brain and other nervous tissues, damaging them. Symptoms, therefore, can be of two kinds, viz (1) those due to vascular blocking, e.g. chest pain (coronary vessel), loin pain (renal vessel), pain in the muscles, and (ii) neurological signs, e.g. paralysis, aphasia etc. etc Treatment is recompression (by sending the subject promptly back again under water) as soon as symptoms appear in the diver, followed by very slow decompression. Dangers of breathing under hyperbaric conditions The lerm 'hyperbarism' means where barometric pres-sure is high. Thus, any barometric pressure > than 760 mm Hg is hyperbaric pressure. At 760 mm Hg (= atmospheric pressure), partial pressure of 02 in inspired air, PI02, is roughly 1 /5th of the barometric pressure (presuming the air is dry, le, there is no water vapour pressure). This is because, volume by volume, 02 accounts roughly 1 /5 th of the total volume of air. In hyperbaric condition, the PI02 being 1 /5th of the barometric pressure will be proportionately higher, so will be the case of every component of the 'air', because their proportions (volume/volume) remain same. This means P02, PN2 and even of any other (normally insignificant) gas component of the air will be higher therefore there will be high partial pressure of all of them in the alveolar air (tolal pressure of alveolar air, recall, has to be equal with the almospheric pressure of the environment which is here, substantially higher) therefore, their partial pressures in blood (eg. Pa02, PaC02, PaN2 etc. etc ) will be higher Question arises, what are the biological effects ? Biological effects (this is not to be confused with de-compression sickness, the subject is still underwater) are of two lypes, (i) nitrogen narcosis, and (ii) high pressure nervous syndrome, HPNS, depending upon the exact condilions. Where there is pure hyperbarism there is excessive PaN2 (partial pressure of dissolved N2 in arterial blood) nitrogen narcosis develops. In this case, it has to be presumed, that the diver (who is perhaps working to prepare a underwater tunnel) is breathing 'air' (through a tube) whose composition is identical with the ordinary room air and it is only compressed (= hyperbaric). If the deplh of ocean water is aboul 100 ft the baromelric pressure will be about 3. 5 limes 760 mm Hg (for every 30 ft, the baromelric pressure increases by one atmosphere) and this is the criti-cal point At a barometric pressure of 3.5 atmosphere or more, the PaN2 rises sufficiently to produce 'nitrogen _narcosis', a condition characterized by loss of higher brain function bul no loss of motor activity. That is, confusion/ euphoria/speech incoherence (resembling a drunk) but his motor activities will be at narcosis, oxygen helium mixture can be used for brealhmg this will lower Ihe PaN2. However althoi,, deeper than 100 ft by brealhmg oxygen-helium gas mixture, if he goes too deep he develops HPNS. HPNS is perhaps produced by gases, (inert gases ), which at sea level are non-significant but at this pressure they are dissolved sufficiently in the blood and brain tissue producing anesthesia. In HPNS there is gross impairment of motor activity but higher cerebral functions remain reasonably intact ASTHMA (BRONCHIAL ASTHMA) This disease is characterized by bronchospasm (i.e. ,